Chapter 11a Nerve Physiology
control systems
•endocrine
•nervous system
communication systems
•cell to cell contact
•adjacent cellsinterstitial fluid
•hormonalblood
•nervousaxons + synapse
nervous system vs endocrine system
•neurotransmittershormones
•axonblood
•receptorsreceptors
•fast actionslower to act
•short term effectlong term
•muscles ; glandsany cell, organ
•depolarize target cell stim specific cellular function
•expensivecheaper
Structural division of nervous system
•Central nervous system (CNS)
•brain
•spinal cord
•Peripheral nervous system (PNS)
•spinal nerves
•cranial nerves
cells of the nervous system
•neuronsconduct impulse
•neurogliasupportive cells don’t conduct impulse
–atrocytesblood-brain barrier
–oligodendrocytesmake myelin CNS
–Schwann cellsmake myelin PNS
2 definitions
•stimulus change in environ that excites a neuron
•impulse wave of excitation along the neuron
functional terms
•direction :
–sensory=afferent impulse to the CNS gets stimulated
–motor=efferent impulse from CNS response
•connection:
–somaticskin , muscles
–visceralorgans
functional divisions
•somatic sensory
•visceral sensory
•somatic motor
•visceral motor to cardiac and smooth muscle, glands= Autonomic nervous system
•interneuronsconnect sensory to motor
Neuron anatomy
•cell body = somanucleus, organelles
•dendriteimpulse begins
–receptors “stimulated” area
•axonconducts impulse
–axon terminal release neurotransmitter
–axon collateralsbranches
–axon hillock“threshold” for axon
•synapsespace betw neuron and target cell
•ganglioncluster of cell bodies in PNS
•nucleuscluster of cell bodies in CNS
neuron specialization
•size and shape
•cell membrane
•receptors
•secretions
•“one way”receptor – dendrite – axon – axon terminal
structural classes of neurons
•number of processes from the cell body
•multipolarmultiple dendrites1 axon all motor neurons ; most interneurons
•bipolar1 dendrite + 1 axon sensory - eye , smell
•unipolar1 process - short dendrite ; long axon most sensory neurons
remember ?
•3 steps of cell communication :
•signal senderneuron
•signal transportersynapse
•signal receiverneuron muscle gland
•signal=neurotransmitter
neuron functional areas
•receptive regiondendrites chemical gated ion channels graded potential
•conductive regionaxon voltage gated Na channels action potential
•secretory regionaxon terminal voltage gated Ca channels release neurotransmitters
•neuron is a target cell and controlling cell
•neuron is a signal receiver and signal sender
electricity
•charge
–electrons - protons +
–we use ions + / -
•voltage=potential energy
–opposite charges are separated = electrical gradient
–energy = work required to maintain electrical gradient
–“V = voltsmV = millivolts
•current=flow of electric charge (ions)
•resistance =slows or prevents flow of charges
resting membrane potential
•resting neuron membrane has potential of -70mV
•+ outside vs - inside=polarized
•K+ and A- insideK channel open
•Na+ outsideNa channels closed Na / K ATPase pumps Na out huge Na gradient = rubberband
what changes?
•resting = closed Na channel
•action = open Na channel
•what happens?
•goal of stimulation = open Na channel
•excitability = changed ion permeability
what causes this change?
•stimulation of receptors alters ion permeability
•external changestouch, pressure temp
•internal changeschemical levels O, CO, pH BP neurotransmitters hormones
types of channels
•leakage channels (passive)always open
•gated channels (active)
–chemically gated
•open/close when receptor binds with ligand
–voltage gated
•opens with change in membrane potential
•other :
–mechanically gated
–temperature
–photon
electrical activity
•depolarizationdecrease potential(toward 0 mV)> -70 mV Na+ moves in excitatory
•repolarizationreturn to membrane potential K+ moves out
•hyperpolarizationincrease potential (more negative)< -70 mV + ion out and/or - ion in inhibitory
•Na / K pumprestores ion gradients
graded potentials
•local depolarizationdendrite, cell body
–Na+ in / K+ out
•repolarizationquickly returns to resting potential
•gradedamount of depolarization varies
–stim strength# channels open
–stim frequency time channels are open
•various stimuli open ion channels
–sensations generator potential
–neurotransmitterspost-synaptic potential
–gated channelsligand; mechanical; light; temperature
graded potentials are decremental
•cable property - depolarization spreads from ion channel
•charge (Na+) moves through cytoplasm ~ 1 – 2 mm
•charge leaks out
•returns to resting potential
•decremental - decrease strength with distance
•stronger depolarization travels further
•dendrites, cell body
•GOAL:get to the axon hillock
action potentials
•brief, complete reversal of potential + 30 mV
•voltage-gated channels
–depolarization opens channels
•axons , muscle cells
action potentials
•restingvoltage-gated channels closed
–K leaks out
–Na / K pump maintains membrane potential -70mV
•depolarization phaseincrease Na permeability
•increase potential opens Na channels voltage-gated
•Na rushes in-decreases potential if threshold - all Na channels open
•reversal of potential+ 30mV
•repolarizationdecrease Na permeability
•+ insideresists further Na+ moving in (nature)
•+ insidecloses Na+ inactivation gates
•K channels open rushes outWhy?
•hyperpolarization
•K movement continues out
•undershoot
•membrane repolarized , but Na and K reversed
•Na / K pumprequires ATP
•return to resting state
AP propogation
•depolarization opens adjacents Na voltage-gated channels
•no decrease of strength
•starts at axon hillock
•entire axon
•= impulse
•one wayto axon terminal
threshold
•threshold = minimum depolarization to produce AP
•= min depol to open a voltage gated channel
•potential ~ 15 – 20 mV above resting
•subthreshold stimuli- don’t depolarize enough
•summation of all graded potentials
–at axon hillock
all – or – none
•if > thresholdalways to + 30mV
–propagates entire axon or not at all
•if < thresholdno further depolarization
refractory periods
•absolute refractory period
–during APneuron cannot be stimulated
–Na channels open
–ensures one way transmission
•relative refractory period
–during repolarization
–very strong stimulus can reopen Na channels
–allows more AP with stronger stimuli
graded potential vs action potential
•stimuluschemicalvoltage
•effectNa, K openNa channel open
•distance1-2 mmentire axon
•max depol0 mV+30 mV
•strengthvariesall or none
•thresholdnone+ 15 mV
•refractory periodnoyes
•summationyesno
stimulus intensity
•all action potentials are the same
•intensity ~ frequency of AP to CNS
–amt neurotransmitter released
myelin
•covers section of axon
•increase speed of AP conduction
•Nodes of Ranviervoltage gated Na channels
•AP only at Nodessaltatory conduction
•insulates neuronsprevents ion leakage
•next :
–the neuron as controlling cell
–action at axon terminal
–physiology of synapses and neurotransmitters