THE NERVE OF IT ALL
COMMUNICATION
CONTROL AND INTEGRATION
COURTESY OF BRAIN, SPINAL
CORD (CNS) AND NERVES
(PNS)
KINDS OF CELLS
• NEURONS
– Classified by direction of impulse
• Afferent (sensory) – transmit signal to spinal
cord
• Efferent (motor) – transmit signal away from
spinal cord or brain to glands or muscles
• Interneurons – lie within CNS; transmit from
afferent to efferent
CELLS, cont’d
– Can also be classified by # of processes
• Multipolar – (CNS and neurons that carry
AWAY from CNS) one axon, several
dendrites (brain and spinal cord)
• Bipolar – (receptors) one axon, one dendrite
(retina, inner ear, olfactory pathways)
• Unipolar – (to CNS) sensory neurons
CELLS, cont’d
• Large myelinated neurons (eg. Those
that innervate muscles) conduct AP
up to 100 m/sec
• Small unmyelinated neurons (visceral
organs) ~.5 to 1 m/sec
More on Neurons
• CELL BODIES (gray matter)
– In CNS = nuclei
– Outside CNS = ganglia
• PROCESSES (white matter)
– In CNS = tracts
– Outside CNS = nerves
Epineurium – tough and cordlike; perineurium –
around fascicles; endoneurium – delicate CT
CELLS, cont’d
• Neuroglia – CT cells
– Astrocytes – largest and most numerous; form
sheaths around blood capillaries of brain
– Microglia – small phagocytes
– Oligodendrocytes – lay down myelin sheath
around brain and spinal cord neurons (CNS)
– Schwann – form neurolemma and myelin sheath
only on neurons OUTSIDE of CNS = PNS
– Ependymal – line central canal and ventricles;
help produce and circulate CSF in choroid
plexus
Dendrite – receivers - take
impulse to soma
Cell body = soma
Axon – away from soma
Nissl bodies
Myelin sheath
Neurilemma – Sheath of
Schwann – only in PNS
Myelin sheath – segmented
wrapping around an axon –
concentric layers
Saltatory conduction
WHY?
NEURAL PATHWAYS
• Stimulus -> dendrites of sensory
neuron -> soma -> axon -> axon
terminus -> release neurotransmitters
-> synapse -> dendrites of
interneuron -> interneuron ->
interneuron releases
neurotransmitters across synapse to
motor neuron -> effect ( contraction
or secretion)
ACTION POTENTIAL
• An electrical gradient exists between inner
and outer membrane
• At rest, neuron is –70mV inside membrane;
courtesy of Na+/K+ pump (3Na+ out/2K+ in)
• Membrane is polarized
• Stimulus – Na+ rushes into cells –
membrane is depolarized – reverse
polarization
• Inside cell membrane is now +30mV
• This current flows down membrane to
axon terminals
• Lasts for a millisecond, then
repolarization
• Na+ gates close and membrane is
impermeable to Na+ again, but very
permeable to K+; K+ exits
• Absolute refractory period = neuron
resists stimulation
REFLEX ARC
• Route traveled by nervous impulses
• Afferent neuron -> interneuron in
CNS -> efferent neuron
• Vesicles in synaptic knob have
~10,000 molecules of neurotransmitters
NEUROTRANSMITTERS
1. Acetylcholine – autonomic NS and
brain - neuromuscular junctions –
excite muscles – inactivated by
cholinesterase
- flea collars, pest strips, nerve gas
contain cholinesterase inhibitors
2. Norepinephrine – sympathetic NS;
regulates activity of of visceral
organs and controls certain brain
functions
3. Dopamine – Brain - motor functions;
too little – muscle rigidity, tremors,
forward gait (Parkinson’s ); too much
– mental disorders (schizophrenia)
4. Serotonin – CNS - mental functions
– wakefulness and sleep and other
Circadian rhythms
5. GABA – inhibitory
6. Glycine – inhibitory in spinal cord
Net effect is what determines if a
neuron will fire
Neuropharmacological agents
• Exogenous substances that can affect
neuron excitability by either mimicking or
blocking action of neurotransmitter
• Curare = blocks Ach receptors
• Theobromine, caffeine increase
excitability by affecting membrane perm.
to Ca++
• Strychnine and tetanus toxin inhibit
inhibitory neurotransmitters and cause
convulsions
• Ether = inc K+ perm causing hyperpol.
Making membrane less responsive to
stimulation
• Valium enhances inhibitory effects of
GABA
• LSD mimics serotonin and binds to its
receptors – effects are quite
different