5-Nervous system II: Physiology of Neurons. Text REVIEW ORGANIZATION AXON ION GRADIENTS. ACTION POTENTIAL (axon conduction)

Transcription

1 5-Nervous system II: Physiology of Neurons REVIEW ORGANIZATION AXON ION GRADIENTS ACTION POTENTIAL (axon conduction) GRADED POTENTIAL (cell-cell communication at synapse) Text SYNAPSE STRUCTURE & FUNCTION NEURAL INTEGRATION

2 Brain Structure Major Landmarks: Forebrain -Cerebrum -Diencephalon Corpus callosum Brainstem -Midbrain -Pons -Medulla oblongata Cerebellum 2

3 Cranial nerves: 12 pairs 3

4 Spinal Nerves 4

5 Spinal Nerves Spinal nerve structure: (simple version) - gray matter = nerve cell bodies - white matter = nerve cell axons Anterior view of one vertebra and the nearby section of the spinal cord 5

6 CNS PNS CNS = brain + spinal cord; all parts of interneurons are in the CNS PNS: (1) afferent neurons (their activity affects what will happen next) into the CNS & (2) efferent neurons ( effecting change: movement, secretion, etc.) projecting out of the CNS. 6

7 Contrast autonomic and somatic components of the nervous system 7

8 Contrast autonomic and somatic components of the nervous system Voluntary Command: MOVE! Skeletal Muscle Contraction Involuntary Command: Rest/Digest Heart, smooth muscle, glands, etc. Involuntary Command FIGHT! FLIGHT! Heart, smooth muscle, glands, etc. 8

9 Schematic diagram of the mammalian autonomic nervous system 9

10 Another schematic diagram of the mammalian autonomic nervous system 10

11 Spinal Nerves Q: What integrates the afferent and efferent signals? 11

12 Evolution of the spinal nerve pattern Not all vertebrates have the mammalian pattern

13 Note the physical separation of the dorsal and ventral roots in lamprey 13

14 14

15 Contrast autonomic and somatic components of the nervous system Voluntary Command: MOVE! Skeletal Muscle Contraction Involuntary Command: Rest/Digest Heart, smooth muscle, glands, etc. Involuntary Command FIGHT! FLIGHT! Heart, smooth muscle, glands, etc. 15

16 Physiology of the nervous system 16

17 Physiology of the nervous system Only a very thin shell of charge difference is needed to establish a membrane potential. 17

18 Membrane Channels K+ ion Shut Open From Above 18

19 Membrane Channels Change conformation in response to voltage change in the surrounding membrane: voltage gated Change conformation in response to binding by an ion or other compound: ligand gated Are selective in which ions pass through the pore in the center Amino acid charges around the pore can attract specific ions May have 3 states: deactivated (closed), activated (open), inactivated (closed) 19

20 Sodium Potassium Pump (Na+/K+-ATPase) 3 Sodium ions moved out 2 Potassium ions moved in Uses ATP to power protein conformational change 20

21 Graded potentials can be excitatory (an action potential is more likely to occur), or inhibitory where an action potential is less likely. The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance. Membrane Potential (mv) 21

22 Action potentials ~100mV An action potential is an all-or-none sequence of changes in membrane potential (and an example of positive feedback). Action potentials result from an all-or-none sequence of changes in ion permeability due to the operation of voltage-gated Na+ and K + channels. Text The rapid opening of voltage-gated Na+ channels allows rapid entry of Na+ The slower opening of voltage-gated K+ channels allows K + exit 22

23 Flash animations: 1.Voltage gated channels in the axon membrane links on site voltage.swf 23

24 Action potentials Four action potentials, each the result of a stimulus strong enough to cause depolarization, are shown. Note that all are the same height. 24

25 Action potential propagation down the axon The propagation of the action potential from the dendrites to the axon-terminal end is typically one-way because the absolute refractory period follows along in the wake of the moving action potential; the AP starts at the neuron initial segment. 25

26 Flash animations 2. action_potential_un.swf 3. action_potential_myelinated.swf (on web site under links )

27 Action potential propagation down the axon in myelinated axons Saltatory conduction: action potentials jump from one node to the next as they propagate along a myelinated axon. 27

28 Remember the synapse when the action potential arrives at the axon terminals The synapse is the point of communication between two neurons. Chemical synapses have a synaptic cleft (about nm wide) and neurotransmitter diffuses across the cleft to bind to receptors on the postsynaptic neuron membrane. Chemical synapses are one-directional. 28

29 NOTE: 1. The role of voltage-gated calcium channels 4. chemical_synapse.swf 2. Vesicles with neurotransmitter 3.Neurotransmitter binding to postsynaptic receptors (often ligand-gated ion channels) 4.Re-uptake and enzymatic breakdown of neurotransmitter 5. At an excitatory synapse nonselective ion channels open and ions, mostly Na+ move down the gradient 6. At an inhibitory synapse Cl- and K+ channels open 29

30 1.EPSP: excitatory postsynaptic potential (glutamate is the main neurotransmitter) Key point: An action potential in a presynaptic neuron results in a graded potential in the postsynaptic neuron. 2. IPSP: inhibitory postsynaptic potential (GABA and glycine are the main neurotransmitters) 30

31 Neural integration Real neurons receive as many as 200,000 synapses each Ion flows from all inputs summate or average at the initial segment An action potential in the postsynaptic neuron occurs if the membrane potential at the initial segment reaches threshold Summation at the axon hillock can be spatial or temporal 31