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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:

  • the degree to which muscles are contracted
  • the amount of tension created in tendons
  • the change of position of a joint
  • the orientation of the head relative to the ground and in response to movements
  • 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.

  • 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:

  • Muscle spindle fibers (located within muscles)
  • Golgi tendon organs (located in tendon)
  • Joint kinesthetic receptors (located in joints)
  • Hair cells located within the vestibular apparatus in the inner ear coordinate with somatic integration to provide information for maintaining balance. (Remember the labyrinthinereflex?)

Sensory systems provide the input that keeps the CNS informed of changes in the external & internal environment.

  • Impulses for conscious proprioception pass to the spinal cord (intergrating center) via sensory/afferent neurons
  • Then along ascending tracts(sensory tracts)in the spinal cord to the thalamus(integrating center)
  • Then on to the somatosensory sensory and primary motor cortexes in the cerebrum (integrating centers)
  • At the same time, impulses from proprioceptors also pass the cerebellum (integrating center), along the spinocerebellartracts (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.

  • The most direct motor pathways extend from the cerebral cortexand 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.

  • First-order neurons (primary)
  • Carry info. from the somatic receptors into either the brain stem or spinal cord
  • 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
  • Third-order neurons
  • Project from the thalamus to the primary somatosensory area of the cortex

Somatosensory cortex:

  • 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
  • Note that the larger body region, the more sensitive that body region is.

Somatic Sensory Pathways to the Cerebellum:

  • Two tracts in the spinal cord are major routes for subconscious proprioceptive input to reach the cerebellum:
  • Posterior spinocerebellar tract
  • Anterior spinocerebellar tract
  • 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

  • See Fig. 13-12
  • 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.
  • 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 tractmotor neurons cross over to the contralateral side and innervate skeletal muscles on the opposite side of the body

  • 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 / RECEIVES INPUT FROM: / SENDS INTEGRATIVE OUTPUT TO:
Spinal cord / Spinal reflexes / Sensory receptors / Brain stem, cerebellum, thalamus/cerebral cortex
Brain stem / Posture; hand and eye / Cerebellum, visual and vestibular sensory receptors / Spinal cord
Motor areas of cerebral cortex / Planning & coordinating complex movements / Thalamus / Brain stem; spinal cord (corticospinal tract); cerebellum; basal ganglia
Cerebellum / Monitors output signals from motor areas and adjusts movements / Spinal cord (sensory); cerebral cortex (commands) / Brain stem, cerebral cortex
(Note: All output is inhibitory)
Thalamus / Contains relay nuclei that modulate and pass messages to cerebral cortex / Basal ganglia; cerebellum; 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?)