Biol 216 - Topics 5 + 6

CNS

brain, spinal cord (interneurons)

PNS

afferent and efferent neurons

afferent neurons

sensory neurons - pick up stimulus via sensory receptors → transmit info to interneurons in CNS

interneurons

integrate this info - formulate a response

efferent neurons

carry response signal to muscles/glands → response carried out

motor neuron

type of efferent neuron - carries signals to skeletal muscle

3 types of neurons

afferent, interneurons, efferent

order of info processing in nervous system

stimulus → afferent (sensory neurons) → interneurons → efferent neurons → action

key structures in a neuron

cell body, dendrites, axon

afferent neuron structure

one axon with peripheral branch and central branch - no dendrites

nerve

cordlike structure that contains many axons - found in PNS

tracts

CNS version of nerve

white matter

contain myelinated axons and glial cells

gray matter

contain neuronal cell body

glial cells (neuroglia)

non-neuronal cells - provide nutrition and support to neurons

ependymal cells

produce cerebrospinal fluid

microglia

(CNS) phagocytic cells that ingest and break down pathogens and waste products

astrocytes

(CNS) cover the surfaces of blood vessels, for structural support + help maintain ion concentrations in interstitial fluid surrounding them

satellite cells

(PNS) similar function to astrocytes - structural support + maintain ion concentrations

schwann cells

(PNS) form myelin sheath

oligodendrocytes

(CNS) form myelin sheath

myelin sheath

high lipid content → insulate electrical impulse as it travels along axon → saltatory conduction

nodes of ranvier

gaps in myelin - expose axon to extracellular fluid + speed rate that electrical impulses move along axon

axon hillock

spike initiation zone for action potentials: where signals from dendrites converge and has voltage activated sodium channels

synapse

junction between axon terminals of a neuron and the receiving cell

gap junctions

allow current to flow directly between adjacent cells

connexons

protein tubes in cell membrane for electrical synapses

electrical synapses

action potential of one cell → potential in next cell - fast and in sync, found in cardiac/smooth muscle

chemical synapse

electrical impulse travels along axon → neurotransmitter at terminal released → diffuses across synaptic cleft → binds to receptor on postsynaptic cell → new electrical impulse generated - well modulated

membrane potential

difference in electrical charge - at rest -70mV

ways ions can cross cell membrane

diffusion from concentration gradient + electric fields

electro-chemical gradient

net driving force from combo of concentration and electrical gradient

cause of resting membrane potential

Na+/K+ pump + membrane more permeable to K+ than Na+

Na+/K+ pump

3 Na+ out & 2 K+ in → higher K+ concentration inside

selective permeability of plasma membrane

Na+ & K+ diffuse in/out - more K+ leaves through leak channels + cell contains negatively charge ions → net negative charge inside cell

K+ in membrane potential

K+ leaks out b/c of concentration gradient → unbalance negative charge inside cell → electrical field/membrane potential → stops further efflux of K+

equilibrium potential

membrane potential at which voltage gradient of ion balances concentration gradient - no net flow of ion through channel (Nernst equation)

Nernst equation

predicts equilibrium potential of ion = 62mV log10 ([X] outside / [X] inside)

Goldman equation

predicts membrane potential when permeable to more than one ion - based on concentration gradient and permeability

types of ion channels in neurons

ungated channels (leak), voltage gated channels, ligand gated channels, mechanically gated

resting neuron

more K+ leak channels open than Na+ leak channels and Na+/K+ ATPase → flow of K+ ions across membrane

voltage gated ion channel

diverse integral membrane proteins related by structural and functional motifs - classified by ion conductance, pore gating, regulation

rate of flow through ion channel

determined by maximum channel conductance and electrochemical driving force for that ion

action potential

abrupt and transient change in membrane potential that occurs when a neuron conducts an electrical impulse

general action potential process

1. stimulus → positive charge flow into neuron - membrane depolarized

2. depolarization until membrane potential reaches threshold

3. rapid influx of positive ions → sudden increase in membrane potential

4. membrane potential falls below resting potential - hyperpolarization

5. membrane potential returns to resting potential

Na+/K+ action potential process

1. stimulus raises membrane potential to threshold → Na+ channel activation gate opens

2. more Na+ channels open → Na+ flows along concentration gradient → depolarization

3. Na+ channel inactivates - peak of action potential + activated K+ channels

4. K+ flow out along concentration gradient → membrane potential falls 

5. Na+ inactivation gate opens + K+ activation gate closes - membrane potential reaches resting value

6. close of K+ activation gate → stabilizes membrane potential

key features of action potentials

all or nothing

maintain size - magnitude stays the same as it travels

propagate - triggering of action potentials on neighboring stretches

absolute refractory period

excitable membrane can’t generate an AP in response to any stimulus - since Na+ channels already open and become inactivated + K+ channels open

relative refractory period

excitable membrane will produce action potential only if stimulus of greater strength than usual threshold strength - since K+ channels open and some Na+ channels closed

AP during relative refractory period

size is smaller and threshold required is higher

higher intensity of stimulus…

higher frequency of action potentials

A-alpha nerve fibers

carry info related to proprioception (muscle sense) - largest diameter and fastest conduction velocity

A-beta nerve fibers

carry info related to touch

A-delta nerve fibers

carry info related to pain and temp

C-nerve fibers

carry info related to pain, temp, and itch - no myelin → slowest conduction

larger diameter axons

low internal resistance → greater conduction velocity of AP → fast response

cable theory

neuron is treated as electrically passive, perfectly cylinder transmission cable - calculates flow of electric current using capacitance and resistance

Ohm’s Law

current (I) = change in voltage (V) / resistance (R)

capacitance of neuronal fiber

ability to store electric charge - comes from electrostatic forces that act through phospholipid bilayer

longitudinal/internal resistance

cytosol’s resistance to movement of electric charge from proteins and organelles inside - lower resistance/larger diameter → longer lamda

lambda length constant

scale on which the voltage across a membrane decays - decr by 37% of its original size → larger lambda = greater conduction velocity

lambda formula

square root (membrane resistance / longitudinal resistance)

myelin effect on conduction

insulates axon → greater membrane resistance → incr conduction

chemical synapse

allow neurons to receive inputs from numerous axon terminals at the same time using neurotransmitters → allows for modulation of transmission

neurotransmitters

small signal molecules secreted by presynaptic nerve cell to relay signal to postsynaptic nerve cell - can have stimulatory/inhibitory effect

direct neurotransmission

neurotransmitters binds directly to ligand gated ion channel → channel gate opens/closes → affect ion flow into postsynaptic - quick

indirect neurotransmission

neurotransmitters binds to G-protein coupled receptor on postsynaptic membrane → activates second messenger pathway → ion channels open/close - slower but longer effect

metabotropic receptors

indrectly linked w/ ion channels on membrane of cell through signal transduction mechanisms - often g proteins

ionotropic receptors

form ion channel pore for direct neurotransmission

acetylcholine

NT betw nerves and muscle, in the hippocampus, in the heart - associated with Alzheimer’s

Alzheimer Disease

degeneration of acetylcholine releasing neurons

GABA

inhibitor of NT by opening Cl- channels on post synaptic membrane - hyperpolarization

glycine

inhibitor of NT - increases Cl- influx

glutamate

involved w/ learning + memory - excitatory

norepinephrine/epinephrine (adrenaline)

hormones and NT - involved in attention, mental focus, pleasure/reward, memory, motor control - can be excitatory or inhibitory

dopamine

NT involved w/ behavior/cognition, voluntary movement, motivation/reward, inhibits lactation, sleep, mood, attention, learning - linked to Parkinson’s

Parkinson’s

degeneration of dopamine releasing neurons in substantia nigra - loss of muscle control

serotonin

NT regulating intestinal movement, involved in mood, appetite sleep

neuropeptide

indirect neurotransmitters

endorphins

neuropeptide released during pleasurable experience - reduce perception of pain, work on PNS

enkephalins

subset of endorphins and modulate pain response, work in CNS

substance P

neuropeptide released by spinal cord - incr perception of pain

dissolved carbon monoxide

regulate release of hormones from hypothalamus

dissolved nitric oxide

learning, muscle movement, relaxes smooth muscle in walls of blood vessels - causes dilation

synaptic vesicles

store neurotransmitters in cytoplasm of axon terminal

when action potential arrives at axon terminal for chemical synapse…

voltage gated Ca2+ channels open → Ca2+ flow into axon terminal of presynaptic cell → vesicle fuses with membrane → release NT into synaptic cleft → NT released by exocytosis

removal of NT from synaptic cleft

either diffuses away or taken up again by receptors (reuptake)

EPSP

excitatory post synaptic potential: change in membrane potential that moves neuron closer to threshold - depolarize

IPSP

inhibitory post synaptic potential: change in membrane potential that pushes neuron farther away from threshold - hyperpolarize

graded potential

incr/decr in membrane potential below threshold (does not trigger AP) → precursor to AP but have no refractory periods - EPSP/IPSP (seen in sensory/postsynaptic cells)

size of graded potential…

related to stimulus intensity/amount of transmitter (unlike AP) - decr w/ distance

temporal summation

summation of more than one EPSP produced by successive firing of a single presynaptic neuron over a short period of time

spatial summation

summation of EPSPs produced by firing different presynaptic neurons

membrane resistance

leakiness - higher membrane resistance (fewer open channels) → longer length constants

evolution of nervous system

evolution/natural selection → need for more complex nervous system to find food/escape danger

invertebrate nervous system

simple, fewer neurons, less complex networks

cephalization

development of anterior head - where sensory organs + nervous tissues connected

nerve nets

loose mesh of neurons found in symmetrical animals

nerve cord

bundle of nerves - extend from central ganglia to rest of body

ganglia

functional cluster of neurons