Patterns of Connections Made by Local Circuit Neurons in the Intermediate Zone of the Spinal Cord Gray Matter
Long distance interneurons project bilaterally, innervate multiple segments.
Used for posture control.
Short distance interneurons project over a few segments, remain ipsilateral.
Used for distal muscle groups.

Descending Projections from the Brainstem to the Spinal Cord

• Able to predict changes in posture, and generate an appropriate stabilizing response.
• Some muscles fire in anticipation of a need for postural adjustment.
– For example, the gastrocnemius muscle needs to adjust for the anticipation of contracting the biceps that would naturally pull the body forward – stabilizing response
• Reticular formation and Reticulospinal tract important for this.
– Lesion or pharmacologically block it in a cat and compensatory muscle changes do not occur.
• Stimulate motor cortex in the right place can induce paw lifting, also induces other limbs’ muscles to fire. Inhibit reticulospinal tract, paw still moves but other legs do not.

Anticipatory Maintenance of Body Posture
EMG= elecromyography. Measure muscle APs
Gastrocnemius stimulation happens before biceps cut reticulospinal tract Biceps will fire but not Gastrocnemius

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Motor cortex makes two types of projections.
• A direct pathway to the ventral lateral spinal cord
• An indirect pathway to the reticular formation (which subsequently goes to medial spinal cord).
• For example a direct pathway will move the hand and the indirect pathway will posture the body.
• Cutting the direct pathway, leaving the indirect pathways intact will not affect ability to walk, run, etc, but will prevent the distal parts of limbs from being used.

Pathways from the Motor Cortex to the Spinal Cord
Direct pathway: lateral corticospinal and rubrospinal tracts, distal fine motor movements.
“Pyramidal System”

Pathways from the Motor Cortex to the Spinal Cord
Indirect pathway: postural adjustments.
Ventral corticospinal tract, cortico-reticulospinal tract.
“Extrapyramidal System”

• in the frontal lobe
• several interconnected areas
• Primary motor cortex in the precentral gyrus.
• Gets input from basal ganglia, cerebellum and other cortical areas.
• Has 6 layers, layer V is the output layer (pyramidal cells or Betz cells).
• Primary pathway- the pyramidal system.

Architecture of motor cortex is different than the sensory cortex.
Betz cells

The corticospinal tract.

• Stimulation of a neuron in primary motor cortex activates several muscles, and inhibits other muscles.
• Multiple neurons can activate the same muscle.
• The “motor field” of a cortical motor neuron has to do with organized movements rather than specific muscle groups.

Monkey trained to move joystick in response to light

movement starts at 0.
Yellow – activity of neuron is increased.
Purple – activity is decreased.
Neurons have
“preferred direction”
Activity of a single neuron in motor cortex anticipates future movement.

Directional Tuning of an Upper Motor Neuron in the Primary Motor Cortex
Individual neurons tuned too broadly to accurately predict movement direction.
P opulations of neurons can calculate a direction. Summed population responses give the actual direction of movement.

• Rostral to primary motor cortex
• Extensive reciprocal connections with primary motor cortex
• Also projects directly to spinal cord (30% of axons in the pyramidal tract).

• mediates selection of movements
• specified by internal rather than external cues.
• Cells fire when just thinking about an event.

Lateral Premotor Cortex-
• Neurons fire earlier than primary motor cortex.
Important in conditional motor tasks that pair a movement with a visual cue.
• Neurons fire before initiation of the task. Used for intentions.
• Lesions in monkey prevent vision-conditioned tasks, although vision is ok and the task could be performed in other ways.

Motor cortex areas

The Primary Motor Cortex and Premotor Area in the
Human Cerebral Cortex

Photograph of sagittal section through brain stem and cerebellum

Cerebellar cortex

Cerebellar circuits