NERVOUS SYSTEM/SENSES: response to stimuli -irritability; (also via
neurons (fig 34.6): receive & transmit
messages across synapses
- dendrite: short, highly branched; receives neural impusle,
transmits to cell body.
- nerve [or soma or cell] body: bulk of cytoplasm, nucleus,
- axon: long (can be > 1m) transmits message to other neurons,
muscles, or glands.
- sensory (afferent) neuron: receive; sense organs to cns;
long dendrite, short axon
- motor (efferent) neuron: transmit; cns to muscle fibers or
glands; long axon, short dendrite
- interneuron: relay; central nervous system only
- nerve: nerve fibers surrounded by connective tissue
- ganglia: bundles of nerve cells outside the CNS
neuroglial [glial] cells:
- 10 X as many as neurons; some envelop neurons and form insulating sheaths;
others phagocytotic; others line cavities of brain & spinal cord.
- Schwann cells: line neurons outside the CNS; form myelin sheath
[fig. 10_3]: white, lipid rich Schawann membrane on axons > 2 microns;
- nodes of Ranvier: gaps between the myelin sheath
- multiple sclerosis, Parkinsonism: deterioration of the myelin
- dendrites & cell body add up incoming information
- impulse fired
- chemicals released
giant axon of the squid (fig 34.9)
Transmission along a Neuron [fig.
potential: -70 mV (- inside, + outside); Na pumped out,
only a small amount diffuses in; K pumped in, leaks out on a diffusion
local potential: small change [depolarize = excitory] or [hyperpolarize =
inhibitory]; contributes to firing or non_firing
action potential : neural impulse generated by
depolarization; membrane becomes permeable, Na gates open; change from -65
mV to +40 mV; self_propagating wave of depolarization down axon. large
diameter or myelin sheath increases velocity.
saltatory conduction (fig ): between nodes of ranvier
threshold level: depolarization to -40 mV required for firing of action
potential; transmisions of hundreds of impulses/second
all or none law: intensity depends on number of neurons stimulated and
frequency of response
refractory period : until membrane is repolarized, a few milliseconds;
absolute r.p.: no signal can be transmitted; relative r.p. requires greater
than threshold to stimulate; membrane briefly hyperpolarized.
Transmission Across a Synapse (fig 34.10)