bio 112 lecture 20: the nervous system

what is a nervous system?

network of interconnected nerve cells; they allow an animal to sense the environment around it

nervous system evolution

1. sponges have no nervous system

2. cnidarians have a simple nervous system

3. bilaterians have complex nervous systems

neurons

nerve cells; share certain features regardless of function

neuron anatomy

1. dendrites

2. axon

3. axon hillock

4. axon terminals

5. myelin sheath

6. nodes of Ranvier

7. synaptic cleft

dendrites

tree-like structures that extend away from the cell body to receive messages from other neurons

axon

extends as a narror conduit from the soma to a branching terminal

axon hillock

located at the transition point from the cell body to the axon; where signals multiple dendrites are compiled

axon terminals

produce neurotransmitters to signal the dendrites of the adjacent cells to which they connect

myelin sheath

lipid rich layers that wrap around the neuron axon; provide insulation for the electrically conducted signal

nodes of Ranvier

regular gaps in the myelin sheath where the axon is exposed; allows for the exchange of ions, namely Na+ and K+ to actively be exchanged when signals fire

*Na+ and K+ voltage-gated ions channels are concentrated here

synaptic cleft

gap between connected neurons

glial cells

support neurons by providing a variety of functions including nutrition and support (*more glial cells in the NS than neurons)

types of neurons

1. sensory

2. interneurons

3. motor

sensory neurons

activated by sensory input from the environment

interneurons

short; convey signals between neurons or process information from sensory neurons

motor neurons

long; originate in the brain or spinal cord that may trigger muscle movement or effect change in internal body systems

nerve signal transmission

nerve cell membrane segregates ions by charge:

1. negative charge inside

2. positive charge outside

neuron membrane states

1. polarized

2. membrane potential

3. resting membrane potential

polarized

difference in relative charge across the membrane

membrane potential

due to difference in number of charged ions across the membrane (more Na+ outside the cell, more K+ inside the cell)

resting membrane potential

negative at rest (voltage depends on nerve cell type)

action potential

electrical nerve impulse that travels down the axon (axon all-or-nothing firing response)

threshold potential

minimal stimulation needed to trigger the firing of an action potential above the resting

depolarization

once a threshold potential is achieved, the axon depolarizes; begins at the end of the axon closest to the dendrites, and then continues towards the axon terminal

*the voltage across the membrane rapidly changes; Na+ rushes into the cell and K+ rushes out

voltage-gated channels

will only open when right amount of voltage is reached

the Na+ and K+ channels are voltage-gated, the action potential travels down the length of the axon

positive-feedback loop

created by voltage-gated channels; depolarization at one point in the plasma membrane ultimately causes depolarization across the whole of the membrane of the axon

refractory period

slowly closing potassium channels result in hyperpolarization; until the membrane returns to its resting potential, the neuron cannot fire again

saltatory propagation

myelin sheaths and the nodes of Ranvier cause nerve impulses (action potential) to jump from node to node

synaptic signal transmission

voltage-gated Ca2+ channels at the axon terminals are triggered by the action potential; Ca2+ flooding into the axon terminal triggers release of neurotransmitters

*these neurotransmitters connect to ligand-gated ion channels in the receptor cell

**these are excitatory postsynaptic potentials (EPSP)

inhibitory postsynaptic potentials (IPSP)

cause Cl- ions to enter a neuron or by causing K+ ions to exit it; this makes the voltage more negative and therefore harder to fire (neuron membrane becomes hyperpolarized)

nervous system organization

1. central nervous system

   1. brain, spinal cord

2. peripheral nervous system

   1. nerves connecting to sensory organs and muscles

peripheral nervous system

1. somatic (voluntary): under conscious control, sensing and responding to external stimuli

2. autonomic (involuntary): NOT under conscious control, regulate internal bodily functions

autonomic nervous system

1. sympathetic: decreases response time and increase action; flight, flight or freeze response

2. parasympathetic: tends to decrease activity and stimulates digestion; triggers rest and digest behaviors (calms body after stimuli)

reflex responses

coded for in our spines; signals don’t have enough time to go to brain to respond to stimuli