Download Neuron Function notes

Neuron (bundles of individual neurons) function
Nerve fiber = axon of single neuron
3 sheets of CT
epineurium – around the entire nerve
perineurium – ct around bundles of nerve fibers(fascicle)
endoneurium – surrounds each nerve fiber
Sensory nerve – only sensory fibers, afferent nerves , carry
impulses to CNS
Motor nerves – efferent, contain only motor fibers, carry impulse
away from CNS
Mixed nerves – have both sensory and motor nerve fibers
Ganglia = clusters of neuron cell bodies
Located ouside of CNS
Also wrapped by CT
Nuclei = clusters of cell bodies INSIDE CNS
Plexus = a complex network of nerves
Comprised of nerves that are combinations of sensory
and motor nerves
Because of multiple branching, damage to a single root or
spinal cord section DOES NOT lead to complete motor or sensory
loss in the body part that is supplied
Root = ventral roots are motor(efferent) nerves
Dorsal roots are sensory(afferent) nerves
Ventral or dorsal relative to the spinal cord
Ventral toward body of vertebra
Dorsal toward spinous process of vertebra
Function of a nerve
To generate a nerve impulse
Flows through cell/ passed from cell to cell
Potential difference occurs across cell membranes
Ion channels in membrane maintain potential difference
Na+, K+ create the potential difference
***Neuron at rest – more Na+ outside than K+ inside
Cl- and negative proteins add to – inside
Polarized state = resting membrane potential (+out, - in)
ACTION POTENTIAL (nerve impulse)
SEQUENCE OF EVENTS [AT CHOLINERGIC
SYNAPSE(acetylcholine is neurotransmitter)]
1. Arriving AP depoliarizes the synaptic knob and the
presynaptic membrane
2. Ca+2 ions enter the cytoplasm of the synaptic knob –
membrane channels in synaptic vesicles – release Ach
3. Ach diffuses across synaptic cleft – bind to postsynaptic
membrane(muscle sarcolemma) – Na+ channels activated
– depolarized –
ADRENERGIC SYNAPSES
Same process as cholinergic
Release norepinephrine(NE) – in brain and in autonomic nervous
system
Ach and NE both excitatory – cause depolarization – AP
Activity of neuron depends on BALANCE between excitation and
inhibition – synapses at cell body and dendrite may involve TENS
of THOUSANDS of other neurons – if more excitatory
neurotransmitters than inhibitory – AP; if more inhibitory NT than
excitatory NT then no AP
Excitability , the ability to respond to stimulus
Stimulus alters the RMP
****Stimulus, Membrane increases permeability to Na+ into
cell
Cell membrane DEPOLARIZED
Quickly switches back to POLARIZED
POLARIZED/DEPOLARIZED/POLARIZED = Action
Potential aka Nerve impulse
Occurs all along neuron, a WAVE of ionic reversals
SALTATORY CONDUCTION – impulse jumps across
nodes of Ranvier
High speed of conduction, 130m/sec = 300mph
ALL OR NONE RESPONSE – just like muscle contraction
If not enough stimulus no action potential
THRESHOLD STIMULUS – minimum stimulus needed
SUBTHRESHOLD – not enough stimulus
SUMMATION – consecutive subthreshold will lead to AP
PRESYNAPTIC NEURON - SYNAPSE -POSTSYNAPTIC
NEURON
Presnyaptic neuron with synaptic end bulbs, synaptic vesicles hold
neurotransmitters
Acetylcholine, norepinephrine, - excitatory
dopamine, serotonin, glutamate, GABA – all inhibitory
Synaptic cleft in the postsynaptic membrane
Neurotransmitter contacts postsynaptic membrane
Response is either excitatory or inhibition
Excitatory – causes AP
Postsynaptic membrane – dendrites
May have thousands of presynaptic end bulbs
Combos of excitatory and inhibition neurotransmitters
Result is sum
If greater excitatory – AP
If greater inhibition – no AP