Motor Systems

General Features: Somatotopic organization, "sensory-motor" integration, convergence

Organization – "series" and "parallel"

Series – spinal cord, brainstem, motor cortex, basal ganglia, cerebellum

Parallel – a -motorneurons receive and integrate input from multiple sources (peripheral afferent projections, spinal cord interneurons, brainstem, motor cortex); basal ganglia and cerebellum influence indirectly through these systems which have direct input to motoneurons

Spinal Cord

Segmented structure: posterior (dorsal) Þ sensory; anterior (ventral roots) Þ motor (a and g neurons)

Transverse cross-section: "butterfly-shaped" core (gray matter) surrounded by white matter

Gray matter – neuronal cell bodies and dendrites, neuronal substrate for reflexive and ‘automatic’ movements

White matter – ascending axonal tractsÞ sensory info; descending tractsÞ motor command for skeletal and smooth musc.

both have somatotopic organization

Direct Inputs to a-motor neurons

Motorneurons – "final common pathway" because they integrate input from multiple sources: (1) Group 1A sensory fibers form muscle spindle (stretch receptor), (2) Spinal interneurons, (3) Brainstem, (4) Motor cortex

located in the anterior horn of spinal cord and organized in a somatotopic fashion

Lateral motorneurons – innervate distal musculature (flexors located more dorsal and extensors more ventral)

Medial motorneurons – innervate axial musculature
cervical lumbosacral enlargement – can locate location of transverse section by size of ventral horn
ventral horn smallest at thoracic segments due to lack of distal muscle innervation

Sensory Fibers – reflex arc (group 1A)

Interneurons – many types, coordinates movements over multiple segments of the spinal cord

segmental (located in gray matter), involved in inhibition (inhibition of extensor muscle in flexion); propriospinal interneurons (leaves gray, enters white matter and synapses different segments of spinal cord)

Brainstem – medulla, pons, midbrain

central sulcus: anterior has primary motor cortex, posterio-lateral has sensory functions

contains a -motorneurons that control muscles of head (receive input form motor cortex)

motor functions: directly influences a -motorneurons from 4 nuclei:

(1) Vestibular nuclei – 4 of them located in medulla, receive input from vestibular system; lateral vestibular nuclei project to EXTENSOR motorneurons (falling and extension of arm to break fall)

(2) Reticular system (Pons) – network of neurons located in central core of brainstem; receives ascending afferent input from spinal cord; involved in wide range of motor functions (controls breathing, posture, etc)

(3) Red Nucleus – located in midbrain; projects to spinal cord and activates a -motorneurons that innervate FLEXOR motorneurons; not involved in specific function

(4) Tectum – "roof" of the midbrain, rostral aspect of brainstem (colliculi means hill); motor function via locus coeruleus; formed by 2 structures: inferior colliculi – receives auditory input and superior colliculi

controls eye movement – together orients head to auditory and visual input via tectal spinal tract

Cerebral Cortex – motor and pre-motor cortex

Motor Cortex: located in the pre-central gyrus (cortical tissue just rostral to central sulcus)

organization: somatotopic fashion – medial Þ legs, lateral Þ trunk arms and face

pyramidal system – axons arise in cortex and project to spinal tracts that are contained in "pyramids" visible on the anterior medullary surface

Pre-motor cortex: highest level in hierarchical system

Indirect influences to a -motor neurons

Basal Ganglia – large groups of neurons deep within cerebral hemispheres; surround diencephalon rostrally and laterally

Function: receives input from all cortical areas and project to premotor cortex to initiate and maintain movement

5 major components:

(1) caudate

(2) putamen

(3) globus pallidus

(4) subthalamus

(5) substantia nigra (midbrain below red nucleus – closely related to basal ganglia, involved in Parkinson’s)

striatum = caudate + putamen; corpus striatum = striatum + globus pallidus

Cerebellum – located in posterior cranial fossa posterior to the brainstem

3 functional subdivisions: (all three control balance, posture, and coordination of eye movement with head movement)

(1) Cerebrocerebellum – receives input from widespread areas of the cortex and projects its output to premotor and motor cortex; complements basal ganglia coordinating complex movements involving multiple joints

(2) Spinocerebellum – receives sensory input from the brainstem, spinal cord, vestibular, and visual systems; output is via brainstem primary control nuclei

(3) Vestibulocerebellum – same as above