D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MOTOR SYSTEMS II
Lecture 18: Motor Cortex, Brainstem
1.
Motor Cortex: (chapter 55)
• organization (figs 55 – 1, 55 – 2 & ppts 1 - 3)
- precentral gyrus located in posterior frontal lobe immediately in front of
central sulcus – site of primary motor cortex
- premotor, supplementary motor areas reside in frontal gyri immediately
anterior to primary motor area; not as discretely organized as primary area
- homunculus map – areas of motor control laid out like somatosensory
cortex (face, oral cavity, etc. – lateral, extending into lateral sulcus or Sylvian fissure;
trunk lower limb – medial, extending into median longitudinal fissure; more than
half occupied with control of muscles of hand and muscles of speech
- primary motor cortex is responsible for sending commands to groups of muscles
for execution of complex maneuvers; signals from primary cortex govern more discrete
movements; signals from premotor areas assist in positioning body parts for fine motor
activity; premotor may connect directly or via basal ganglia & thalamus to primary
motor cortex; signals from supplementary motor areas appear to control bilateral
maneuvers as well as positioning of body in support of fine motor function (ppts. 4 & 5)
• regions of specialization for specific muscular activity: (fig. 55 – 3 & ppt. 6)
- Broca’s area controls word formation, assisted by closely associated
cortex controlling muscles of larynx, tongue & oral cavity to coordinate respiratory
activity, phonation & articulation of words
- eye movement field controls voluntary shifting of gaze to different objects
in visual field as opposed to fixating on objects, which occurs when this area is
damaged
- head rotation field coordinates with eye movement field to control rotation
of head to view different objects in visual field
- hand skills field exercises control over hand activities
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- p. 2 -
• inputs: sensory cortex areas (somatosensory, visual, auditory), thalamus
(including signals relayed from cerebellum & basal ganglia) send signals to modulate
motor cortex output; received in layers 2 – 4 of motor cortex; once sensory input is
received, cerebellum & basal ganglia collaborate with cortex to develop motor
response
2.
Brainstem:
• midbrain: red nucleus located in midbrain; organized similarly to, but less
precisely than motor cortex
- red nucleus receives fibers directly from cortex (corticorubral tract) as well as
collaterals from corticospinal tract; appears capable of similar but cruder control of motor
activity compared to cortex
• pons & medulla (fig. 55 – 7 & ppt. 7): nuclei for respiratory control,
cardiovascular control & some GI control
- reticular nuclei control postural muscles (modulated by other brain regions)
- reticular nuclei of pons send excitatory signals to postural muscles
- reticular nuclei of medulla send inhibitory signals to postural muscles (fig.
55 – 8 & ppt. 8)
- vestibular nuclei facilitate pontine reticular system in excitation of postural
muscles; activity of these nuclei is determined by inputs from vestibular (equilibrium)
apparatus (see below)
3.
Output Tracts:
• corticospinal tracts (fig. 55 – 4 & ppt. 9): descend from layer 5 of cortex (30%
primary motor cortex, 30% premotor & supplementary motor & 40% somatosensory fibers)
to pyramids of medulla (pyramidal system)
- majority of fibers cross to opposite side in pyramids (contralateral fibers)
and continue down lateral corticospinal tract
- smaller number of ipsilateral fibers descends in ventral corticospinal tract
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- p. 3 -
- collaterals communicate with basal ganglia, brainstem (red nucleus) &
cerebellum
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- p. 4 -
• rubrospinal tract (fig. 55 – 5 & ppts. 10 & 11) closely parallels corticospinal
fibers of lateral column of cord; like corticospinal tract, maintains prominent
communication with cerebellum; together with corticospinal tract, constitutes lateral
motor system of the cord
• reticulospinal & vestibulospinal tracts: from reticular nuclei & vestibular nuclei
of pons & medulla; descend in medial columns of cord (medial motor system) (fig. 55
– 6 & ppt. 12)
- reticulospinal may be responsible for coordination of movements (requiring less
dexterity & less balance) involving many segments of the cord (upper & lower limb
movements for walking, swimming, etc.)
- vestibulospinal is responsible for maintenance of balance (ppt. 13)
4.
Equilibrium Apparatus (fig. 55 – 9 ppt. 14): bony & membranous
labyrinths of internal ear, posterior to cochlea
• utricle & saccule (ppt. 15): pouch-like chambers in vestibule of bony labyrinth
- maculae – sensory spots in utricle (horizontal) & in saccule (vertical)
involve hair cells (fig. 55 – 10 & ppts. 16 & 17) with overlying gelatinous layer with
statoconia (otoliths) (ppt. 18); positive & negative signals are produced in vestibular
nerve (branch of cranial n. VIII) in response to orientation of head relative to gravity
(static equilibrium); differing orientation of hair cells accounts for differing patterns
of signals for different orientations of head relative to gravity
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- p. 5 -
• semicircular canals: three mutually perpendicular loops of membranous
labyrinth (anterior – 'near sagittal’, posterior – ‘near coronal’ & lateral – horizontal)
- crista ampullaris (fig. 55 – 11 & ppts. 19 & 20) – swelling at end of each
semicircular canal contains a cupula (sensory hairs embedded in gelatinous mass) that
responds to fluid movements through the semicircular canal; generates signals in
vestibular nerve that carry information about ‘start-and-stop’ movements of the head
(dynamic equilibrium) (ppt. 21) ; this provides an ‘anticipatory’ function (i. e., a
change in direction during running anticipates an impending disturbance of
equilibrium before it happens); static equilibrium organ only responds after
disturbance has occurred