Unit 2 The Nervous System

Objectives List the organs and divisions of the nervous system and describe the generalized functions of the system as a whole Identify the major types of cells in the nervous system and discuss the function of each Identify the anatomical and functional components of a three-neuron reflex arc. Compare and contrast the propagation of a nerve impulse along a nerve fiber and across a synaptic cleft.

Objectives Identify the major anatomical components of the brain and spinal cord and briefly comment on the function of each Identify and discuss the coverings and fluid spaces of the brain and spinal cord Compare and contrast spinal and cranial nerves Discuss the structure and function of the two divisions of the autonomic nervous system Describe major nervous system disorders

Organs and Divisions of the Nervous System Central nervous system (CNS)—brain and spinal cord Peripheral nervous system (PNS)—all nerves Autonomic nervous system (ANS) What are the two principal divisions of the nervous system? The central nervous system and the peripheral nervous system

Divisions of the nervous system.

Cells of the Nervous System Neurons Consist of three parts Cell body of neuron—main part Dendrites—branching projections that conduct impulses to cell body of neuron Axon—elongated projection that conducts impulses away from cell body of neuron What are the two types of cells found in the nervous system? Neurons, or nerve cells, and glia, specialized connective tissue cells What is the direction of neural transmission from sensory neurons and motor neurons? Sensory neurons transmit impulses TO the spinal cord and brain; motor neurons transmit impulses AWAY from the brain and spinal cord. What are interneurons? Interneurons conduct impulses from sensory neurons to motor neurons. Also called central or connecting neurons.

Structure of a neuron. A, Diagram of a typical neuron showing dendrites, a cell body, and an axon. B, Segment of a myelinated axon cut to show detail of the concentric layers of the Schwann cell filled with myelin. C, Photomicrograph of neuron. (C, Dennis Strete.)

Cells of the Nervous System Neurons Neurons classified according to function or direction of impulse Sensory neurons: conduct impulses to the spinal cord and brain Motor neurons: conduct impulses away from brain and spinal cord to muscles and glands Interneurons: conduct impulses from sensory neurons to motor neurons

Cells of the Nervous System Disorders of nervous tissue Multiple sclerosis—characterized by myelin loss in central nerve fibers and resulting conduction impairments Tumors General name for nervous system tumors is neuroma Where is myelin produced in the central nervous system? In oligodendrocytes. Where is myelin produced in the peripheral nervous system? In Schwann cells. How might symptoms differ according to where myelin production is being impaired? Symptoms will depend on the nerve affected as well as the area of the nerve affected.

Effects of multiple sclerosis (MS) Effects of multiple sclerosis (MS). A, A normal myelin sheath allows rapid conduction. B, In those with MS, the myelin sheath is damaged, disrupting normal nerve conduction.

Reflex Arcs Nerve impulses are conducted from receptors to effectors over neuron pathways or reflex arcs; conduction by a reflex arc results in a reflex (i.e., contraction by a muscle or secretion by a gland) The simplest reflex arcs are two-neuron arcs—consisting of sensory neurons synapsing in the spinal cord with motor neurons; three-neuron arcs consist of sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons What is the difference between a neuron pathway and a reflex arc? A reflex arc is the simplest type of neuron pathway. What is a two-neuron arc? The simplest type of reflex arc consisting of only two types of neurons: sensory neurons and motor neurons. What is a three-neuron arc? It consists of three different types of neurons: sensory neurons, motor neurons, and interneurons. What is an effector, and how does it relate to the reflex arc?

Patellar reflex. The neural pathway involved in the patellar (“knee-jerk”) reflex.

Nerve Impulses Definition— wave of electrical disturbance that travels along the surface of a neuron membrane; sometimes called action potentials Where does impulse conduction originate? It normally starts in receptors, the beginnings of dendrites of sensory neurons. The end of the sensory neuron’s axon synapses first with an interneuron before chemical signals are sent across a second synapse, resulting in conduction through the motor neuron. For example, application of an irritating stimulus to the skin of the thigh initiates a three-neuron reflex response that causes contraction of muscles to pull the leg away from the irritant. What are some types of stimuli that initiate nerve impulses? Pressure, temperature, chemical changes

Conduction of nerve impulses Conduction of nerve impulses. A, In an unmyelinated fiber, a nerve impulse (action potential) is aself-propagating wave of electrical disturbance. B, In a myelinated fiber, the action potential “jumps” around the insulating myelin in a rapid type of conduction called saltatory conduction.

Nerve Impulses Mechanism At rest, the neuron’s membrane is slightly positive on the outside—polarized—from a slight excess of Na+ on the outside A stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron

Nerve Impulses Mechanism Inward movement of Na+ depolarizes the membrane by making the inside more positive than the outside at the stimulated point; this depolarization is a nerve impulse (action potential) The stimulated section of membrane immediately repolarizes, but by that time the depolarization has already triggered the next section of membrane to depolarize, thus propagating a wave of electrical disturbances (depolarizations) all the way down the membrane

The Synapse Definition—the place where impulses are transmitted from one neuron to another (the postsynaptic neuron) Synapse made of three structures—synaptic knob, synaptic cleft, and plasma membrane Neurotransmitters bind to specific receptor molecules in the membrane of a postsynaptic neuron, opening ion channels and thereby stimulating impulse conduction by the membrane What are the three structures that make up a synapse? A synaptic knob, a neurotransmitter, and a synaptic cleft How does a nerve impulse travel from one neuron to another? Through a synapse via a neurotransmitter Ask students to draw a schematic diagram correctly charting the following structures and chemicals: axon terminal, synaptic knob, presynaptic neuron, postsynaptic neuron, neurotransmitter, synaptic cleft, plasma membrane, receptor molecules.

Components of a synapse Components of a synapse. Diagram shows synaptic knob or axon terminal of presynaptic neuron, the plasma membrane of a postsynaptic neuron, and a synaptic cleft. On the arrival of an action potential at a synaptic knob, neurotransmitter molecules are released from vesicles in the knob into the synaptic cleft. The combining of neurotransmitter and receptor molecules in the plasma membrane of the postsynaptic neuron opens ion channels and thereby initiates impulse conduction in the postsynaptic neuron.

The Synapse Names of neurotransmitters—acetylcholine, catecholamines (norepinephrine, dopamine, and serotonin), endorphins, enkephalins, nitric oxide (NO), and other compounds Parkinson disease (PD)—characterized by abnormally low levels of dopamine in motor control areas of the brain; patients usually exhibit involuntary trembling and muscle rigidity What is a neurotransmitter and how many are there? Neurotransmitters are chemicals by which neurons communicate. At least 30 different compounds have been identified as neurotransmitters. Name some of the neurotransmitters. Acetylcholine, norepinephrine, dopamine, serotonin, catecholamines Acetylcholine is released at some of the synapses in the spinal cord and at neuromuscular junctions. Norepinephrine, dopamine, and serotonin belong to a group of compounds called catecholamines, which may play a role in sleep, motor function, mood, and pleasure recognition. Two morphine-like neurotransmitters called endorphins and enkephalins are natural painkillers.

Parkinsonism. Parkinsonism is a syndrome typically found in individuals with Parkinson disease (PD). The signs include (but are not limited to) rigidity and trembling of the head and extremities, a forward tilt of the trunk, and a shuffling gait with short steps and reduced arm swinging. (Rolin Graphics.)

Central Nervous System Divisions of the brain Brainstem Consists of three parts, named in ascending order: medulla oblongata, pons, and midbrain What are the three main parts of the brainstem? The medulla oblongata, pons, and midbrain Structure—white matter with bits of gray matter scattered through it. What is the function of the brainstem? It functions as a two-way conduction pathway. Many important reflex centers (cardiac, respiratory, and vasomotor centers – “vital centers”) are located in the brainstem.

The nervous system. The brain and spinal cord (highlighted green) constitute the central nervous system (CNS), and the nerves (yellow) make up the peripheral nervous system (PNS).

Central Nervous System Divisions of the brain Brainstem Functions All three parts of brainstem are two-way conduction paths Many important reflex centers lie in the brainstem

Central Nervous System Structure and function of the hypothalamus Consists mainly of the posterior pituitary gland Acts as the major center for controlling the ANS; therefore, helps control the functioning of most internal organs Controls hormone secretion by anterior and posterior pituitary glands; therefore, it indirectly helps control hormone secretion by most other endocrine glands Contains centers for controlling appetite, wakefulness, pleasure, etc. What is the structure of the hypothalamus? One of the most important brain structures. Lies below thalamus. What is the function of the hypothalamus? Manufactures hormones, part of the mechanism for maintaining body temperature, involved in regulation of water balance, involved in sleep cycle, involved in control of appetite and many emotions of pleasure, fear, anger, sexual arousal, and pain.

Central Nervous System Structure and function of the thalamus Dumbbell-shaped mass of gray matter extending into each cerebral hemisphere Relays sensory impulses to cerebral cortex sensory areas In some way produces the emotions of pleasantness or unpleasantness associated with sensations What is the structure of the thalamus? Dumbbell-shaped section of gray matter above the hypothalamus. What is the function of the thalamus? (1) Helps produce sensations – relays impulses to the cerebral cortex from sense organs; (2) associates sensations with emotions; (3) plays a part in the arousal or alerting mechanism

Central Nervous System Cerebellum Second largest part of the human brain Helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal postures

Central Nervous System Cerebrum Largest part of the human brain Composed mainly of dendrites and cell bodies of neurons Functions of the cerebrum—mental processes of all types, including sensations, consciousness, memory, and voluntary control of movements

Major regions of the central nervous system Major regions of the central nervous system. A, Sagittal sections of the brain and spinal cord. B, Section of preserved brain. (B, Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

Central Nervous System Brain disorders Dementia—progressive loss of memory, shortened attention span, personality changes, reduced intellectual capacity, motor control deficit Alzheimer disease (AD)—brain disorder of the middle and late adult years characterized by dementia Huntington disease (HD)—inherited disorder characterized by chorea (purposeless movement) progressing to severe dementia HIV (also causes AIDS) can infect neurons and thus cause dementia

Alzheimer disease (AD) Alzheimer disease (AD). The CT scan on the left shows a horizontal section of a normal brain. In the CT scan on the right, however, you can see the dark patches in the cerebral cortex that show damage to brain tissue typical of AD. (James King-Holmes and Science Photo Library.)

Central Nervous System Brain disorders Seizure disorders Seizure—sudden burst of abnormal neuron activity that results in temporary changes in brain function Epilepsy—many forms, all characterized by recurring seizures Electroencephalogram—graphic representation of voltage changes in the brain used to evaluate brain activity

Peripheral Nervous System Cranial nerves 12 pairs—attached to undersurface of the brain Connect brain with the neck and structures in the thorax and abdomen Spinal nerves 31 pairs—contain dendrites of sensory neurons and axons of motor neurons Conduct impulses necessary for sensations and voluntary movements What are some of the structures included in the peripheral nervous system? Includes cranial and spinal nerves that connect the brain and spinal cord, respectively, to peripheral structures such as the skin surface and the skeletal muscles. Other structures in the autonomic nervous system are considered part of the peripheral nervous system; they connect the brain and spinal cord to various glands in the body and to the cardiac and smooth muscles in the thorax and abdomen.

Autonomic Nervous System Autonomic nervous system—motor neurons that conduct impulses from the central nervous system to cardiac muscle, smooth muscle regulates body’s automatic or involuntary functions Motor nerves that control the voluntary actions of skeletal muscles are sometimes called the somatic nervous system. What are the two divisions of the autonomic nervous system (ANS)? Sympathetic nervous system, parasympathetic nervous system Spinal nerves conduct impulses between the spinal cord and parts of the body not supplied by cranial nerves. Spinal nerves function to make possible sensations and movements.

Innervation of the major target organs by the autonomic nervous system Innervation of the major target organs by the autonomic nervous system. The sympathetic pathways are highlighted with orange, and the parasympathetic pathways are highlighted with green.

Autonomic Nervous System Composed of two divisions: the sympathetic system and the parasympathetic system

Autonomic Nervous System Functions of the sympathetic nervous system Serves as the emergency or stress system, during strenuous exercise and when strong emotions (anger, fear, hate, or anxiety) are elicited Group of changes induced by sympathetic control is called the fight-or-flight response What are the functions of the sympathetic nervous system? It functions as an emergency system. It takes control of many internal organs when we exercise strenuously and when strong emotions are elicited. It other words, it functions during stress. See Table 9-3. What physiological changes are associated with the fight-or-flight response? Heart beats faster, blood vessels constrict causing blood pressure to increase, blood vessels in muscle dilate delivering more blood to the muscles, sweat glands and adrenal glands secrete more abundantly, salivary and other digestive glands secrete more sparingly, peristalsis becomes sluggish, and we are ready for “flight or flight.” The sympathetic nervous system controls visceral effectors during strenuous exercise and strong emotions (such as anger, fear, hate, or anxiety).

Autonomic Nervous System Parasympathetic nervous system Function—dominates control under normal, everyday conditions What are the functions of the parasympathetic nervous system, and how do they differ from the functions of the sympathetic nervous system? It dominates control of many visceral effectors during normal, everyday conditions. Impulses tend to slow heartbeat, increase peristalsis, and increase secretion of digestive juices and insulin. See Table 9-3.

Autonomic Nervous System Autonomic nervous system as a whole Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis