Nervous System. Unit 6.6 (6 th Edition) Chapter 7.6 (7 th Edition)

Transcription

1 Nervous System Unit 6.6 (6 th Edition) Chapter 7.6 (7 th Edition) 1

2 Learning Objectives Identify the main parts (anatomy) of a neuron. Identify the 2 divisions of nervous system. Classify the major types of nerves. Describe the function (physiology) of each of the 6 main parts of the brain. Explain 3 functions (physiology) of spinal cord. Name the 3 meninges. Contrast the actions of the sympathetic and parasympathetic nervous systems. Describe major diseases of the nervous system. 2

3 Fun Facts About the Nervous System The nervous system is a complex, highly organized system that coordinates all activities in the body. The nervous system allows the body to change/adapt. The left side of your brain controls the right side of your body. The right side of your brain controls the left side of your body. There are ~ 100 billion neurons in the human brain. The average speed of a signal transferred through a neuron is miles per hour. If you lined up all of the neurons in the human body, they would stretch 600 miles! 3

4 Neurons Neurons basic structural unit of nervous system Dendrites nerve fibers that carry impulses toward the cell body Axon single nerve fiber that carries impulses away from the cell body Synapses spaces between axon of one neuron & dendrite of other neurons Neurotransmitters chemicals at the end of each axon that allow nerve impulses to travel 4

5 Nerves Nerves combination of many nerve fibers (cells) outside the brain and spinal cord Afferent (Sensory) Nerves carry messages from all parts of the body to the brain and spinal cord Efferent (Motor) Nerves carry messages from the brain and spinal cord to the muscles and glands Associative (Internuncial) Nerves carry both sensory and motor messages 5

6 Two Divisions of Nervous System Central Nervous System brain and spinal cord Peripheral Nervous System consists of nerves subdivided into 2 parts (1) Autonomic Nervous System-controls involuntary body functions (2) Somatic Nervous System-carries messages between central nervous system & body 6

7 Central Nervous System Brain mass of nerve tissue protected by membranes & skull divided into six main parts (discussed below) Cerebrum largest and highest section of brain separated into lobes reasoning, thought, memory, speech, sensation, sight, smell, hearing, and voluntary body movement Cerebellum section below back of cerebrum muscle coordination, balance, posture, &muscle tone 7

8 Brain Components Continued Diencephalon between cerebrum and midbrain regulates temperature, appetite, water balance, sleep, blood vessel constriction and dilation emotions: anger, fear, pleasure, pain, & affection Midbrain located below cerebrum at top of brain stem conducts impulses between brain parts certain eye and auditory reflexes Pons below midbrain and in brain stem conducts messages to other parts of brain reflex actions: chewing, tasting, saliva, & respiration Medulla Oblongata lowest part of brain stem that connects with spinal cord regulates heartbeat, respiration, swallowing, coughing, BP 8

9 Spinal Cord Spinal Cord continues down from medulla oblongata ends at first or second lumbar vertebrae surrounded and protected by vertebrae responsible for reflex actions carries sensory (afferent) messages up to the brain carries motor (efferent) messages from brain to the nerves that go to muscles and glands Meninges three membranes that cover &protect brain & spinal cord dura mater is thick, tough outer layer arachnoid is middle layer, delicate and weblike pia mater is closely attached to brain and spinal cord and contains blood vessels that nourish nerve tissue 9

10 Peripheral Nervous System Somatic Nervous System 12 pairs of cranial nerves 31 pairs of spinal nerves and their branches each nerve goes directly to a particular part of the body or networks with other spinal nerves & forms plexus Autonomic Nervous System maintains balance in involuntary functions allows body to react in emergencies sympathetic nervous system-in emergencies, increases heart rate, respiration, and BP (fight or flight) parasympathetic nervous system-after emergencies counteracts by slowing heart rate, lowering BP, etc. 10

11 Diseases & Abnormal Conditions Meningitis inflammation of the linings of the brain and spinal cord Encephalitis inflammation of the brain caused by germ or chemicals Epilepsy seizure disorder caused by excessive discharge from neurons 1 in 200 suffer grand mal or petit mal seizures Cerebral Palsy disturbance in voluntary muscular action caused by brain damage Parkinson s Disease decreased neurotransmitter results in tremors usually after age 50 11

12 Diseases & Abnormalities Continued Cerebrovascular Accident (CVA) also called stroke when blood flow to brain is impaired &destroys brain tissue Hydrocephalus excessive accumulation of cerebrospinal fluid in brain MS (Multiple Sclerosis) chronic, disabling condition resulting from degeneration of myelin sheath occurs between ages of there is no cure Paralysis Results from brain or spinal cord injury that destroys neurons Results in loss of function below the level of the injury 12

13 Action Potentials and Conduction QuickTime and a DV/DVCPRO - NTSC decompressor are needed to see this picture.

14 Neuron F8-2 Axons carry information from the cell body to the axon terminals.

15 Changes in the Membrane Potential Produce Electric Signals in Nerve Cells Ion Intracellular Extracellular Normal Plasma Value K Na Cl Organic Anions 65 0 T3-5 Difference in ion concentration between compartments gives rise to the resting membrane potential (RMP). Membrane permeability to these ions also influences the RMP. Transient changes from the RMP produce electrical signals which transmit information in nerve cells.

16 Terminology Associated with Changes in Membrane Potential F8-7, F8-8 Overshoot- when the inside of the cell becomes +ve due to the Depolarization- a decrease in the potential difference between the inside and outside of the cell. Hyperpolarization- an increase in the potential difference between the inside and outside of the cell. Repolarization- returning to the RMP from either direction.

17 Gated Channels Are Involved in Neuronal Signalling In the nervous system, different channel types are responsible for transmitting electrical signals over long and short distances: A) Graded potentials travel over short distances and are activated by the opening of mechanically or chemically gated channels. B) Action potentials travel over long distances and they are generated by the opening of voltage-gated channels.

18 Graded Potentials F8-9 Graded potentials are depolarizations or hyperpolarizations whose strength is proportional to the strength of the triggering event. Graded potentials lose their strength as they move through the cell due to the leakage of charge across the membrane (eg. leaky water hose).

19 Graded Potentials Above Threshold Voltage Trigger Action Potentials Graded potentials travel through the neuron until they reach the trigger zone. If they depolarize the membrane above threshold voltage (about -55 mv in mammals), an action potential is triggered F8-10

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21 Spatial Summation F8- A neuron may receive greater than 10, 000 inputs from presynaptic 12 neurons. The initiation of an action potential from several simultaneous subthreshold graded potentials, originating from different locations,

22 Action Potential (AP) F8-14 They are initiated in an all-or-none manner when the summed graded potential exceed threshold voltage. They remain the same size as they travel along the axon over long distances. They are identical to one another.

23 F8-20a Graded potential triggers AP. Opens voltage-gated Na + channels.

24 The Na + spreads in all directions attracted by the -ve ions in adjacent regions (3,4). Opens Na + channels and initiates AP in the adjacent region along the axon (4), but not in the cell body where there are no voltage-gated Na + channels (3). F8-20b

25 F8-20c K + channels have opened in the initial segment (5) and the Na + (6) ions cannot trigger an AP in that region since its absolutely refractory. Na + ions initiate action potentials in segment (7).

26 Factors Influencing Conduction Speed of APs The resistance of the membrane to current leak out of the cell and the diameter of the axon determine the speed of AP conduction. Large diameter axons F8- provide a low 6 resistance to current Myelin sheath which wraps around vertebrate axons prevents flow within the axon current leak out of the cells. Acts like an insulator, for example, and this in turn, speeds plastic coating surrounding electric wires. up conduction. However, portions of the axons lack the myelin sheath and these +

27 Saltatory Conduction When depolarization reaches a node, Na + enters the axon through open channels. At the nodes, Na + entry reinforces the depolarization to keep the amplitude of the AP F8-22 constant, but slows the However, it speeds up again when the depolarization current flow encounters due to a the next node. loss of charge to the extracellular fluid. The apparent leapfrogging of APs from node to node along the axon is called saltatory conduction.

28 are needed to see this picture. Jacqueline du Pré died of Multiple Sclerosis QuickTime and a DV/DVCPRO - NTSC decompressor QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. In demylinating diseases, such as multiple sclerosis, the loss of myelin in the nervous system slows down the conduction of APs. Multiple sclerosis patients complain of muscle weakness, fatigue, difficulty with walking and a loss of vision.