Sensory and Motor Pathways
Somatic Sensory Pathways
• The pathways consist of first-order,
second-order, and third-order neurons
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
Somatic Sensory Pathways
Two Pathways;
1. Posterior Column Pathway to the Cortex
2. Spinothalamic Pathways to the Cortex
Posterior Column Pathway to the
Cortex
• The nerve impulses for conscious
proprioception and most tactile sensations
ascend to the cortex.
Posterior Column Pathway to the
Cortex
• The first neuron goes from the receptor to
the cord and ascends to the medulla
Posterior Column Pathway to the
Cortex
• The second order neuron crosses in the
medulla and ascends to the thalamus
Posterior Column Pathway to the
Cortex
• The third order neuron goes to the
parietal lobe
Posterior Column Pathway to the
Cortex
• The posterior columns are damaged by B12 deficiency
Anterolateral Pathways to the
Cortex
• Carry mainly pain and temperature
impulses
Anterolateral Pathways to the
Cortex
• The first order neuron goes from the
receptor to the cord and releases
substance P
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)
Anterolateral Pathways to the
Cortex
• The second order neuron ascends either
via the anterior or lateral spinothalamic
tracts
Anterolateral Pathways to the
Cortex
• The third order neuron goes to the
parietal lobe where we interpret the pain
Anterolateral Pathways to the
Cortex
• Modulation of pain by the brain: release
of substance P is inhibited by descending
pathways that release enkephalins
Somatic Motor Pathways
• The primary motor area is the major
control region for initiation of voluntary
movements
Somatic Motor Pathways
• Different muscles are not represented
equally in the motor cortex
Somatic Motor Pathways
• The degree of representation is
proportional to the number of motor units
in a particular muscle of the body
Somatic Motor Pathways
• Voluntary motor impulses are propagated
from the motor cortex to somatic efferent
neurons that innervate skeletal muscles
via the direct pathways
Somatic Motor Pathways
Direct pathways;
1. Corticospinal Tracts
2. Corticobulbar Tracts
Corticospinal Tracts
• They go from the primary motor cortex
down through the cerebral peduncles of
the midbrain
Corticospinal Tracts
• They cross in the medulla (if lateral) or in
the spinal cord (if anterior)
Corticospinal Tracts
• Then they run down the cord to the
anterior grey horn cells where the cell
bodies of the lower motor neuron lie
Corticospinal Tracts
• Lower motor neurons = somatic efferent
neurons
Corticospinal Tracts
• Efferent = Motor
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
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)