Chapter 15: Respiration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The respiratory tract The respiratory tract extends from the nose to the lungs, which are composed of air sacs called alveoli. Gas exchange occurs between air in the alveoli and blood within a capillary network that surrounds the alveoli. Notice that the pulmonary arteriole is colored blue – it carries O2-poor blood away from the heart to the alveoli. The pulmonary venule is colored red – it carries O2-rich blood from alveoli toward the heart.

The Respiratory Tract Air is cleansed, warmed, and moistened as it passes the cilia and mucus in the nostrils and nasal cavity. In the nose, the hairs and the cilia act as a screening device. In the trachea, the cilia beat upward, carrying dust and mucus into the pharynx. Exhaled air carries out heat and moisture.

The path of air This drawing shows the path of air from the nose to the trachea.

The Nose The two nasal cavities are divided by a septum.
They contain olfactory cells, receive tear ducts from eyes, and communicate with sinuses. The nasal cavities empty into the nasopharynx. Auditory tubes lead from the middle ears to the nasopharynx.

The Pharynx The pharynx (throat) is a passageway from the nasal cavities to oral cavities and to the larynx. The pharynx contains the tonsils; the respiratory tract assists the immune system in maintaining homeostasis. The pharynx takes air from the nose to the larynx and takes food from the oral cavity to the esophagus. The pharynx has three parts: the nasopharynx, the oropharynx, and the laryngopharynx. The tonsils contain lymphocytes that protect against invasion of foreign antigens that are inhaled.

The Larynx The larynx is a cartilaginous structure lying between the pharynx and the trachea. The larynx houses the vocal cords. A flap of tissue called the epiglottis covers the glottis, an opening to the larynx. In young men, rapid growth of the larynx and vocal cords changes the voice. Vocal cords are folds of mucous membrane supported by elastic ligaments, which are stretched across the glottis. Pitch of the voice is controlled by regulating the tension on the vocal cords. Volume of the voice depends on the amplitude of vibrations of the vocal cords.

Placement of the vocal cords
a. Frontal section of the larynx shows the location of the vocal cords inside. b. Viewed from above, it can be seen that the vocal cords are stretched across the glottis. When air passes through the glottis, the vocal cords vibrate, producing sound. The glottis is narrow when we produce a high-pitched sound (top), and it widens as the pitch deepens (bottom).

The Trachea The trachea, supported by C-shaped cartilaginous rings, is lined by ciliated cells, which sweep impurities up toward the pharynx. Smoking destroys the cilia. The trachea takes air to the bronchial tree. Blockage of the trachea requires an operation called a tracheostomy to form an opening.

Cilia in the trachea

The Bronchial Tree The trachea divides into right and left primary bronchi which lead into the right and left lungs. The right and left primary bronchi divide into ever smaller bronchioles to conduct air to the alveoli. An asthma attack occurs when smooth muscles in the bronchioles constrict and cause wheezing.

The Bronchial Tree The respiratory tract extends from the nose to the lungs, which are composed of air sacs called alveoli. Gas exchange occurs between air in the alveoli and blood within a capillary network that surrounds the alveoli. Notice that the pulmonary arteriole is colored blue – it carries O2-poor blood away from the heart to the alveoli. The pulmonary venule is colored red – it carries O2-rich blood from alveoli toward the heart.

The Lungs Lungs are paired, cone-shaped organs.
Lungs are functionally composed of tiny air-sacs called aveoli The right lung has three lobes, and the left lung has two lobes, allowing for the space occupied by the heart. The lungs are bounded by the ribs and diaphragm.

The Alveoli Alveoli are the tiny air sacs of the lungs made up of squamous epithelium and surrounded by blood capillaries. Alveoli function in gas exchange, oxygen diffusing into the bloodstream and carbon dioxide diffusing out. Infant respiratory distress syndrome occurs in premature infants where underdeveloped lungs lack surfactant (thin film of lipoprotein) and collapse. Surfactant lowers the surface tension within the alveoli and prevents them from collapsing.

Gas exchange in the lungs
The lungs consist of alveoli surrounded by an extensive capillary network. Notice that the pulmonary artery carries O2-poor blood (colored blue), and the pulmonary venule carries O2-rich blood (colored red).

Inspiration and Expiration
There is a continuous column of air from the pharynx to the alveoli, and the lungs lie within the sealed-off thoracic cavity. The thoracic cavity is bounded by the rib cage and diaphragm. A infection of the pleural membranes is called pleurisy.

Inspiration When we inhale (inspiration) the rib cage rises and the diaphragm lowers, causing the thoracic cavity to expand. The negative pressure or partial vacuum in the alveoli causes the air to come in. Chemoreceptors in the carotid bodies, located in the carotid arteries, and in the aortic bodies, located in the aorta, are sensitive to the level of oxygen in blood. Decreasing oxygen causes these bodies to communicate with the respiratory center to increase the rate and depth of breathing. Carbon dioxide (CO2) and hydrogen ions (H+) are the primary stimuli that cause changes in the activity of the medullary respiratory center; the center itself is not affected by low oxygen levels.

Control of breathing During inspiration, the respiratory center stimulates the external intercostal (rib) muscles to contract via the intercostal nerves and stimulates the diaphragm to contract via the phrenic nerve. Should the tidal volume increase above 1.5 liters, stretch receptors send inhibitory impulses to the respiratory center via the vagus nerve. In any case, expiration occurs due to lack of stimulation from the respiratory center to the diaphragm and intercostal muscles.

Inspiration/Expiration
During inspiration, the thoracic cavity and lungs expand so that air is drawn in.

Expiration When we exhale (expiration), the rib cage lowers and diaphragm to resume dome shape. Expiration is passive, while inspiration is active.

Internal Respiration Internal respiration is the diffusion of O2 from systemic capillaries into tissues and CO2 from tissue fluid into systemic capillaries through hemoglobin in Red Blood Cells. Oxyhemoglobin gives up O2, which diffuses out of the blood and into the tissues because the level of O2 in tissues is lower than that of the blood. CO2 diffuses from tissue cells into the blood, it enters red blood cells where a small amount is taken up by hemoglobin

Internal Respiration All this “internal respiration” occurs at the capillaries (single-cell thickness allows for complete diffusion) Blood leaving capillaries (which become “venules” and then “veins” is a dark maroon color because red blood cells contain reduced hemoglobin.

External and internal respiration
During external respiration in the lungs, CO2 leaves the blood and O2 enters the blood. During internal respiration in the tissues, O2 leaves the blood and CO2 enters the blood. External Respiration: At the pulmonary capillaries, O2 enters red blood cells where it combines with hemoglobin (Hb) to form oxyhemoglobin (HbO2). Also, bicarbonate (HCO3-) is converted inside red blood cells to H2O and CO2. CO2 leaves red blood cells and capillaries and diffuses into the lungs to be exhaled. Internal Respiration: At the systemic capillaries, oxyhemoglobin (HbO2) inside red blood cells gives up its oxygen and becomes Hb and O2. Hemoglobin (Hb) now combines with H+ to form reduced hemoglobin (HHb). O2 leaves red blood cells and capillaries and enters tissue cells. At the same time, CO2 enters red blood cells. Some combines with Hb to form carbaminohemoglobin (HbCO2). Most CO2 is converted to bicarbonate (HCO3-), which is carried in the plasma.

Sites of upper respiratory infections
A nasal infection, more commonly called rhinitis, is the usual symptom of a common cold due to a viral infection, but rhinitis can also be due to a bacterial infection. Secondary to an URI, the sinuses, middle ear, tonsils, and vocal cords can become infected. Allergies also cause runny nose, blocked sinuses, and laryngitis.

Sinusitis Sinusitis is infection of the cranial sinuses within the facial skeleton that drain into nasal cavities. It occurs when nasal congestion blocks the sinus openings and is relieved when drainage is restored. Pain and tenderness over the lower forehead and cheeks, and toothache, accompany this condition.

Otitis Media Otitis media is bacterial infection of the middle ear.
Children suffer when a nasal infection spreads to the middle ear by way of the auditory tube and antibiotics are usually used to clear the infection. Sometimes drainage tubes (called tympanostomy tubes) are inserted into the eardrums of children with recurrent infections.

Tonsillitis Laryngitis
Tonsillitis is infection of tonsils and recurrent infections that make breathing or swallowing difficult may be relieved by a tonsillectomy. Laryngitis Laryngitis is an infection of the larynx and usually results in a loss of voice. Persistent hoarseness is a warning sign of cancer.

Lower Respiratory Tract Disorders
Lower respiratory infections include: acute bronchitis, an infection of primary and secondary bronchi; pneumonia involving a bacterial or viral infection of the lungs; and pulmonary tuberculosis (infection caused by tubercle bacillus). Acute bronchitis is usually preceded by a viral URI that has led to a secondary bacterial infection. Pneumonia causes the lungs to fill with fluid; high fever and chills, headache, and chest pain are symptoms of pneumonia. When tubercle bacilli invade the lung tissue, the lung cells build a protective capsule about the foreign cells, isolating them from the rest of the body. This capsule is called a tubercle (hence the name tuberculosis). If the person’s immune system is functioning well, the bacteria are killed. If not, the bacteria are eventually liberated from their capsules. Tuberculosis was a major killer in the United States before the middle of the twentieth century, after which antibiotic therapy brought it under control. New, antibiotic-resistant strains, plus the rise of tuberculosis among AIDS patients, the homeless, and the rural poor are contributing to the new increase in tuberculosis cases.

Restrictive Pulmonary Disorders
In restrictive pulmonary disorders, vital capacity is reduced because the lungs have lost their elasticity due to inhaled particles such as silica, coal dust, or asbestos. Fibrous connective tissue builds in the lungs in pulmonary fibrosis, caused by exposure to inhaled particles, including those of fiberglass. It has been projected that two million deaths caused by asbestos exposure – mostly in the workplace – will occur in the United States between 1990 and Asbestos exposure is also associated with the development of lung cancer. Asbestos was formerly widely used as a fireproofing and insulating material.

Obstructive Pulmonary Disorders
In obstructive pulmonary disorders, air does not flow freely in the airways, and inhalation and exhalation are difficult. Chronic bronchitis with inflamed airways, emphysema where alveolar walls break down, and asthma with constricted bronchioles obstruct the airways and tend to get progressively worse or recur.

Lower respiratory tract disorders
Exposure to infectious pathogens and/or air pollutants, including cigarette and cigar smoke, can cause the diseases and disorders shown here. Pneumonia: Alveoli fill with thick fluid, making gas exchange difficult. Pulmonary fibrosis: Fibrous connective tissue build up in the lungs, reducing lung elasticity. Pulmonary tuberculosis: Tubercles encapsulate bacteria, and the elasticity of the lungs is reduced. Emphysema: Alveoli burst and fuse into enlarged air spaces. Surface area for gas exchange is greatly reduced. Asthma: Airways are inflamed due to irritation, and bronchioles constrict due to muscle spasms. Bronchitis: Airways are inflamed due to infection (acute) or an irritant (chronic). Coughing brings up mucus and pus.

Lung Cancer Lung cancer follows this sequence of events: thickening of airway cells, loss of cilia on the lining, cells with atypical nuclei, tumor development, and finally metastasis. Removal of a lobe or lung, called pneumonectomy, may remove the cancer. Smoking, whether active or passive, is a major cause of lung cancer.

Normal lung versus cancerous lung
On the left is a normal lung with the heart in place. Note the healthy red color. On the right are the lungs of a heavy smoker. Notice how black the lungs are except where cancerous tumors have formed.

During inspiration, the pressure in the lungs decreases and air comes rushing in; during expiration, increased pressure in the thoracic cavity causes air to leave the lungs. External respiration occurs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out of the blood. Internal respiration occurs in the tissues where oxygen diffuses out of the blood into tissue cells and carbon dioxide diffuses into the blood.

The respiratory pigment hemoglobin transports oxygen from the lungs to the tissues and aids in the transport of carbon dioxide from the tissues to the lungs. The respiratory tract is especially subject to disease because it is exposed to infectious agents; also, cigarette smoking contributes to two major lung disorders—emphysema and cancer.

Chapter 17: Nervous System

Nervous Tissue The nervous system is divided into a central nervous system (CNS), consisting of the brain and spinal cord, and a peripheral nervous system (PNS), consisting of nerves carrying sensory and motor information between the CNS and muscles and glands. Both systems have two types of cells: neurons that transmit impulses.

Organization of the nervous system
In paraplegics, messages no longer flow between the lower limbs and the central nervous system (the spinal cord and brain). The sensory neurons of the peripheral nervous system take nerve impulses from sensory receptors to the central nervous system (CNS), and motor neurons take nerve impulses from the CNS to the organs, muscles, and glands.

Neuron Structure Neurons are composed of dendrites that receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away. Sensory neurons take information from sensory receptors to the CNS. Interneurons occur within the CNS and integrate input. Motor neurons take information from the CNS to muscles or glands.

Types of neurons A sensory neuron, an interneuron, and a motor neuron are drawn here to show their arrangement in the body. (The breaks indicate that the fibers are much longer than shown.) How does this arrangement correlate with the function of each neuron?

Myelin Sheath Long axons are covered by a protective myelin sheath formed by another type of cell. This sheath acts like insulation on a wire, increasing the speed of transmission. The sheath contains lipid myelin which gives nerve fibers their white, glistening appearance. Multiple sclerosis is a disease of the myelin sheath. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains an serves as a passageway for new fiber growth. In multiple sclerosis (MS) lesions develop in the myelin sheath of the CNS and become hardened scars that interfere with normal conduction of nerve impulses, and the result is various neuromuscular symptoms.

Myelin sheath In the PNS, a myelin sheath forms when cells wrap themselves around an axon. The inset shows an electron micrograph of a cross section of an axon surrounded by a myelin sheath.

The Nerve Impulse The nervous system uses the nerve impulse to convey information. The nature of a nerve impulse has been studied by using excised axons and a voltmeter. Voltage (in millivolts, mV) measures the electrical potential difference between the inside and outside of the axon.

Resting Potential When an axon is not conducting a nerve impulse, the inside of an axon is negative (-65mV) compared to the outside; this is the resting potential. A sodium-potassium pump in the membrane actively transports Na+ out of the axon and K+ into the axon to establish resting potential. The membrane is more permeable to K+ and much of the resting potential is due to the excess of K+ outside of the neuron.

Resting potential An oscilloscope, an instrument that records voltage changes, records a resting potential of -65 mV. There is a preponderance of Na+ outside the axon and a preponderance of K+ inside the axon. The permeability of the membrane to K+ compared to Na+ causes the inside to be negative compared to the outside.

Action Potential An action potential is a rapid change in polarity as the nerve impulse occurs. The action potential occurs if a stimulus causes the membrane to depolarize past threshold. An intense stimulus causes many firings (reaching action potential) in an axon; a weak stimulus may cause only a few. The action potential requires two types of gated channel proteins: one each for Na+ and K+.

Sodium Gates Open Potassium Gates Open
The gates of sodium channels open first and Na+ flows into the axon. The membrane potential depolarizes to +40 MV. Potassium Gates Open The gates of potassium channels open next and K+ flows to the outside of the axon. The membrane potential repolarizes to –65 MV.

Action potential A depolarization occurs when Na+ gates open and Na+ moves to inside the axon; a repolarization occurs when K+ gates open and K+ moves to outside the axon.

This graph shows the enlargement of the action potential as seen by an experimenter using an oscilloscope.

Propagation of an Action Potential
The action potential travels the length of an axon, with each portion of the axon undergoing depolarization then repolarization. A refractory period ensures that the action potential will not move backwards. In myelinated fibers, the action potential only occurs at the nodes of Ranvier. This “jumping” from node-to-node is called saltatory conduction. Saltatory conduction on myelinated axons causes the axon potential to travel much faster than it does on nonmyelinated axons.

Transmission Across a Synapse
The tip of an axon forms an axon bulb that is close to a dendrite or cell body of another neuron; this region of close proximity is called the synapse. Transmission of a nerve impulse takes place when a neurotransmitter molecule stored in synaptic vesicles in the axon bulb is released into a synaptic cleft between the axon and the receiving neuron.

When a nerve impulse reaches an axon bulb, gated channels for calcium open and Ca2+ flow into the bulb. This sudden rise in Ca2+ causes synaptic vesicles to move and merge with the presynaptic membrane, releasing their neurotransmitter molecules into the cleft. The binding of the neurotransmitter to receptors in the postsynaptic membrane causes either excitation or inhibition.

Synapse structure and function
Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released at the presynaptic membrane, diffuses across the synaptic cleft, and binds to a receptor in the postsynaptic membrane. After the action potential arrives at an axon bulb, synaptic vesicles fuse with the presynaptic membrane. Next, neurotransmitter molecules are released and bind to receptors on the postsynaptic membrane. Finally, when a stimulatory neurotransmitter binds to a receptor, Na+ diffuses into the postsynaptic neuron.

Synaptic Integration Many synapses per single neuron is not uncommon.
Excitatory signals have a depolarizing effect, and inhibitory signals have a hyperpolarizing effect on the postsynaptic membrane. Integration is the summing up of these excitatory and inhibitory signals.

Integration Inhibitory signals and excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. In the example shown in the graph, threshold was not reached.

Neurotransmitter Molecules
Out of 25, two well-known neurotransmitters are acetylcholine (ACh) and norepinephrine (NE). Neurotranmitters that have done their job are removed from the cleft; the enzyme acetylcholinesterase (AChE) breaks down acetylcholine. Neurotransmitter molecules are removed from the cleft by enzymatic breakdown or by reabsorption, thus preventing continuous stimulation or inhibition. Many drugs that affect the nervous system act by interfering with of enhancing the action of neurotransmitters. A drug can either block the release of a neurotransmitter, mimic the action of a neurotransmitter or block the receptor, or interfere with the removal of a neurotransmitter from a synaptic cleft.

The Central Nervous System
The central nervous system (CNS) consists of the spinal cord and brain. Both are protected by bone, wrapped in protective membranes called meninges, and surrounded and cushioned with cerebrospinal fluid that is produced in the ventricles of the brain.

The ventricles are interconnecting cavities that produce and serve as a reservoir for cerebrospinal fluid. The CNS receives and integrates sensory input and formulates motor output. Gray matter contains cell bodies and short, nonmyelinated fibers; white matter contains myelinated axons that run in tracts.

Organization of the nervous system
The CNS, composed of the spinal cord and brain, communicates with the PNS, which contains nerves. In the somatic system, nerves conduct impulses from sensory receptors to the CNS and motor impulses from the CNS to the skeletal muscles. In the autonomic system, consisting of the sympathetic and parasympathetic divisions, motor impulses travel to smooth muscle, cardiac muscle, and the glands.

Structure of the Spinal Cord
The spinal cord extends from the base of the brain through the vertebral canal. Structure of the Spinal Cord A central canal holds cerebrospinal fluid. Gray matter of the spinal cord forms an “H” and contains interneurons and portions of sensory and motor neurons. White matter consists of ascending tracts taking sensory information to the brain and descending tracts carrying motor information from the brain.

Spinal cord The spinal cord passes through the vertebral canal formed by the vertebrae.

The spinal cord has a central canal filled with cerebrospinal fluid, gray matter in an H-shaped configuration, and white matter around the outside. The white matter contains tracts that take nerve impulses to and from the brain. The photomicrograph shows a cross section of the spinal cord.

Functions of the Spinal Cord
The spinal cord is the center for many reflex arcs. It also sends sensory information to the brain and receives motor output from the brain, extending communication from the brain to the peripheral nerves for both control of voluntary skeletal muscles and involuntary internal organs. Severing the spinal cord produces paralysis.

The Brain The brain has four cavities called ventricles.
The cerebrum has two lateral ventricles, the diencephalon has the third ventricle, and the brain stem and cerebellum have the fourth ventricle.

The human brain The cerebrum, seen here in longitudinal section, is the largest part of the brain in humans. Note the locations of the four ventricles (one lateral ventricle lies within each cerebral hemisphere).

The Cerebrum The cerebrum or telencephalon has two cerebral hemispheres connected by the corpus callosum. Learning, memory, language and speech take place in the cerebrum. Sulci divide each hemisphere into lobes including the frontal, parietal, occipital, and temporal lobes.

Cerebral hemispheres Viewed from above, the cerebrum has left and right cerebral hemispheres that are connected by the corpus callosum.

The Cerebral Cortex The cerebral cortex is a thin, highly convoluted outer layer of gray matter covering both hemispheres. The primary motor area is in the frontal lobe; this commands skeletal muscle. The primary somatosensory area is dorsal to the central sulcus or groove. The primary visual area is at the back occipital lobe. The temporal lobe has the primary auditory area.

The parietal lobe provides taste sensation.
All have adjacent association areas that integrate signals; the prefrontal area is an important association area for appropriate behavior. White matter consists mostly of long myelinated axons forming tracts; these cross over so the left side of the brain handles right side information. Basal nuclei are masses of gray matter deep within the white matter integrate motor commands. Huntington disease and Parkinson disease, which are both characterized by uncontrollable movements, are believed to be due to malfunctioning of the basal nuclei. (Basal nuclei were formerly referred to as basal ganglia.)

The lobes of a cerebral hemisphere
Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. The cerebral cortex of a frontal lobe has motor areas and an association area called the prefrontal area. The cerebral cortical areas of the other lobes have both sensory areas and association areas.

The Diencephalon The hypothalamus and thalamus are in the diencephalon that encircles the third ventricle. The hypothalamus controls homeostasis and the pituitary gland, and the thalamus receives all sensory input except smell and integrates it and sends it to the cerebrum. The pineal gland is also located here and secretes melatonin that may regulate our daily rhythms. The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The Cerebellum The cerebellum receives sensory input from eyes, ears, joints and muscles and receives motor input from the cerebral cortex. It integrates this information to maintain posture and balance. The cerebellum is involved in learning of new motor skills, such as playing the piano. A thin layer of gray matter covers the white matter.

The Brain Stem The brain stem contains the medulla oblongata, pons, and midbrain. The medulla oblongata and pons have centers for vital functions such as breathing, heartbeat, and vasoconstriction. The medulla also coordinates swallowing and some other automatic reactions. The midbrain acts as a relay station between the cerebrum and spinal cord or cerebellum. The medulla oblongata contains reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing.

The Reticular Formation
The reticular formation is a complex network of nuclei and fibers that extend the length of the brain stem. One portion of the reticular formation, called the reticular activating system, arouses the cerebrum via the thalamus causing alertness. An inactive reticular activating system results in sleep. A severe injury to the reticular activating system can cause a person to be comatose.

The reticular activating system
The reticular formation receives and sends on motor and sensory messages to various parts of the CNS. One portion, the reticular activating system (see arrows), arouses the cerebrum and in this way controls alertness versus sleep.

The Limbic System and Higher Mental Functions
The limbic system is involved in our emotions and higher mental functions. The limbic system is a complex network of tracts and nuclei involving cerebral lobes, basal nuclei and the diencephalon. Two structures, the hippocampus and amygdala are essential for learning and memory. The hippocampus is well situated in the brain to make the prefrontal area aware of past experiences stored in association areas. The amygdala can cause these experiences to have emotional overtones. The prefrontal area consults the hippocampus in order to use memories to modify our behavior. However, since the frontal lobe is included in the limbic system, the ability to reason prevents us from acting out strong feelings.

The limbic system Structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions such as reasoning with more primitive feelings such as fear and pleasure.

Higher Mental Functions
Animal research, MRI, and PET scans allow researchers to study the functioning of the brain. Memory and Learning Memory is the ability to hold a thought in mind or recall events from the past. Learning takes place when we retain and utilize past memories.

Short-term memory involves activity in the prefrontal area.
Long-term memory includes semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.). Skill memory involves ability to ride a bike, for example, and involves all motor areas of the cerebrum below the level of consciousness.

Long-term Memory Storage and Retrieval
Our long-term memories are stored in bits and pieces throughout the sensory association areas of the cerebral cortex. The hippocampus is a bridge between sensory association areas and the prefrontal area where memories are utilized. The amygdala associates danger with sensory stimuli.

Long-term memory circuits
The hippocampus and amygdala are believed to be involved in the storage and retrieval of memories. Semantic memory (red arrows) and episodic memory (black arrows) are stored separately, and therefore you can lose one without losing the other.

Long-Term Potentiation
Long-term potentiation is increased response at synapses within the hippocampus and is essential to long-term memory. However, a postsynaptic neuron in the hippocampus can become too excited and then die. Excitotoxicity, a form of cell death, is due to the neurotransmitter glutamate rushing in too quickly. Neuroprotective drugs are being developed in the hope they will prevent disorders like Alzheimer disease that might be due to excitotoxicity.

Language and Speech Language and speech are dependent upon Broca’s area (a motor speech area) and Wernicke’s area (a sensory speech area) that are involved in communication. These two areas are located only in the left hemisphere; the left hemisphere functions in language in general and not just in speech. The left brain and right brain have different functions. Functions associated with the left brain center around those that are verbal, logical, analytical, and rational; functions associated with the right hemisphere are nonverbal, visuo-spatial, intuitive, and creative. Recent studies in hemisphere dominance suggest that rather than hemisphere dominance, the hemispheres simply process the same information differently.

Language and speech Wernicke’s area and Broca’s area are thought to be involved in speech comprehension and use, as are the other labeled areas of the cerebral cortex.

The Peripheral Nervous System
The peripheral nervous system (PNS) contains nerves (bundles of axons) and ganglia (cell bodies). Sensory nerves carry information to the CNS, motor nerves carry information away, and mixed nerves have both types of fibers. Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Nerve structure

Cranial nerves Here is the ventral surface of the brain showing the attachment of the 12 pairs of cranial nerves.

The dorsal root of a spinal nerve contains sensory fibers that conduct sensory impulses from sensory receptors toward the spinal cord. Dorsal root ganglia near the spinal cord contain the cell bodies of sensory neurons. The ventral root of a spinal nerve contains motor fibers that conduct impulses away from the spinal cord to effectors.

Spinal nerves Here is a cross section of the spinal cord, showing 3 pairs of spinal nerves. The human body has 31 pairs of spinal nerves altogether, and each spinal nerve has a dorsal root and a ventral root attached to the spinal cord.

Somatic System The somatic system serves the skin, skeletal muscles, and tendons. The brain is always involved in voluntary muscle actions but somatic system reflexes are automatic and may not require involvement of the brain.

The Reflex Arc Involuntary reflexes allow us to respond rapidly to external stimuli. In reflexes, sensory receptors generate nerve impulses carried to interneurons in the spinal cord. Next, interneurons signal motor neurons which conduct nerve impulses to a skeletal muscle that contracts, giving the response to the stimulus. Pain is not felt until the brain receives nerve impulses.

A spinal nerve reflex arc
A stimulus (e.g., a pinprick) causes sensory receptors in the skin to generate nerve impulses that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons. Motor axons convey nerve impulses from the spinal cord to a skeletal muscle, which contracts. Movement of the hand away from the pin is the response to the stimulus.

Autonomic System The autonomic system of the PNS regulates the activity of cardiac and smooth muscle and glands. The system is divided into sympathetic and parasympathetic divisions that: Function automatically and involuntarily; Innervate all internal organs; and Use two neurons and one ganglion. Table 17.1 (page 337) contrasts the two divisions of the autonomic system with the features of somatic motor pathways. Reflex actions of the autonomic system, such as those that regulate the blood pressure and breathing rate, are especially important to the maintenance of homeostasis.

Sympathetic Division The sympathetic division is associated with responses that occur during times of stress, including “fight or flight” reactions. The postganglionic axon releases mainly norepinephrine which acts similar to adrenaline, the hormone from the adrenal medulla. The preganglionic fibers of both the sympathetic and parasympathetic divisions release the neurotransmitter acetylcholine.

In the sympathetic division, preganglionic fibers arise from the middle, or thoracic-lumbar, portion of the cord and synapse in ganglia close to the cord. The preganglionic fiber is short, while the postganglionic fiber is long.

Parasympathetic Division
The parasympathetic system is associated with responses that occur during times of relaxation and promotes “housekeeper” activities. The postganglionic neurotransmitter used by the parasympathetic division is acetylcholine.

Preganglionic fibers of the parasympathetic division arise from the base of the brain or from the sacral spinal cord. The preganglionic fiber is long, the postganglionic fiber is short, and ganglia lie close to the effector organ.

Autonomic nervous system
Sympathetic preganglionic fibers (left) arise from the cervical, thoracic, and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Drug Abuse Stimulants increase excitation, and depressants decrease excitation; either can lead to physical dependence. Each type of drug has been found to either promote or prevent the action of a particular neurotransmitter. Medications that counter drug effects work by affecting the release, reception, or breakdown of dopamine, a neurotransmitter responsible for mood. Physical dependence was formerly referred to as addiction.

Drug actions at a synapse
A drug can affect a neurotransmitter in these ways: (a) cause leakage out of a synaptic vesicle into the axon bulb; (b) prevent release of the neurotransmitter into the synaptic cleft; (c) promote release of the neurotransmitter into the synaptic cleft; (d) prevent reuptake by the presynaptic membrane; (e) block the enzyme that causes breakdown of the neurotransmitter; or (f) bind to a receptor, mimicking the action or preventing the uptake of a neurotransmitter.

Drug use Blood-borne diseases such as AIDS and hepatitis B pass from one drug abuser to another when they share needles.

Alcohol Alcohol may affect the inhibiting transmitter GABA or glutamate, an excitatory neurotransmitter. Alcohol is primarily metabolized in liver and heavy doses can cause liver scar tissue and cirrhosis. Alcohol is an energy source but it lacks nutrients needed for health. Cirrhosis of the liver and fetal alcohol syndrome are serious conditions associated with alcohol intake. Alcohol disrupts the normal workings of the liver so that fats cannot be broken down. Fat accumulation, the first stage of liver deterioration, occurs after only one night of heavy drinking. The second stage of deterioration id the presence of fibrous scar tissue. If heavy drinking stops, the liver can still recover and become normal once again. If not, the final and irrevocable stage, cirrhosis of the liver, occurs: liver cells die, harden, and turn orange (cirrhosis means orange).

Nicotine Nicotine is an alkaloid derived from tobacco.
In the CNS, nicotine causes neurons to release dopamine; in the PNS, nicotine mimics the activity of acetylcholine and increases heart rate, blood pressure, and digestive tract mobility. Nicotine induces both physiological and psychological dependence. Nicotine adversely affects a developing embryo and fetus.

Cocaine Cocaine is an alkaloid derived from the shrub Erythroxylum cocoa, often sold as potent extract termed “crack.” Cocaine prevents uptake of dopamine by the presynaptic membrane, is highly likely to cause physical dependence, and requires higher doses to overcome tolerance. This makes overdosing is a real possibility; overdosing can cause seizures and cardiac arrest. It is possible that long-term cocaine abuse causes brain damage. Babies born to addicts suffer withdrawal symptoms and may have neurological and developmental problems.

Heroin Derived from morphine, heroin is an alkaloid of opium.
Use of heroin causes euphoria. Heroin alleviates pain by binding to receptors meant for the body’s own pain killers which are the endorphins. Tolerance rapidly develops and withdrawal symptoms are severe.

Marijuana Marijuana is obtained from the plant Cannabis sativa that contains a resin rich in THC (tetrahydrocannabinol). Effects include psychosis and delirium and regular use can lead to dependence. Long-term marijuana use may lead to brain impairment, and a fetal cannabis syndrome has been reported. Fetal cannabis syndrome is similar to fetal alcohol syndrome. Some psychologists believe that marijuana use among adolescents is a way to avoid dealing with the personal problems often characteristic of that stage of life.

Chapter Summary The nervous system consists of two types of cells: neurons and mesoglia. Neurons are specialized to carry nerve impulses. A nerve impulse is an electrochemical change that travels along the length of a neuron fiber. Transmission of signals between neurons is dependent on neurotransmitter molecules.

The central nervous system is made up of the spinal cord and the brain.
The parts of the brain are specialized for particular functions. The cerebral cortex contains motor areas, sensory areas, and association areas that are in communication with each other. The cerebellum is responsible for maintaining posture; the brainstem houses reflexes for homeostasis.

The reticular formation contains fibers that arouse the brain when active and account for sleep when they are inactive. The limbic system contains specialized areas that are involved in higher mental functions and emotional responses. Long-term memory depends upon association areas that are in contact with the limbic system.

There are particular areas in the left hemisphere that are involved in language and speech.
The peripheral nervous system contains nerves that conduct nerve impulses toward and away from the central nervous system. The autonomic nervous system has sympathetic and parasympathetic divisions with counteracting activities. Use of psychoactive drugs such as alcohol, nicotine, marijuana, cocaine, and heroin is detrimental to the body.

Ex 22: Annelida (segmented worms)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Evolutionary tree All animals are believed to be descended from protists; the Porifera (sponges) with the cellular level of organization may have evolved separately.

Annelids Annelids are segmented both externally, and internally by partitions called septa. Annelids have a hydrostatic skeleton, and partitioning of the coelom permits each body segment to move independently. The tube-within-a-tube body plan allows the digestive tract to have specialized organs. Each segment of an earthworm has its own set of longitudinal and circular muscles and its own nerve supply, so each segment or group of segments can function independently.

Annelids have an extensive closed circulatory system with blood vessels that run the length of the body and branch to every segment. The brain is connected to a ventral solid nerve cord with ganglia in each segment. The excretory system has nephridia in each segment. A nephridium is a tubule that collects wastes and excretes through an opening in the body wall.

Marine Worms Polychaetes are marine worms with paddlelike parapodia at the side of each segment. Some polychaetes are sessile tube worms. A clam worm is a predaceous marine worm with a defined head region. During breeding seasons, some worms form sex organs in special segments and shed these segment during breeding.

Polychaete diversity a. Clam worms are predaceous polychaetes that undergo cephalization. Note also the parapodia, which are use for swimming and as respiratory organs. b. Fan worms (a type of tube worm) are sessile filter feeders whose ciliated tentacles spiral in this example.

Earthworms Earthworms are oligochaetes having few setae per segment.
Most scavenge for food in the soil and the moist body wall functions in gas exchange. When muscles contract in each segment, setae anchor in the soil, and aid locomotion. Five “hearts” pump blood and a branch blood vessel reaches each segment. These worms are hermaphroditic. External segmentation reflects a coelom divided by septa. The brain connects to the ventral nerve chord with a lateral nerve on most segments. Nephridia in most segments remove wastes.

Segmentation in earthworms is evidenced by: Body rings
Coelom divided by septa Setae on most segments Ganglia and lateral nerves in each segment Nephridia in most segments Branch blood vessels in each segment

Earthworm, Lumbricus The top diagram shows the external anatomy of an earthworm. Note the clitellum. The bottom diagram is a cross section of an earthworm showing the internal anatomy and how the setae project through the body wall.

When earthworms mate, they are held in place by a mucus secreted by the clitellum. The worms are hermaphroditic, and when mating, sperm pass from the seminal vesicles of each to the receptor vesicles of the other.

Leeches Most leeches are fluid feeders that attach themselves to open wounds using suckers. Bloodsuckers, such as the medicinal leech, can cut through tissue. An anticoagulant (hirudin) in their saliva keeps blood from clotting. The Health Focus (page 625) discusses the medicinal value of leeches.

Evolutionary tree All animals are believed to be descended from protists; the Porifera (sponges) with the cellular level of organization may have evolved separately.

Arthropods Arthropods are the most varied and numerous of animals.
The success of arthropods is largely attributable to a flexible exoskeleton, jointed appendages, and specialization of body regions. Three body regions – head, thorax, and abdomen – with specialized appendages in each region, and a well-developed nervous system characterize this group. Over one million species of arthropods have been described but many more may exist. Each of the five major groups of arthropods contains species that are adapted to terrestrial life. The exoskeleton contains chitin.

Arthropod diversity a. A millipede (flat-backed millipede, Sigmoria) has only one pair of antennae, and the head is followed by a series of segments, each with two pairs of appendages. b. The hairy tarantulas of the genus Aphonopelma are dark in color and sluggish in movement. Their bite is harmless to people. c. A crab (dungeness crab, Cancer) is a crustacean with a calcified exoskeleton, one pair of claws and four other pairs of walking legs. d. A wasp (paper wasp, Polistes) is an insect with two pairs of wings, both used for flying, and three pairs of walking legs. e. A centipede (stone centipede, Lithobius) has only one pair of antennae, and the head is followed by a series of segments, each with a single pair of appendages.

Crustaceans Crustaceans are largely marine and have a head that bears compound eyes, two pair of antennae, and specialized mouth parts. Five pairs of walking legs include a first pair of pinching claws. In the crayfish, head and thorax are fused into a cephalothorax which is covered on the top and sides by carapace. The abdominal segments have swimmerets.

The crayfish has an open circulatory system in which the heart pumps blood into a hemocoel consisting of sinuses where the hemolymph flows about the organs. Respiration takes place by gills under the hard carapace, and there is a ventral solid nerve cord. Sexes are separate in the crayfish.

Male crayfish, Cambarus
a. Externally, it is possible to observe the jointed appendages, including the swimmerets, the walking legs, and the claws. These appendages, plus a portion of the carapace, have been removed from the right side so the gills are visible. b. Internally, the parts of the digestive system are particularly visible. The circulatory system can also be clearly seen. Note the ventral solid nerve cord.

Insects The head of an insect usually bears a pair of antennae, compound eyes, and simple eyes. The thorax bears three pairs of legs and up to two pairs of wings, and the abdomen contains most of the internal organs. The insect exoskeleton is lighter and contains less chitin than that of many other arthropods.

Insect diversity a. Walking sticks (Diapheromera) are herbivorous, with biting and chewing mouthparts. b. Bees (Apis) have four translucent wings and a thorax separated from the abdomen by a narrow waist. c. Flies (housefly, Musca) have a single pair of wings and lapping mouthparts. d. Dragonflies (Aeshna) have two pairs of similar wings. They catch and eat other insects while flying. e. Butterflies (American copper butterfly, Lycaena) have forewings larger than their hindwings. Their mouthparts form a long tube for siphoning up nectar from flowers.

Grasshoppers are examples of insects adapted to a terrestrial life; they respire by tracheae and have wings that allow them to evade enemies; the third pair of legs is suitable for jumping. There is a tympanum for the reception of sound waves and a male penis for passing sperm to the female without desiccation.

Malpighian tubules function in excretion in grasshopper.
Grasshoppers undergo gradual metamorphosis from nymph to adult. Butterflies undergo complete metamorphosis, changing from larva to pupa to adult.

Female grasshopper a. Externally, the tympanum receive sound waves, and the hopping legs and the wings are for locomotion. b. Internally, the digestive system is specialized. The Malpighian tubules excrete a solid nitrogenous waste (uric acid). A seminal receptacle receives sperm from the male, which has a penis.

Arachnids The arachnids include terrestrial spiders, scorpions, ticks, and mites. The cephalothorax bears six pairs of appendages: the chelicerae and the pedipalps, and four pairs of walking legs. Scorpions are the oldest terrestrial arthropods. Ticks and mites are parasitic. Chiggers are the larvae of certain mites that feed on the skin of vertebrates.

Spiders are well-adapted to life on land and have Malphigian tubules – they secrete uric acid, helping to conserve water. Spiders spin silk used in various ways. Where spiders spin webs, the type of web is a feature that demonstrates the evolutionary relationship among spiders.

Arachnid diversity a. A scorpion (Kenyan giant scorpion, Pandinus) has pincerlike chelicerae and pedipalps; its long abdomen ends with a stinger that contains venom. b. Most spiders are harmless, but the venom of the black widow spider (Latrodectus) is harmful to humans. c. In the western United States, the wood tick (Ixodes) carries a debilitating diseases called Rocky Mountain spotted fever. d. Arachnids breathe by means of book lungs, in which the “pages” are double sheets of thin tissue (lamellae).

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Chapter 15: Respiration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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