Nervous System Notes

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Central Nervous System

brain, spinal cord

peripheral nervous system

motor, sensory

autonomic

involuntary, smooth muscle, cardiac muscle, glands

somatic

voluntary, skeletal muscle

autonomic system

sympathetic and parasympathetic

sympathetic

responds to alarm (fight or flight); speeds heartbeat

parasympathetic

returns body to normal; slows heartbeat

neuron

responds to stimulus; initiate and conduct electrical impulses

neuron cell body

contains nucleus and most cytoplasm, contains axon hillock

axon hilock

area where most impulses are generated, located where cell body leads to axon

cell processes or extensions

axon and dendrite

axon

conveys impulse away from cell body, most cells have 1

dendrite

conveys impulse toward the cell body, most cells have many branching ones

synaptic knob or terminal

at end of axon, contains synaptic vesicles which store and release neurotransmitters

neurotransmitters

chemicals that aid in conduction of nerve impulses across a ynapse

synapse, synaptic cleft

junction between to nerve cells or between nerve cells and glands or muscle cells

types of synapses

electrical - cell to cell contact via gap junctions (heart)
chemical- NT's from presynaptic cell pass across synapse (most)

information is transmitted

electrically down presynapttic cell, chemically across synapse, electrically down post synaptic cell

membrane potential

difference in charge or voltage between 2 sides of a membrane, potential energy used to run nerve impulse

resting potential

-70 mV, polarized

polarized

difference in electric charge across the membrane

cause of difference in electric charge across membrane

1. Na/K pump - membrane not very permeable to these ions
2. cell membrane is slightly more permeable to K so some diffuses out down its gradient, making it more negative inside
3. negatively-charged ions in the cytosol

nerve impulse/action potential

many action potential propagated down the length of the neuron

action potential

local change in charge across the membrane due to Na and K gates closing in succession in just one area of the membrane
1. Na gates open
2. Reversal of charge
3. Na gates close
4. K gates open
5. back to resting potential
lasts approx. one millisecond

polarized state

-70 mV

depolarized state

+35 mV

3 types of gates

1. ligand-gated channels
2. mechanical-gated channels
3. Voltage-gated channels

ligand-gated channels

respond to the binding of NT's

mechanical-gated channels

respond to touch or pressure

Voltage-gated channels

respond to a change in charge

sections of a neuron during a nerve impulse

1. polarized/resting (Na + K gates closed) -70
2. Action Potential/Depolarized (Na open K closed) -70:+35
3. Repolarized (Na closed K open) +35:-90
4. polarized/resting (Na + K gates closed) -90:-70

threshold potential

if the stimulus is strong enough to cause the cell to change from -70 mV to -50 mV - causes the charge of the cell to completely reverse, and the inside becomes + 35 mV
(if not strong enough, membrane can still become partially depolarized)
-50 causes Na voltage-sensitve gates to open which makes it go from -50 to +35 as more Na+ diffuses into cell

causes of depolarization

1. stimulus (binding of NT) causes Na ligand-sensitive gates there to open, and pressure will cause mechanical-sensitive gates to open. Cell becomes permeable to Na (-70:-50)
2. If threshold is reached, initial partial depolarization will lead to complete depolarization = reversal of charge across membrane (+35).
3. Action potential is all-or-none event
4. an action potential is always the same strength +35 regardless of strength of a stimulus - stronger stimulus just makes more action potentials

repolarization

cell membrane becomes impermeable to Na as Na gates close behind action potential. membrane becomes more permeable to K as K gates open. K moves out of cell and original polarized state is reestablished. Na/K pump then restores resting cell condition where high Na outside and high K inside by pumping.

review of action potential

A resting cell is in a polarized state.
It can become depolarized due to a stimulus, and may reach threshold potential.
This causes an action potential.
This is followed by depolarization.
Then another action potential may be sent.

refractory period

when Na gates close after action potential, they cannot be reactivated for a brief time, to prevent the Na that entered during depolarization from re-exciting the region behind the action potential. This insures that the impulse can only travel in one direction: from the body to the axon knobs.

hyperpolarization

some NTs cause a membrane potential to become greater than resting potential (cell goes from -70 to -90); more negative not he inside. This reduces the chances of an action potential being initiated because -90 is further from the threshold level.
Due to a particular type of NT that binds to K gates instead of Na gates - when the K gates open, K diffuses out of the cell making the interior more negative than resting potential.

graded potential

a local voltage change proportional to the strength of the stimulus. type of change in membrane potential that is a single stimulus and usually not enough to bring the membrane to threshold level, therefore an action potential does not generate.
Most EPSPs and IPSPs at the synapse are graded potentials. Alone neither will initiate or inhibit a nerve impulse.

EPSP

excitatory post synaptic potential
If after binding NTs, the membrane becomes depolarized, an EPSP is generated.
Excited = brought closer to threshold level (-70 mV to anything less negative)

IPSP

Inhibitory Post Synaptic Potential
If after binding NTs, the membrane becomes hyper polarized, an IPSP is generated.
Inhibited = brought farther away from threshold level (-70 to anything more negative)

summation

a process at the synapse where EPSPs and IPSPs are integrated
all of the stimulus received by a post-synaptic cell "added together", and when the additive effect of these graded potentials reaches the axon hillock of the neuron an action potential may or may not be initiated.
Can be temporal or spatial

EPSPs and IPSPs

lose strength the farther they travel away from the initial stimulus = the voltage changes from all the EPSPs and IPSPs will diminish as they reach the axon hillock
not considered APs because not at axon hillock

Temporal summation

repeated sub-threshold stimuli from the same neuron that occur in quick succession

spatial summation

takes many synaptic knobs from different neurons releasing NTs simultaneously not eh same post-synaptic membrane to reach threshold

rate of transmission

1. the larger the diameter of the axon, the faster the transmission (less resistance to flow of electric current)
2. saltatory conduction - impulse jumps from node to node - Na and K channels are concentrated at the nodes and action potentials can only be generated in these areas. In a non-myelinated neuron, Na and K gates exist all the way down the length of the axon so there are many more action potentials generated. this increases transmission rate because there are far fewer action potentials that have to be generated before the axon knob is stimulated to release NTs

saltatory conduction

occurs in myelinated neurons (areas with myelin- high in lipid content) are poor conductors of electrical impulses

Transmission across synapse

when action potential reaches the synaptic knob, it causes Ca++ channels to open, which diffuses into axon job and causes synaptic vesicles to fuse with presynaptic membrane, and NTs are released into synapse.
NTs bind to receptors on postsynaptic membrane, Na+ Gates open, Na difffuses into cell = causes graded potential in postsynaptic cell
NTs remain only briefly in synapse so there is not a constant stimulation to post-synaptic nerve.

After transmission, NTs are

1. returned to presynaptic knob
2. degraded by enzymes from presynaptic cell(acetylcholinesterase degrades acetylcholine at synapse)

types of neurons

1. sensory neurons
2. motor neurons
3. interneurons

sensory neurons

transmits info to CNS from sensory receptors

motor neurons

transmits info away from CNS to effectors

effectors

cells that cause an effect or elicit a response
can be glands (secrete hormones) or muscles (contract)

interneurons

connects sensory and motor neurons
Located within CNS
most neurons are interneurons (90%) in the brain or spinal cord

sensory receptors

receive stimulus
1. specialized nerve cells (rods and cones of eye)
2. dendrites of a sensory neuron

stimulus

any factor that induces a change in environment and evokes a response
ex. light, pressure, sound

Reflex arc

stimulus - sensory receptor - sensory neuron - interneuron - motor neuron - effector - response

Glail cells

brain cells
supporting cells- help assist, protect, insulate, and reinforce neurons
most nerve cells are glial cells, not neurons
don't transmit messages

Astrocytes

provide support for neurons and other jobs
during development these cells induce the formation of tight junctions between capillary endothelial cells, forming the blood-brain barrier

blood-brain barrier

a barrier that helps prevent harmful substances from entering the brain and spinal cord from the circulatory system

schwann cells and oligodendocytes

wrap around some axons in many concentric layers like a jelly roll
form myelin sheaths that insulate axon
lots of phospholipids in membrane

nodes of ranvier

spaces along axon with no myelin covering

internode

portion of axon covered by myelin sheath

myelin sheath

lipid
cause poor conduction of electrical impulses
causes impuse to jump from node to node = increase rate of nerve impulse

nerve

bundles of axons from many nerve cells
motor, sensory or combo

ganglia

clusters of nerve cell bodies located outside CNS

Acetylcholine

triggers contraction of muscle and is also used in other parts of the nervous system

catecholamines

made of amino acids
epinephrine (adrenaline), norepinephrine (noradrenaline), dopamine, serotonin
too much or to little of these cause illness (parkinson's, schizo, sleep disorders, depression, etc)
drugs can interfere with this chemical communication

how drugs affect catecholamines

1. drugs bind to receptors for NTs and block impulse
2. drugs mimic NTs and induce impulse (LSD and barbiturates- bind receptors and alter brain activity)

peptides/neuropeptides

small chains of amino acids
GABA and Endorphines

GABA

present at most inhibitory sun apses - hyper polarizes the membrane to inhibit action potentials; most abundant NT in brain

Endorphines

neural analgesic (pain reliever)
1. decreases pain
2. elevates mood (opium binds same receptors and has similar but stronger effect)

gases

some cases are NTs - Nitric Oxide (NO)

NTs

1. Acetylcholine
2. Catecholamines - AAs (epinephrine/adrenaline, norepinephrine/noradrenaline, dopamine, seratonin)
3. Pepties/neuropetides - AAs (GABA, Endorphines)
4. Gases (NO)

Energy needed to run a nerve impulse

ATP - indirectly - Na/K pump to establish a gradient
ATP - directly - to run Na/K pump
Potential E (ATP) --> Pot E (Gradient) --> gradient used to do work
many times cell uses proton gradients as pot E
requires a proton gradient to produce ATP
Active transport directly to help protein change configuration Na or H+ gradients as an energy source
Facilitated diffusion - many molecules bumping into channel is enough to change configuration of protein

Steps of action potential

1. depolarization
1a. threshold
2. action potential
3. repolarization
4. resting potential

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