Nervous System

general functions of a neuron

1) respond to chemical and physical stimuli
2) conduct electrochemical impulses
3) release chemical regulators
4) enable perception of sensory stimuli, learning, memory, and control of muscles and glands

basic structure of a neuron

1) soma
2) dendrites
3) axon hillock
4) axon
5) axon terminals

axon

connected to cell body/soma by the axon hillock- where action potentials are generated at the initial segment of the axon

covered in myelin with empty spots called "nodes of ranvier"; insulate signal and allow for quicker propagation of signal

can form many branches called "axon collaterals"

vary in length from a few millimeters to a meter

nerve

neurons+glia cells

glia cells

supportive cells

oligodendrocytes and schwann cells

oligodendrocyte

supply myelin in CNS; insulation

mostly membrane and proteins (and some cytoplasm) cover the axon

one wraps around many axons (1: many)

schwann cells

supply myelin in PNS

one wraps around axon (1:1)

microglia

supportive cell

phagocytosis of foreign object/debris (similar to immune cell)

astrocyte

helps regulate external environment of neurons; regulate blood- brain barrier; keep certain thing out of the brain

end-feet cover capillary surfaces or are adjacent to synapses between two neurons

guide neuron growth, control chemical environment around neurons, and absorbs extra neurotransmitters from synapse (moderates interaction and monitor communication of neurons)

capillary cells in the brain are joined by tight junctions

information flow of a neuron

reception: dendrites

integration/summation: axon hillock

conduction: axon

transmission: axon terminals

functional classification of neuron

1) efferent/motor neuron (signal "exits")
2) interneuron
3) afferent/sensory neuron (send signal inward)

structural classification of neuron

unipolar or multipolar

based on number of "poles" connecting to the soma

membrane/resting potential

the difference in electrical potential (change in charge) between the interior and exterior of the cell

requires an electrochemical gradient

mostly negative inside the cell- greater amount of K+ inside the cell; greater amount of Na+ outside the cell

cell has a negative membrane potential (polarized): -70mV

electrochemical gradient

chemical and electrical gradients

chemical gradient: difference in solute concentration across a membrane

electrical gradient: difference in charge across a membrane

why is membrane potential of neurons important?

potential energy, when activated, can be used to send signals; ions that shift, stimulate

the Na+/K+/ATP pump (active transport)

1) Na+ binds to the sodium potassium pump. the affinity for Na+ is high when the protein has this shape
2) Na+ binding stimulates phosphorylation by ATP
3) phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside the cell; becomes more negative inside the cell
4) the new shape has a high affinity fro K+, which binds on the extracellular side and triggers release of the phosphate group
5) loss of the phosphate group restores the protein's shape, which has a lower affinity for K+
6) K+ is released; affinity fro Na+ is high again, and the cycle repeats

leak channel proteins

allow ions to "leak" back in to keep charge balance

slower rate than being pumped in/out by sodium-potassium pump

action potential

large and rapid changes in the membrane potential made possible by voltage gated Na+ and K+ channels

allows for rapid conduction down the axon

graded potentials- initiation

activation of post-synaptic receptors allow for + or - charges to flow into the cell which changes the membrane potential

chemically gated ion channels

open in response to binding of appropriate neurotransmitter

voltage gated channels

change in voltage allows channel to open, allowing for ions to rush through, down the concentration gradient

stage 1 of action potential

resting/membrane potential

-70mV

channels involved:
leak channels for K+ and Na+
Na+/K+/ATPase

stage 2 of action potential

depolarization; rising beyond threshold

channels involved:
1) leak channels K+ and Na+
2) Na+/K+/ATPase
3) voltage gated Na channel (VGNaC): open immediately and close quickly; Na+ rush in quickly, making the inside of the cell more positive

stage 3 of action potential

repolarization; falling

channels involved:
1) leak channels K+ and Na+
2) Na+/K+/ATPase
3) VGKC: K+ rush out of cell to bring cell back to more negative; open after delay and takes more time to close

stage 4 of action potential

hyperpolarization

part of refractory period

cannot fire at this point

channels involved:
1) leak channels K+ and Na+
2) Na+/K+/ATPase
3) VGKC: remain open, close slowly

stage 5 of action potential

recovery; goes back up to resting state/membrane potential

channels involved:
1) leak channels K+ and Na+
2) Na+/K+/ATPase

as the inside of the cell becomes more positive, is this depolarization or hyperpolarization?

depolarization

the more _____ the inside of the cell, the stronger the electrical attraction for Na+ ions to enter the cell

negative

Na+ and K+ are present on both sides of the cell. which of these factor into the chemical driving force fro Na+ to enter the cell?

Na+; bc the chemical concentration gradient

why does it take a stronger stimulus to re-fire the neuron during the relative refractory period?

the inside of the cell is much more negative/farther from the threshold

K+ is flooding out (losing positives) of the cell quickly, therefore making the inside of the cell more negative quickly; need to let more positives into the cell, but would also be trying to get Na+ ions to go against its concentration gradient

absolute refractory

a depolarized membrane can't depolarize again once it's just fired; can't fire again

occurs during depolarization and most of repolarization

relative refractory period

once the VGNaC reset, you could depolarize again IF the signal is strong enough to reopen the Na+ channels and overcome the K+ mvmt

occurs during some of repolarization, hyperpolarization, and refractory

function of the refractory period

propagate depolarization down the axon; help push the action potential down

when Na floods into the axon, some ions drift to the right (toward more negative), which causes a change in voltage and activates the next VGNaC

some ions drift to the left, but doesnt fire bc need a much bigger stimulus (too negative in the refractory period)

optimize speed of action potential conduction

low resistance (larger axon diameter): with a wider diameter, there is less resistance and pressure, therefore allowing quick flow

insulation: myelin sheath insulates axon

saltatory conduction

signal "jumps" down the axon

nodes of ranvier

gaps in myelin on the axon; allow signal to "jump"

mechanism of nodes of ranvier

1) when an action potential comes down the axon and reaches a node/gap, it causes an influx of Na at the node
2) Na rushes into the axon at the node, creating an electrical force that pushes on the ions already inside the axon
3) the signal reaches the next node and creates another action potential, therefore refreshing the signal

synapse

1) axon terminus (presynaptic cell)
2) synaptic cleft
3) post synaptic cell

neurotransmitter release (transmission)

1) action potential (change in voltage) arrives at the axon terminal
2) Voltage-gated calcium channels open; calcium enters the axon terminal
3) calcium entry causes synaptic vesicles to release neurotransmitter by exocytosis
4) neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane
5) binding of neurotransmitters opens ion channels in postsynaptic cell

neurotransmitter elimination

1) diffusion; some get absorbed by astrocytes and other diffuse out in to the body
2) enzyme degradation
3) re-uptake: repackage left over NT
ex. SSRI= re-uptake inhibitor ( results in more NT spending more time in synaptic cleft; increases time and ability to bind to receptors)

ionotropic receptors (ligand-gated ion channels)

direct-acting NT (similar to primary messenger)

1) channel closed until NT binds to it
2) open channel allows diffusion of specific ions

G- protein coupled receptors

indirect-acting NT (similar to second messenger)

1) NT binds to its receptor
2) G-protein subunits dissociate
3) adenylate cyclase activated
4) cAMP activates protein kinase, which opens ion channels

excitatory postsynaptic potential (EPSP)

local depolarization of the postsynaptic membrane; brings the neuron closer to action potential threshold

one won't be enough to fire

inhibitory postsynaptic potential (IPSP)

hyperpolarization of the postsynaptic membrane

drive neuron away from action potential threshold

what must occur for the postsynaptic neuron to reach the threshold to fire?

EPSPs must outweigh IPSPs

no summation

presynaptic neuron causes EPSPs that are too far apart or too weak --> they do not add together and is not strong enough --> threshold is not reached, so no action potential is generated

signals summated --> too weak/far --> no fire

temporal summation

a rapidly firing presynaptic neuron causes EPSPs that are close in time --> summation brings axon to threshold --> action potential fires

multiple rapid signals--> summation --> reaches threshold --> neuron fires

spatial summation

more than one presynaptic neuron fires at the same time --> EPSPs are generated at different locations of the neuron --> add together/summation brings axon to threshold --> neuron fires

spatial summation of EPSPs and IPSPs

presynaptic neuron creates an IPSP it can override the EPSP created by another neuron --> EPSP brings neuron closer to threshold, IPSP brings the neuron farther from threshold --> together, (nearly) cancel each other out

EPSPs and IPSPs compete; if EPSPs win, neuron fires

serial neural processing

whole system works in a predictable, all or nothing manner

reflexes: stimulus always gives the same response (ex. touch something hot --> hand pulls back)

parallel neural processing

input is segregated into many pathways; different parts of the nervous system can respond differently to same info

helps brain put parts together to understand the whole

ex. smelling a pickle may invoke a memory, remind you of a dislike, remind you to buy some, all at once

general organization: structural

CNS: brain and spinal cord (higher functions)

PNS: spinal nerves and cranial nerves (basic functions)

general organization: functional

CNS: information processing

PNS: sensory (afferent) division and motor (efferent) division

sensory: somatic sensory (sense outside the body; touch, temp, sharp) and visceral sensory (sense inside the body; organs; urge to pee, eat, etc)

motor: somatic nervous system (muscles; info going out)
autonomic nervous system --> sympathetic (fight or flight) and parasympathetic (rest and digest)

what do the sympathetic and parasympathetic systems control?

glands, cardiac muscle, and smooth muscle

reflex arc

the neural path of a reflex

the sensory pathway: somatosensations --> CNS

the motor pathway: CNS --> neuromuscular junction

patellar reflexe

1) physically stretching the patellar tendon stretches the quads; activates the sensory neuron
2) nerve impulse goes to motor neuron to quad, telling it to contract, causing the knee to jerk

keeps us upright as knees buckle

somatosensations

REWATCH LECTURE

upper motor neurons

efferent pathways primarily relaying info from the cerebrum to the brainstem or spinal cord

synapse with interneurons

if damaged, partial recovery possible

lower motor neurons

neurons having direct influence on muscles

cell bodies originate in gray matter of spinal cord, but their axons extend into the PNS

if destroyed, permanent paralysis