Motor Cortex and Brain Stem Control

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Questions and Answers

What percentage of the corticospinal tract originates from the somatic sensory areas?

20%

40% (correct)

30%

50%

What is the transmission rate of the large fibers arising from the giant pyramidal cells (Betz cells)?

20m/sec

100m/sec

70m/sec (correct)

40m/sec

What is the function of the small diameter fibers that make up 97% of the total number of fibers in the corticospinal tract?

Conduct fast transmission signals

Provide feedback signals to the brain

Conduct background tonic signals (correct)

Regulate the intensity of sensory signals

What is the destination of the fibers from the red nucleus in the corticospinal tract?

Cord in the rubrospinal tract Signup and view all the answers

What is the function of the Betz collaterals?

Sharpen the boundaries of the excitatory signal Signup and view all the answers

What is the primary function of the premotor area?

To provide attitudinal, fixation, or positional movement for the body Signup and view all the answers

Damage to which area of the motor cortex can cause motor apraxia?

Hand skills area Signup and view all the answers

What is the primary difference between the direct and indirect pathways of cortical motor signals?

The direct pathway is for discrete, detailed movement, while the indirect pathway is for coordinated movement Signup and view all the answers

What is the characteristic of the topographic representation in the primary motor cortex?

Unequal representation of all body parts Signup and view all the answers

What is the function of the supplemental motor area?

To provide background for fine motor control of the arms and hands Signup and view all the answers

Which of the following nuclei of the thalamus is involved in the coordination of function between motor cortex, basal ganglia, and cerebellum?

Ventral lateral and ventral anterior nuclei Signup and view all the answers

What type of fibers are transmitted from the magnocellular portion of the red nucleus to form the rubrospinal tract?

Primary motor cortex fibers Signup and view all the answers

What is the result of stimulating the red nucleus?

Relatively fine motor movement Signup and view all the answers

Which of the following is NOT a source of incoming sensory pathways to the motor cortex?

Dorsal column nuclei Signup and view all the answers

What is the organization of the magnocellular portion of the red nucleus?

Somatotopic organization Signup and view all the answers

What is the primary function of the semicircular ducts in maintaining equilibrium?

To detect rotational movements of the head Signup and view all the answers

What happens to the endolymph in the semicircular duct when the duct rotates?

It remains stationary due to inertia Signup and view all the answers

What is the primary function of the predictive function of the semicircular ducts?

To predict situations in which equilibrium will be affected Signup and view all the answers

What is the role of the crista ampullaris in the semicircular duct?

To activate receptors in the crista ampullaris Signup and view all the answers

What is the relationship between the semicircular ducts and equilibrium?

The semicircular ducts play a crucial role in maintaining equilibrium Signup and view all the answers

What is the primary function of sensory feedback from muscle spindle, tactile receptors, and proprioceptors in motor control?

To fine-tune muscle movement Signup and view all the answers

Which type of neurons in the spinal cord are directly synapsed by some corticospinal and rubrospinal neurons?

Alpha motor neurons Signup and view all the answers

What is the result of a lesion in the basal ganglia?

Muscle spasticity from loss of inhibitory input Signup and view all the answers

What is the primary function of the brainstem in motor control?

To perform motor and sensory functions for the face and head Signup and view all the answers

Which brainstem nuclei excite the antigravity muscles?

Pontine reticular nuclei Signup and view all the answers

Study Notes

Motor Cortex

Divided into 3 sub-areas: primary motor cortex, premotor area, and supplemental motor area
Primary motor cortex:
- Unequal topographic representation
- Elicits fine motor movement with stimulation
Premotor area:
- Topographical organization similar to primary motor cortex
- Stimulation results in movement of muscle groups to perform specific tasks
- Works in concert with other motor areas
Supplemental motor area:
- Topographically organized
- Stimulation often elicits bilateral movements
- Functions in concert with premotor area to provide attitudinal, fixation, or positional movement for the body

Specialized Areas of the Motor Cortex

Broca's area:
- Damage causes decreased speech capability
- Closely associated area controls appropriate respiratory function for speech
Eye fixation and head rotation area:
- Coordinates head and eye movements
Hand skills area:
- Damage causes motor apraxia (inability to perform fine hand movements)

Transmission of Cortical Motor Signals

Direct pathway:
- Corticospinal tract
- For discrete detailed movement
Indirect pathway:
- Signals to basal ganglia, cerebellum, and brain stem nuclei

The Corticospinal Tract

Originates in primary motor cortex (30%), supplementary motor areas (30%), and somatic sensory areas (40%)
Majority of fibers cross to opposite side in medulla and descend in lateral corticospinal tracts
Corticospinal fibers synapse with interneurons, anterior motor neurons, and a few sensory relay neurons in the cord gray matter
Giant pyramidal cells (Betz cells) give rise to large fibers with fast transmission rates (70m/sec)

Corticospinal Fibers

34,000 Betz cell fibers make up only about 3% of the total number of fibers
97% of the 1 million fibers are small-diameter fibers that conduct background tonic signals and feedback signals from the cortex to control intensity of sensory signals to the brain

Other Pathways from the Motor Cortex

Betz collaterals back to cortex sharpen the boundaries of the excitatory signal
Fibers to caudate nucleus and putamen
Fibers to the red nucleus, which then sends axons to the cord in the rubrospinal tract
Fibers to reticular substance, vestibular nuclei, and pons, then to the cerebellum
Therefore, the basal ganglia, brain stem, and cerebellum receive a large number of signals from the cortex

Incoming Sensory Pathways to Motor Cortex

Subcortical fibers from adjacent areas of the cortex, especially from somatic sensory areas of parietal cortex and visual and auditory cortex
Subcortical fibers from opposite hemisphere, which pass through corpus callosum
Somatic sensory fibers from ventrobasal complex of the thalamus (i.e., cutaneous and proprioceptive fibers)

Red Nucleus and the Rubrospinal Tract

Substantial input from primary motor cortex
Primary motor cortex fibers synapse in the lower portion of the nucleus (magnocellular portion), which contains large neurons similar to Betz cells
Magnocellular portion gives rise to rubrospinal tract
Magnocellular portion has somatotopic organization similar to primary motor cortex

Sensory Feedback is Important for Motor Control

Feedback from muscle spindle, tactile receptors, and proprioceptors fine-tunes muscle movement
Length mismatch in spindle causes auto-correction
Compression of skin provides sensory feedback to motor cortex on degree of effectiveness of intended action

Excitation of Spinal Motor Neurons

Motor neurons in cortex reside in layer V
Excitation of 50-100 giant pyramidal cells is needed to cause muscle contraction
Most corticospinal fibers synapse with interneurons
Some corticospinal and rubrospinal neurons synapse directly with alpha motor neurons in the spinal cord, especially in the cervical enlargement
These motor neurons innervate muscles of the fingers and hand

Control of Motor Function by the Brainstem

Brainstem as an extension of the spinal cord
Performs motor and sensory functions for the face and head (i.e., cranial nerves)
Similar to spinal cord for functions from the head down
Contains centers for stereotypic movement and equilibrium

Support of the Body Against Gravity

Muscles of the spinal column and the extensor muscles of the legs support the body against gravity
These muscles are under the influence of brainstem nuclei
Pontine reticular nuclei excite the antigravity muscles
Medullary reticular nuclei inhibit the antigravity muscles

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Description

Learn about the cortical and brain stem control of motor function, including the primary motor cortex and premotor area, and their roles in motor movement and control.