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Chapter 13 – Somatic Integration
Proprioceptive Sensations
 Awareness of body position and movements of parts of the body is provided by the proprioceptive
(one’s own), or kinesthetic (motion) sense.
 It informs us of:
o the degree to which muscles are contracted
o the amount of tension created in tendons
o the change of position of a joint
o the orientation of the head relative to the ground and in response to movements
o the location and rate of movement of one body part in relation to others
 So we can walk, type, or dress without using our eyes
 It allows us to estimate the weight of objects and determine the muscular effect necessary to perform a
task.
o E.g. as you pick up a bag, you quickly realize whether it contains feathers or books, and you
exert the correct amount of effort to lift it (Remember motor units and recruitment?).
 Proprioception does not adapt, thus allowing the brain to be informed continually of the status of
different parts of the body so that adjustments can be made to ensure coordination.
 Receptors for proprioception include:
o Muscle spindle fibers (located within muscles)
o Golgi tendon organs (located in tendon)
o Joint kinesthetic receptors (located in joints)
o Hair cells located within the vestibular apparatus in the inner ear coordinate with somatic
integration to provide information for maintaining balance. (Remember the labyrinthine reflex?)
 Sensory systems provide the input that keeps the CNS informed of changes in the external & internal
environment.
o Impulses for conscious proprioception pass to the spinal cord (intergrating center) via
sensory/afferent neurons
o Then along ascending tracts (sensory tracts) in the spinal cord to the thalamus (integrating
center)
o Then on to the somatosensory sensory and primary motor cortexes in the cerebrum
(integrating centers)
o At the same time, impulses from proprioceptors also pass the cerebellum (integrating center),
along the spinocerebellar tracts (ascending/sensory tracts).
 Output from the CNS is then conveyed to motor systems, which enable us to move about and change
our relationship to the world around us.
o The most direct motor pathways extend from the cerebral cortex and basal nuclei (integrating
center) into the spinal cord (integrating center) via descending tracts (efferent/motor tracts)
then out to skeletal muscles (effectors) via nerves (motor neurons) controlling voluntary
movement
Somatic Sensory Pathways
 Somatic sensory pathways from receptors to the cerebral cortex involve 3-neuron sets simultaneously
sending signals to the cerebellum and the reticular formation of the brain stem.
o First-order neurons (primary)
 Carry info. from the somatic receptors into either the brain stem or spinal cord
o Second-order neurons (secondary)
 Carry info. from the spinal cord & brain stem to the thalamus
 Secondary axons cross over (decussate) to the opposite side in the spinal cord or brain
stem before ascending to the thalamus
o Third-order neurons
 Project from the thalamus to the primary somatosensory area of the cortex
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 Somatosensory cortex:
o Areas of the somatosensory cortex receive sensory information from different parts of the body
and have been mapped out. See Fig. 10-10
 E.g. touch finger tip of left hand, signal sent & interpretation in right cerebral cortex
o Note that the larger body region, the more sensitive that body region is.
 Somatic Sensory Pathways to the Cerebellum:
o Two tracts in the spinal cord are major routes for subconscious proprioceptive input to reach the
cerebellum:
 Posterior spinocerebellar tract
 Anterior spinocerebellar tract
o Sensory input conveyed to the cerebellum along these two pathways is critical for:
 Posture
 Balance
 Coordination of skilled movements
Somatic Motor Pathways
 The primary motor area of the cerebral cortex is the major control region for initiation of voluntary
movements
 The adjacent premotor area and somatosensory area, also contribute fibers to the descending motor
pathways
 Like the somatosensory area, different muscles are represented unequally in the primary motor areas
o See Fig. 13-12
o The degree of representation is proportional to the number of motor units in a particular
muscle of the body.
 E.g. muscles in thumb, fingers, lips, tongue, and vocal cords have large representations,
while the trunk has a much smaller representation.
o By comparing the somatosensory cortex (Fig. 10-10) and the primary motor cortex
(Fig. 13-12 ), you can see that somatosensory and motor representations are similar, but not
identical for the same part of the body.
 Nerve impulses for voluntary movements propagate from the motor cortex to somatic motor neurons
that innervate skeletal muscles
 Descending tract motor neurons cross over to the contralateral side and innervate skeletal muscles on
the opposite side of the body
o Thus the motor cortex of the right side of the brain controls the muscles on the left side of the
body, and vice versa
Neural control of Movement
*LOCATION
ROLE
Spinal cord
Spinal reflexes
Brain stem
Motor areas of
cerebral cortex
Cerebellum
Posture; hand and eye
RECEIVES INPUT FROM:
Sensory receptors
Cerebellum, visual and
vestibular sensory receptors
Thalamus
SENDS INTEGRATIVE OUTPUT TO:
Brain stem, cerebellum, thalamus/cerebral
cortex
Spinal cord
Planning & coordinating
Brain stem; spinal cord (corticospinal
complex movements
tract); cerebellum; basal ganglia
Monitors output signals from
Spinal cord (sensory);
Brain stem, cerebral cortex
motor areas and adjusts
cerebral cortex (commands)
(Note: All output is inhibitory)
movements
Thalamus
Contains relay nuclei that
Basal ganglia; cerebellum;
Cerebral cortex
modulate and pass messages to spinal cord
cerebral cortex
Basal nuclei
Motor planning
Cerebral cortex
Cerebral cortex, brain stem
*If any of these areas are damaged, what you be the result (i.e. what “ROLE” would you lose control of)?
What if there was damage to the areas in which “All output is inhibitory?)