Sensory and Motor Pathways

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Sensory and Motor Pathways

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Somatic Sensory Pathways

• The pathways consist of first-order,
  second-order, and third-order neurons

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Somatic Sensory Pathways

• The relative size of the area that the
  somatosensory cortex represents a body part are
  proportional to the number of specialized sensory
  receptors

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Somatic Sensory Pathways

• Two Pathways
• Posterior Column Pathway to the Cortex
• Spinothalamic Pathways to the Cortex

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Posterior Column Pathway to the Cortex

• The nerve impulses for conscious proprioception
  and most tactile sensations ascend to the cortex.

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Posterior Column Pathway to the Cortex

• The first neuron goes from the receptor to the
  cord and ascends to the medulla

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Posterior Column Pathway to the Cortex

• The second order neuron crosses in the medulla
  and ascends to the thalamus

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Posterior Column Pathway to the Cortex

• The third order neuron goes to the parietal lobe

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Posterior Column Pathway to the Cortex

• The posterior columns are damaged by B-12
  deficiency

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Anterolateral Pathways to the Cortex

• Carry mainly pain and temperature impulses

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Anterolateral Pathways to the Cortex

• The first order neuron goes from the receptor to
  the cord and releases substance P

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Anterolateral Pathways to the Cortex

• The second order neuron crosses in the cord and
  ascends to the thalamus (where we become
  conscious of the pain)

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Anterolateral Pathways to the Cortex

• The second order neuron ascends either via the
  anterior or lateral spinothalamic tracts

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Anterolateral Pathways to the Cortex

• The third order neuron goes to the parietal lobe
  where we interpret the pain

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Anterolateral Pathways to the Cortex

• Modulation of pain by the brain release of
  substance P is inhibited by descending pathways
  that release enkephalins

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Somatic Motor Pathways

• The primary motor area is the major control
  region for initiation of voluntary movements

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Somatic Motor Pathways

• Different muscles are not represented equally in
  the motor cortex

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Somatic Motor Pathways

• The degree of representation is proportional to
  the number of motor units in a particular muscle
  of the body

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Somatic Motor Pathways

• Voluntary motor impulses are propagated from the
  motor cortex to somatic efferent neurons that
  innervate skeletal muscles via the direct pathways

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Somatic Motor Pathways

• Direct pathways
• Corticospinal Tracts
• Corticobulbar Tracts

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Corticospinal Tracts

• They go from the primary motor cortex down
  through the cerebral peduncles of the midbrain

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Corticospinal Tracts

• They cross in the medulla (if lateral) or in the
  spinal cord (if anterior)

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Corticospinal Tracts

• Then they run down the cord to the anterior grey
  horn cells where the cell bodies of the lower
  motor neuron lie

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Corticospinal Tracts

• Lower motor neurons somatic efferent neurons

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Corticospinal Tracts

• Efferent Motor

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Corticobulbar Tracts

• Axons of upper neurons run from the primary motor
  cortex to the corticobulbar tracts in the R. and
  L. cerebral peduncles of the midbrain

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Corticobulbar Tracts

• The tracts cross and the axons end in the motor
  nuclei of nine cranial nerves (III, IV,V, VI,
  VII, IX, X, XI, XII)