D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
INTRODUCTION TO NERVOUS SYSTEM
Lecture 9: Anatomical & Functional Organization
1.
Nervous System: (ch. 45)
• general organization (ppt. 1): central nervous system – CNS (brain & spinal
cord) contains billions of neurons (ppts. 2 & 3); peripheral nervous system – PNS
(spinal & cranial nerves & ganglia (that include somas) of sensory or afferent fibers &
motor or efferent fibers) (ppts. 4 to 6)
• neurons: (fig. 45 – 1 & ppts. 7 to 10) cell body (soma) contains nucleus and is
metabolic center; many fibrous processes attach to soma, including numerous
dendrites (receptors of synaptic connections – from hundreds to hundreds of
thousands of other neurons) and a single axon (transmitting process) that may have
few to many branches, leading to separate destinations; because of the property of
synaptic connections, transmission is normally one-way, from axon of input cell to
dendrites (& soma) of output cell
• functional organization: reflex arc (may contain as few as two to perhaps
1000s of neurons (ppt. 11)
- sensory system (fig. 45 – 2 & ppt. 12) involves peripheral receptors &
sensory or afferent fibers; sensory signals pass to multiple destinations in CNS,
including spinal cord, reticular substance of brainstem, cerebellum, thalamus &
cerebral cortex; nerve tracts in cord are ascending
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- p. 2 -
- motor system (fig. 45 – 3 & ppt. 12) involves effectors (muscles or glands)
that respond to signals delivered by motor or efferent fibers; motor signals originate
from many areas including spinal cord, reticular substance of brainstem, cerebellum,
basal ganglia & cortex of cerebrum; nerve tracts in cord are descending
- integrative component involves screening out of majority of sensory
signals, permitting only important sensory information to be distributed to appropriate
destinations that develop desired response
- synapses play major role in controlling input signals – screening out
many & amplifying others through mechanisms of facilitation & inhibition
memory is stored sensory inputs that reside mainly in the cerebral cortex & are
also a function of synapses; repeated sensory inputs may facilitate a certain sequence of
synapses to such a level of sensitivity that other signals in the brain may excite the
same sequence of synapses, independent of any sensory input
• levels of function in CNS:
- spinal level – many neuron circuits that control actions such as walking,
withdrawal movements, postural support & controls over smooth muscle of blood vessels,
urinary & gastrointestinal tracts (ppt. 13)
- higher brain regions issue ‘commands’ that activate programs of output that
reside in circuitry of cord
- brainstem and subcortical level – many functions including control of
respiration, cardiovascular regulation, maintenance of posture & equilibrium and patterns of
emotional and sexual responses may be performed, without the intervention of the cerebral
cortex, by such regions as medulla, pons, midbrain, cerebellum, thalamus, basal ganglia &
hypothalamus
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- p. 3 -
- cerebral cortex – repository of vast amount of stored information that
exerts more precise control of responses delivered by lower levels & by cord; never operates
alone, but in coordination with other levels, choosing to activate or suppress circuitry
patterns present in the lower levels
- brain as a computer: input data (sensory information) is processed
(ignored, stored &/or used to generate an output (motor response) that governs
bodily activity (fig. 45 - 4 & ppts. 14 - 16)