Motor Systems I:
Cortico-spinal, cortico-bulbar
Competencies:
- Diagram the path of the corticospinal tract.
- Diagram a cross section of spinal cord and the location of alpha and gamma motor neurons.
- Illustrate the motor homunculus (somatic mapping in the human motor cortex).
- Appraise the locations and function of the supplementary motor cortex in the cerebral cortex.
To master the material presented in this
lecture:
Read ...
Purves text, Chapters 16 and 17
Haines pp 206 and 250.
Listen to the lecture and focus on the
following points ...
- Major components of the motor systems:
- spinal cord.
- brainstem.
- motor cortex (Brodman's area 4).
- Other important components for motor function:
- cerebellum.
- basal ganglia.
- brainstem-derived upper motor neuron pathways.
Lower Motor Neurons and Muscles.
- Two types of lower motor neurons:
- Alpha motor neurons are large motor
neurons in the anterior horn of the spina cord. These
neurons innervate extrafusal muscle fibers (striated muscle).
An alpha motor neuron and muscle fibers innervated by a motor
neuron constitute a motor unit.
- Gamma motor neurons are smaller motor
neurons scattered amon the alpha motor neurons in the anterior
horn of the spinal cord. These neurons innervate the
intrafusal muscle fibers of the muscle spindle.
- Motor neurons contain acetylcholine as their
neurotransmitter.
- Myasthenia gravis is a disease characterized
by the weakness of skeletal muscles. This is an autoimmune
disease caused by circulating antibodies that react with nicotinic
acetylcholine receptors present in the postsynaptic membrane at
the neuromuscular junction.
Clinical Classification of Lesions.
- Signs of lower motor neuron lesions:
- diminished or absent tendon-jerk reflexes.
- flaccid paralysis or paresis.
- decreased resistance to passive movement.
- progressive atrophy of the affected muscles.
- fibrillation potentials, caused by random
contraction of isolated denervated muscle fibers.
- sprouting neuronal branches at nodes of
Ranvier and at motor end plates.
- Signs of upper motor lesions:
- voluntary movements of the affected muscles
are absent or weak - only the lower half of the face is involved
while muscle action involved with emotional changes is unaffected.
- absence of profound atrophy of affected
muscles.
- spastic paralysis.
- exaggerated tendon reflexes.
- abnormal plantar reflex - a sign of Babinski.
- superficial reflexes (e.g. abdominal reflex,
cremasteric reflex) are suppressed or absent.
- Determinants of activity in motor neurons
may be broadly divided into three overlapping classes:
- preprogrammed patterns of neural connections
- e.g. monsynaptic reflexes.
- descending pathways that modulate the activity
of motor neurons.
- higher centers that influence the activity
of descending pathways.
Motor Cortex and Descending Motor Pathways.
- Primary motor cortex = precentral gyrus
= Brodman area 4.
- Efferents from the motor cortical
areas:
- corticospinal tract.
- corticostriatal efferents.
- corticorubral efferents.
- corticoreticular (medullary and pontine)
projections.
- corticocortical associational projections.
- Secondary motor areas:
- supplementary motor area - medial and
superolateral portion of Brodman's area 6 (initiation, i.e. "thinking" stage of the performance of a sequence
of motor tasks).
- premotor cortex - Brodman's area 6 (bilateral
coordination).
- posterior parietal motor area - Brodman's
area 5 and 7 (coordination of tactile and visual aspects of
reaching and guiding movements).
- frontal eye fields - Brodman's area
8 (coordination of eye movements).
- Corticospinal pathways:
- Corticospinal fibers arise from axons of pyramidal
cells in lamina V (two/thirds arise from Brodman areas 4 and 6,
one/third from parietal motor area and somatosensory areas).
- The tract courses through the posterior limb
of the internal capsule, the middle third of the cerebral peduncle
and through the basal pons to form the medullary pyramids.
- About 85% of the fibers cross in the decussation
of the pyramids to form the lateral corticospinal tract in the
lateral funiculus of the spinal cord; uncrossed fibers descend
in the ventral funiculus.
- Corticospinal fibers from the frontal cortex
terminate in laminae VII, VIII and IX of the spinal gray; fibers
from the parietal cortex terminate in more dorsal aspects of the
cord; few terminate directly on the primary motor neurons per
se but on interneurons in the ventral gray.
Corticobulbar pathways:
- The oculomotor, trochlear, trigeminal, abducens
and hypoglossal motor nuclei as well as the nucleus ambigus
receive bilateral innervation; the spinal accessory receives
ipsilateral innervation.
- Corticobulbar fibers to the facial nucleus
innervate three groups of neurons: neurons for ipsilateral upper
face, neurons for contralateral upper face (thus, corticobulbar
control of the upper face is bilateral), neurons for contralateral
lower face.
Consider the Following Questions ..
- Distinguish the clinical signs of upper motor neuron lesions from those of lower motor neuron lesions, and discuss diseases and traumas which cause upper and lower motor neuron lesions.
- Describe the orientation of the motor homunculus, and the sources of blood supply. What cortical areas contribute to the corticospinal tract?
- Which cranial nerve motor nuclei receive bilateral or only contralateral corticobulbar afferents?
- If the somato-motor component of the facial nerve is interrupted, the clinical signs will differ from interruption of the corticobulbar tract in what way?
