Week 9 Upper Motor Neuron Control of the Brainstem and Spinal Cord PDF

Preview

Summary

Download Week 9 Upper Motor Neuron Control of the Brainstem and Spinal Cord PDF

Description

WEEK 9: UPPER MOTOR NEURON CONTROL OF THE BRAINSTEM AND SPINAL CORD Organization of Descending Motor Control:

Somatotopic organization of the ventral horn The Corticospinal and Corticobulbar Tracts: ➔ Upper motor neurons in the cerebral cortex reside in several adjacent and highly interconnected areas in the posterior frontal lobe, which together mediate the planning and initiation of complex temporal sequences of voluntary movements. ➔ Primary motor cortex refers to location in the precentral gyrus and paracentral lobule. ➔ Axons of upper motor neurons descend into corticobulbar and corticospinal tracts ➔ Medullary pyramids are formed on the ventral surface of the medulla. Corticobulbar tract: upper motor neurons innervating cranial nerve nuclei, reticular information and red nucleus leave pathway at levels of brainstem. originating in M1 (represents movements, not muscles)  and terminating in the brainstem. Involved in controlling the muscles of the face and neck involved in facial expression, mastication, swallowing and other functions. ➔ Corticospinal tract: involved in controlling proximal and distal limb muscles.

Lateral corticospinal tract: located near the caudal end of the medulla, control of the hands. It forms a direct pathway from the cortex to the spinal cord & terminates primarily in the lateral portions of the ventral horn and intermediate gray matter. Some of these axons synapse directly on α motor neurons that govern the distal extremities. 90% of corticospinal axons cross the body midline (decussate) and form the lateral corticospinal tract. Corticospinal axons mostly synapse onto spinal local circuit neurons. Ventral corticospinal tract: arises primarily from dorsal and medial regions of the motor cortex. They terminate ipsilaterally or bilaterally. They serve trunk and axial and proximal limb muscles. Enters spinal cord without crossing. 1  0% terminate ipsilaterally or bilaterally and form the ventral corticospinal tract. Functional Organization of the Primary Motor Cortex:

➔ Theodor Fritsch & Eduard Hitzig had shown that electrical stimulation of the motor cortex elicits contractions of muscles on the contralateral side of the body. ➔ At the same time, John Hughlings Jackson surmised that the motor cortex contains a complete spatial representation of the body’s musculature. ➔ Sir Charles Sherington confirmed this, by publishing classic maps of the organization of the motor cortex in great apes. ➔ The introduction in the 1960’s of intracortical microstimulation allowed a more detailed understanding of motor maps. Microstimulation entails the delivery of brief electrical currents an order of magnitude smaller than those used by Sherrington & Penfield. By passing the current through the sharpened tip of a metal microelectrode inserted into the cortex, the upper motor neurons in layer 5 that project to lower motor neuron circuitry could be stimulated more focally. The Primary motor cortex: can be distinguished in 2 ways ● Architectonics: output layer V contains distinctive large-diameter pyramidal neurons ● Electrophysiology: l ow intensity electrical stimulation elicits movements. ➔ Wilder Penfield was the first to systematically map somatotopic organization of human primary motor cortex

➔ Somatotopic organization also occurs in other cortical areas including the premotor area (PMA)

The Premotor Cortex: ➔ The functional division of the motor cortex includes B  rodmann’s areas 6, 8, and 44/45 on the lateral surface of the frontal lobe and parts of areas 23 and 24 on the medial surface of the hemisphere. ➔ Each division of the premotor cortex receives extensive multisensory input from regions of the inferior & superior parietal lobes, as well as more complex signals related to motivation & intention from the rostral divisions of the frontal lobe. ➔ Premotor neurons help control movement both indirectly via reciprocal connections with primary motor cortex and directly via axons through corticobulbar and corticospinal tracts. 30% of axons in the corticospinal tract originate in the premotor cortex ➔ The difference between the premotor cortex and the primary  motor cortex lies in the strength of their connections to lower motor neurons, with more upper motor neurons in the primary motor cortex making monosynaptic connections to α motor neurons. ➔ Primary motor cortex neurons: c an be directionally selective, the directional control of a movement is coded by the activity of a population of primary neurons AND the directional responses tend to be broadly tuned. Mirror neurons: a subset of neurons in the ventrolateral portion of the premotor cortex that respond in preparation for upcoming movement and when the same action is observed being performed by another individual. They respond much less when actions are pantomimed without the explicit presence of an action goal, such as an object being grasped. Involved in imitation learning & may or may not exist in human Mirror neurons fire in response to observation of a particular motor act performed by others & fire most strongly in response to the act that activates neurons during self-initiated movements. They also encode the intention to make a particular motor act.

Motor Control Centers in the Brainstem: neurons in the reticular information initiate anticipatory or feedforward postural adjustments based on outbound motor commands originating in the cortex. Direct projections from vestibular nuclei to spinal cord provide sensory feedback about postural changes detected by vestibular labyrinth. Superior colliculus: is located in the dorsal midbrain, and contributes to upper motor neuron pathways that govern lower motor neurons in the spinal cord.

In the midbrain, is also the mesencephalic locomotor region which is involved in the initiation of locomotion. Reticular formation: is a complicated network of circuits in the core of the brainstem that extends from the rostral midbrain to the caudal medulla. It provides information to the spinal cord essential for posture maintenance and control. It comprises numerous clusters of neurons that serve a variety of functions such as cardiovascular and respiratory control, governance of myriad sensorimotor reflexes, coordination of eye  movements, regulation of sleep and wakefulness & temporal and spatial coordination of limb  & trunk movements. Neurons in the reticular formation initiate anticipatory or feedforward postural adjustments on outbound motor commands originating in the cortex. Upper Motor Neuron Syndrome ➔ Damage to the motor cortex or descending upper motor axons in the internal capsule typically causes an immediate flaccidity of the muscles on the contralateral side of the body and lower face. ➔ Hypotonia: initial period after injury – decreased activity of spinal circuits suddenly deprived of input from the motor cortex. (spinal shock) ➔ Babinski Sign: response to sharply stroking the foot is the extension of the big toe and fanning of the other toes, rather than flexion of the toes. ( it does not concern normal vs upper motor neuron deficit) ➔ Spasticity: increased muscle tone, hyperactive stretch reflexes and clonus (contractions and relaxations of muscles in response to muscle stretching. Loss of ability to perform fine movements The acute phase of upper motor neuron syndrome is characterized by: the passive dropping of an affected limb that has been elevated and then released. Ipsilateral → same side of body Contralateral → opposite side of body Decussation → crossing of fiber tracts at the midline

Spike-triggered averaging ➔ Correlate timing of cortical neuron firing with onset times of muscle contractions ➔ The technique is useful for determining a  ll the muscles that are driven by a given motor neuron - is a means of correlating upper motor neuron activity with muscle activation. ➔ The group of muscles activated by a individual upper motor neuron is called the muscle field (whose activity is directly facilitated by a given upper motor neuron) Brain-computer interface (BCI) → refers to a broad range of technologies that directly interface to the nervous system. ● Interfaces can be made at many levels of neural organization ● Can provide control signals to support/restore communication, movement, rehabilitation Upper motor neurons involved in the control of axial muscles would most likely project to the spinal cord in which pattern? Medial gray matter over many spinal segments Which statement about directional tuning and population coding by primary motor cortical neurons is true? The vector summation of population responses of primary motor cortical neurons is important for directional control of motor movements. The rubrospinal pathway: might not exist in humans In an anticipatory postural response of a standing person about to tug on a handle, the early response of leg muscles (such as the gastrocnemius) that precedes the actual tug is an example of: feedforward motor control The "indirect pathway" from cortex to spinal cord does not play a role in: post-injury recovery of fine motor functions such as using two fingers to pick up food. Which is not a function of the reticular information: Transmission of spinal nociceptive and tactile sensory signals to the cerebellum Which statement about the reticular activating system is true?: It supports transitions between sleep and wakefulness A patient is diagnosed with a tumor located in the right internal capsule. Which motor dysfunction would you expect to see in this patient?: CHECK ANSWER Cortical areas that plan and initiate motor sequences: comprise several functionally distinct but highly interconnected regions Although the phenomenon is not well understood, the increased muscle tone and spasticity that develop after an upper motor neuron injury appears to be due, at least in part, to: increased responsiveness of motor neurons to Ia afferent inputs.

Which method helped scientists correct a long-standing misconception about the neurological origins of facial weakness deficits seen in humans?: anatomical tract-tracing in primates When Graziano and colleagues extended cortical microstimulation in monkeys to time epochs approximating those of natural movements, they observed: purposeful movements distributed sequentially across multiple joints. TUTORIAL CONTENT REVIEW QUESTIONS GROUP 1 1. Define a motor unit: A motor unit consists of an alpha motor neuron and all of the fibres it innervates. We have more muscle fibers than motor neurons. 2. What are α motor neurons?: are large motor neurons that innervate the striated muscle fibers that generate the forces needed for posture and movement. (can initiate skeletal muscle contraction) Alpha motor neurons send axons directly to skeletal muscles via two different pathways. Via the ventral roots and spinal peripheral nerves or cranial nerves 3. Define contralateral organisation: The organization of how the hemispheres of the brain represent the opposite side of the body. (e.g in vision the right hemisphere is processing information from the left side of the body) 4. Define topographic organisation & describe as many areas as you can that exhibit it in the motor system: Topographic organisation is in the ventral horn of the spinal cord. There are populations of lower motor neurons in the ventral horn that can move specific parts of the body. It is also found in the primary motor cortex. There is a map of the body within it. It is also found in the somatosensory cortex. Somatosensory cortex is responsible for proprioception (information coming from the body e.g if someone touches you and, represents individual movements) GROUP 2 1. Does the primary motor cortex control individual muscles or entire movements? It controls individual muscles. Every neuron can move one specific muscle. 2. What is population vector coding in the primary motor cortex? The vector summation of population responses of primary motor cortical neurons is important for directional control of motor movements.? 3. What's the difference between primary motor cortex and premotor cortex? The primary motor cortex can be distinguished from other motor areas in 2 primary ways: ● Architectonics: output V layer contains distinctive larger-diameter pyramidal neurons (betz cells) ● Electrophysiology: low intensity electrical stimulation elicits movements The main functional difference between the premotor cortex and the primary motor cortex is that the primary motor cortex is responsible for the execution of movements whilst the premotor cortex is involved in the planning and preparation of movements....