34  NEURAL CONTROL

 

NERVOUS SYSTEM/SENSES: response to stimuli -irritability; (also via hormones)

neurons (fig 34.6): receive & transmit messages across synapses

  1. dendrite: short, highly branched; receives neural impusle, transmits to cell body.
  2. nerve [or soma or cell] body: bulk of cytoplasm, nucleus, most organelles.
  3. axon: long (can be > 1m) transmits message to other neurons, muscles, or glands.
  4. sensory (afferent) neuron: receive; sense organs to cns; long dendrite, short axon
  5. motor (efferent) neuron: transmit; cns to muscle fibers or glands; long axon, short dendrite
  6. interneuron: relay; central nervous system only
  7. nerve: nerve fibers surrounded by connective tissue
  8. ganglia: bundles of nerve cells outside the CNS

neuroglial [glial] cells:

  1. 10 X as many as neurons; some envelop neurons and form insulating sheaths; others phagocytotic; others line cavities of brain & spinal cord.
  2. Schwann cells: line neurons outside the CNS; form myelin sheath [fig. 10_3]: white, lipid rich Schawann membrane on axons > 2 microns;
  3. nodes of Ranvier: gaps between the myelin sheath
  4. multiple sclerosis, Parkinsonism: deterioration of the myelin

Nerve Impulse

  1. dendrites & cell body add up incoming information
  2. impulse fired
  3. chemicals released

giant axon of the squid (fig 34.9)

Transmission along a Neuron  [fig. 34.9]

  1. resting potential: -70 mV (- inside, + outside); Na pumped out, only a small amount diffuses in; K pumped in, leaks out on a diffusion gradient
  2. local potential: small change [depolarize = excitory] or [hyperpolarize = inhibitory]; contributes to firing or non_firing
  3. 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.
  4. saltatory conduction (fig ): between nodes of ranvier
  5. repolarization
  6. threshold level: depolarization to -40 mV required for firing of action potential; transmisions of hundreds of impulses/second
  7. all or none law: intensity depends on number of neurons stimulated and frequency of response
  8. 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)