1. The Nervous System By.Brett Bennell And Lisa Matzelle

2. Neuron Structure Three classes of neurons:sensory neurons, interneurons, and motor neurons. Sensory neuron takes messages from sensory receptors to the CNS. Interneuron lies in the CNS. Motor neuron takes messages away from the CNS. Neurons all vary in appearance, but have three of the same parts which are:a cell body, dendrites, and an axon. The cell body contains the nucleus, as well as other organelles. An axon conducts nerve impulses away from the cell body toward other neurons or target structures. Dendrites are extensions from a cell body that receive signals from other neurons and send them on to the cell body.

3. Myelin Sheath Myelin Sheath is formed by a type of neuroglia called Schwann cells, which contain the lipid substance myelin in their plasma membranes. A Myelin sheath develops when Schwann cells wrap themselves around an axon many times. Gaps where there are no myelin sheath are called nodes of Ranvier. Myelin gives fibres their white, glistening appearance and serves as an excellent insulator. It also helps accelerate nerve impulses. Only long axons tend to have a myelin sheath. These axons carry messages from one part of the nervous system to another.

4. Resting Potential Resting potential is the polarity across the plasma membrane of a resting neuron due to and unequal distribution of ions. The unequal distribution of these ions is due to the action of the sodium-potassium pump. The pump is always working because the membrane is somewhat permeable to these ions, and they tend to diffuse toward their lesser concentration. The sodium-potassium pump, makes a greater concentration of Na+ outside an axon and a greater concentration of K+ inside an axon.

5. Action Potential An action potential is a rapid change in polarity across an axomembrane as the nerve impulse occurs. Action potential is an all-or-none phenomenon. If a stimulus causes the axomembrane to depolarize to a certain level, which is called a threshold, an action potential occurs. The action potential requires two types of gated channel proteins in the membrane. One gated channel protein opens to allow Na+ to pass through the membrane, and another opens to allow K+ to pass through the membrane.

6. Synaptic Integration A single neuron has many dendrites plus the cell body, and both can have synapses with many other neurons. Integration is the summing up of inhibitory and excitatory signals received by a postsynaptic neuron. If a neuron receives both inhibitory and excitatory signals, the summing up of these signals may prohibit the axon from firing. Excitatory signals have a depolarizing effect, and inhibitory signals have a hyperpolarizing effect.

7. Neurotransmitter Molecules There have been at least 25 different neurotransmitters that have been identified. Two very well-known ones are acetylcholine (Ach) and norepinephrine (NE). Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft. The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes. Many drugs that affect the nervous system act by interfering with or potentiating (enhancing) the action of neurotransmitters. Drugs can enhance or block the release of a neurotransmitter.

8. Central Nervous System The spinal cord and the brain make up the central nervous system (CNS), where sensory information is received and motor control is initiated. Bone, and cerebrospinal fluid is used to protect both the spinal cord and the brain. The CNS is composed of two types of nervous tissue—gray matter and white matter. The gray matter is gray because it contains cell bodies and short, nonmyelinated fibers. The white matter is white because it contains myelinated axons that run together in bundles called tracts.

9. Spinal Cord The spinal cord extends from the case of the brain through a large opening in the skull called the foramen magnum and into the vertebral canal formed by openings in the vertebral. The spinal cord serves as a means of communication between the brain and much of the body. The spinal cord is also a center for reflex actions. Spinal nerves project form the cord between the vertebrae that make up the vertebrae column. A cross section of the spinal cord shows a central canal, gray matter, and white matter. Gray matter is centrally located and shaped like the letter H.

10. The Brain The human brain has been called the last great frontier of biology. Modern neurosciences modern goal is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans beings of a normal life. The brain has four ventricles called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. The cerebrum is associated with the two lateral ventricles, the diencephalons with the third ventricle, and the brain stem and the cerebellum with the fourth ventricle.

11. Diagrams of the Brain

12. The Cerebrum The cerebrum is also known as the telencephalon, is the largest portion of the brain in humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. Cerebrum has two halves. The cerebrum carries out higher thought processes required for learning and memory and for language and speech. Shallow grooves called sulci divide each hemisphere into lobes.

13. The Cerebral Cortex The cerebral cortex is a thin but highly convoluted outer layer of gray matter that covers the cerebral hemisphere. It contains over one billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness. Voluntary commands to skeletal muscles begin in the primary motor area. The premotor area organizes motor functions for skilled motor activities. Broca’s area is a motor speech area in the lledt frontal lobe.

14. The Diencephalon The hypothalamus and the thalamus are in the diencephalon. The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The hypothalamus serves as a link between the nervous and endocrine system. The thalamus is on the receiving end for all sensory input except smell, visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts form the spinal cord. The pineal gland, which secretes the hormone melatonin is also located in the diencephalon.

15. The Cerebellum The cerebellum is separated from the brain stem. It has two portions that are joined by a narrow median portion. The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts, and it also receives motor output form the cerebral cortex about where these parts are located. It also insures that all of the muscles work together to produce smooth, coordinating voluntary movements. The cerebellum also assists with the learning of a new skill.

16. The Brain Stem The brain stem contains the midbrain, the pons, and the medulla oblongata. The midbrain acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum. It also has a reflex centers for visual, auditory, and tactile responses. The pons functions with the medulla oblongata to regulate breathing rate and has reflex centers concerned with head movements in response to visual and auditory stimuli. The medulla oblongata contains a number of reflex centers for regulation heartbeat, breathing, and vasconstriction, vomiting, coughing, sneezing, hiccupping, and swallowing. The medulla oblongata lies just superior of the spinal cord.

17. Limbic System The limbic system is a complex network of tracts and nuclei that incorporates medial portions of the cerebral lobes, the basal nuclei, and the diencephalon. It blends primitive emotions an higher mental functions into a united whole. Two significant structures within the limbic system are the hippocampus and the amygdala, which 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, in particular, can cause these experiences to have emotional overtones. The prefrontal area consults the hippocampus in order to use memories to modify our behavior.

18. Memory and Learning Memory is the ability to holds a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and utilize past memories. Short term memory lies just dorsal to our forehead. Long term memory is like remembering lots of phone numbers. Long term memory can be a mixture of semantic memory (numbers, words, etc) and episodic memory (persons, events, etc).

19. Language and Speech Language is dependent upon memory; special areas in the left hemisphere help account for our ability to comprehend and use speech. People with speech disorders have been known for some time that damage to a motor speech area called Broca’s area results in an inability to speak. The right hemisphere is more global in its approach to things as in the left hemisphere it is more specific.

20. The Peripheral Nervous System The peripheral nervous system lies outside the central nervous system, and is composed of nerves and ganglia. Sensory fibers carry information to the CNS, and motor fibers carry information away from the CNS. Ganglia are swellings associated with nerves that contain collections of cell bodies. Humans of 12 pairs of cranial nerves attached to the brain. The vagus nerve has branches not onluy to the pharynx and larynx, but also to most of the internal organs. The spinal nerves of humans emerge in 31 pairs form either side of the spinal cord. In the PNS, cranial nerves take impulses to and from the brain, and spinal nerves take impulses to and from the spinal cord.

21. Somatic System The PNS is divided into the somatic system and the autonomic system. The somatic system serves the skin, skeletal muscles, and tendons. Some actions in the somatic system are due to reflexes, autonomic responses to a stimulus. A reflex occurs, quickly, without out even having to think about it. Some other actions that are voluntary, originate in the cerebral cortex, as when we decide to move a limb.

22. Autonomic System The autonomic system of the PNS regulates the activity of cardiac and smooth muscle and glands. The system is divided into the sympathetic and parasympathetic divisions. These two divisions have several features in common:1)they function automatically and usually in an involuntary manner; 2)they innervate all internal organs; and 3)they utilize two neurons and one ganglion for each impulse. Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis.

23. Sympathetic Division Most preganglionic fibers of the sympathetic division arise form the middle, or thoracic-lumbar, portion of the spinal cord and almost immediately terminate in ganglia that lie near the spinal cord. The sympathetic division is the most important during emergency situations. The sympathetic division brings about those repsonses we associate with “fight or flight”.

24. Parasympathetic Division The parasympathetic division includes a few cranial nerves as well as fibers that arise form the sacral (bottom) portion of the spinal cord. The preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ. This division promotes all the internal responses we associate with a relaxed state. The neurotransmitter utilized by the parasympathetic division is acetylcholine (Ach).

25. Drug Abuse A wide variety of drugs affect nervous system and cal alter the mood and /or emotional state. Such drugs have two general effects:1)they impact the limbic system, and 2)they either promote or decrease the action of a particular neurotransmitter. Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. Taking drugs that affect the nervous system lead to physical dependence and withdrawal symptoms.

26. A Small List of Some Harmful Drugs Alcohol Nicotine Cocaine Heroin Marijuana All these drugs listed above have two things in common they are very addictive, and they can be very damaging to the brain if there is constant use with them.

27. THE END By. Brett Bennell and Lisa Matzelle