5-Nervous system II: Physiology of Neurons

Transcription

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

2 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. 2

3 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. 3

4 Schematic diagram of the mammalian autonomic nervous system 4

5 Another schematic diagram of the mammalian autonomic nervous system 5

6 6

7 Physiology of the nervous system 7

8 Physiology of the nervous system Only a very thin shell of charge difference is needed to establish a membrane potential. 8

9 Membrane Channels K+ ion Shut Open From Above 9

10 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) 10

11 Sodium Potassium Pump (Na+/K+-ATPase) 3 Sodium ions moved out 2 Potassium ions moved in Uses ATP to power protein conformational change 11

12 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) 12

13 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. The rapid opening of voltage-gated Na+ channels allows rapid entry of Na+ The slower opening of voltage-gated K+ channels allows K + exit 13

14 Action potentials mediated by voltage gated channels in the axon membrane 14

15 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. 15

16 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. 16

17 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. Ca++ Chemical synapses are one-directional. 17

18 18

19 Excitatory vs Inhibitory Influences: Neural Integration But also Ach, serotonin and others EPSP: excitatory postsynaptic potential IPSP: inhibitory postsynaptic potential

20 1.EPSP: excitatory postsynaptic potential 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) 20

21 NOTE: 1. The role of voltage-gated calcium channels 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 non-selective ion channels open and ions, mostly Na+ move down the gradient 6. At an inhibitory synapse Cl- channels open 21

22 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 22