Flashcards in Lesson 6: Efferent Tracts and Relationships Deck (39):
What is a mixed peripheral nerve?
The (somatic) peripheral nerves carry afferent and efferent information. They are mixed nerves
What are the three options once sensory input info arrives at the spinal cord? (3)
Initiate a simple reflex, initiate a complex reflex, or initiate a pathway to send sensory information to higher centers.
How are the alpha motor neurons stimluated for output? (3)
The first two involve the 2 neurons, sensory neuron to motor neuron reflex called the monosynaptic (one synapse) reflex, or it may be part of a more complex reflex in which at least one inter-neuron is included in the sensory-motor reflex.
Third, the motor neuron may receive instructions from higher centers.
What is a motor unit?
A single alpha motor neuron and all the muscle fibres it innervates
Why is a motor unit also called the final common pathway?
Because, with many different sources to stimulate it (sensory neuron, inter-neuron or upper motor neuron), once it fires, the same thing “final” and “common” to all sources happens – it sends its message along the alpha motor neuron to its motor unit muscle fibres and they contract.
Where does the upper motor neuron span?
The upper motor neuron spans from the cerebral cortex to the ventral horn of the spinal cord
Where does the lower motor neuron span?
The second motor neuron is the alpha motor neuron (final common pathway)
What does "bulbar" mean in corticobulbar?
Refers to structures around the head and neck, so “cortex-to-neck” pathway
What does "nuclear" refer to in corticonuclear?
Refers to the cranial nerve nuclei in the brainstem, so (motor) cortex to brainstem (via internal capsule and pes pedunculi)
What is the difference between the corticobulbar/corticonuclear pathways?
For some of the cranial nerve nuclei there is extra input from the cerebral cortex on the same side – “ipsilateral backup”.
What is the mechanism for motor movement in the corticonucear/corticobulbar pathways?
Typically a 2-neuron pathway, and occasionally there is a tertiary motor neuron as with some of the parasympathetic functions. For voluntary motor movement, the signal originates with the upper motor neuron in the contralateral motor cortex and synapses on the lower motor neuron in the brainstem
Where does the lateral corticospinal tract travel?
Travels in the lateral white column in the spinal cord and crosses in the pyramids
Where does the anterior corticospinal tract travel?
Travels in the anterior part of the cord and doesn’t cross until it reaches the spinal level it will innervate
In the pyramidal and extrapyramidal pathways, where do the descending motor fibres travel?
Fibres stay on the ventral (anterior) aspect of the brainstem. They pass through the crus cerebri of the midbrain, then make their way between the thick bundle of crossing fibres of the pons (basis pons), form the pyramids in the open medulla and then in the pyramidal decussation just between the closed medulla and spinal cord) they lose the pyramid shape as the fibres cross the cord to continue to descend in the lateral white matter.
What do the anterior and lateral corticospinal and corticobulbar pathways control?
Voluntary movement of the trunk, limbs, head, and neck (everything) is achieved by the two-neuron system.
What does the extrapyramidal pathway control?
The pyramidal tract is only the lateral corticospinal; are all the other influences on movement that aren’t in those pathways.
What defines an upper/lower motor neuron lesion?
What does each affect?**
- If the first neuron in the pathway is damaged, that is an Upper Motor Neuron Lesion (UMNL) - delayed muscle spasticity, increased tone and hyperreflexes, paralysis
- If the second neuron in the pathway (alpha motor neuron) is damaged, that is a Lower Motor Neuron Lesion (LMNL - flaccid paralysis, decreased tone and reflexes, muscle atrophy.
If the upper motor neuron is damaged, will the sensory sensory input from the reflex still stimulate the intact lower motor neuron?
How does the Babinski reflex occurs?
If you stroke the bottom of the foot from the side of the foot, toward the toes and then across to the big toe, a primitive reflex (the Babinski Reflex) occurs (is present) and the big toe extends upward and the rest of the toes “splay” outward. If there is not damage to the upper motor neuron the Babinski reflex will be absent
What is spastic paralysis?
The paralysis (lack of voluntary movement) is called “spastic paralysis” because there are other sources than just the upper motor neuron and the usual inhibitory instructions resulting in “spasticity” and increased tone “hypertonia”.
What other sources of input are there to the lower motor neuron when it is damaged?
(other reflexes for example);
muscles fire involuntarily such that they do become weak from lack of purposeful movement, but they retain some strength because of the involuntary contraction.
What are UMNL characteristics?
Hyperreflexia, spastic paralysis, minimal wasting, and clonus and the Babinski reflex will also be present.
What is clonus?
Clonus is the rapid reversal of the hyperactive stretch reflex causing a bouncing movement.
What are LMNL characteristics?
LMN is cut, there will be NO activity in that neuron. The reflex cannot stimulate it because the axon is cut and there can be no response.
There will be flaccid paralysis (floppy paralysis) hyporeflexia (areflexia), rapid and marked atrophy, and the Babinski reflex will not be present. There will be no clonus.
How many neurons are there for sensory/motor pathways?
3 (sensory); 2 (motor)
Where is the PMC? What important structure does it have?
PMC is in the precentral gyrus of the frontal lobe. It contains large Betz cells, which are unique to this cortical area and are very important in voluntary motor movement.
How is the homunculus organized in the PMC?
Face, speech muscles, and head are in the lower third of the motor cortex, the arms and trunk are in the upper motor cortical region, and legs and toes are in the midsagittal area.
Where do motor neural impulses that travel in the pyramidal tract originate?
PMC, PreMC, and primary sensory cortex (PSC)
What cell is: are pyramidal cells whose long axons extend to the lower limbs and thus require large cell bodies for metabolic support
To maintain the precision, accuracy, smoothness, and sequential nature of the motor activity, the PMC depends on?
Constant feedback from the adjacent cortical and subcortical regions
Cortical Input: includes the PreMC, PFC, what do they do together?
Iincludes afferent from the PreMC, which with input from the PFC is concerned with setting up a motor plan of a skilled movement pattern involving specific limbs. This area adds to the quality of judgment and foresightedness to motor movements.
What motor cortex: Regulated planning and the implementation of bilateral aspects of movements.
Supplementary motor cortex
What is the association somesthetic cortex vs. somesthetic cortex?
Projections from the somesthetic cortex modulate sensory feedback
Fibers from the association somesthetic cortex regulate higher-order spatial aspects of the movement plan
Where do the corticonuclear and corticobulbar efferent projections go from motor cortex?
Cross the midline to innervate contralateral cranial nerve and spinal output motor nuclei.
What is the difference between a lesion of motor fibres above/below point of decussation?
Above the point of decussation: produces clinical signs contralateral to the site of damage.
Below the decussation point: clinical signs of the spinal UMNs and LMNs are ipsilateral to the locus of the damage
Coritconuclear/Corticobulbar path: What nerves are involved?
Trigeminal, facial, glossopharyngeal, vagus, spinal accessory and hypoglossal nerves
Fibers form the left motor cortex innervate which motor nuclei?
Fibres from the right motor cortex control what?
Both the left and right motor nuclei of some of the cranial nerves
Projections from the right motor cortex control the functioning of some cranial nerve nuclei on both sides.
What are some characteristics of UMN Syndrome? (6)
• Paralyzed muscles are initially flaccid, but there is increased muscle tone
• Loss of voluntary movements in the affected muscles
• Contralateral spinal reflexes are hyperactive
• Muscle tone profoundly increases, contributing to muscle spasticity.
• Altered reflexes – Babinski reflex is present = corticospinal abnormality. (Toe dorsiflexes)
• Abdominal and cremasteric reflexes are two reflexes that are lost