Spinal cord and descending tracts + Motor cortex (wk 4) Flashcards
Describe the overview of motor systems:
The spinal cord (alpha)-motor neuron is the final common pathway of motor control. Receives input from local circuit neurons within the spinal cords (interneurons) and directly from the brainstem and motor cortex.
Describe the cerebellum and basal ganglia modulate motor output:
-> Basal nuclei assists initiation and termination of movement. Cerebellum assists posture and movement coordination by detecting error signals. Thalamus is the gateway for cerebella and basal nuclei input to the cerebral cortex.
Describe the spinal cord
A ‘neural’ highway. Information is sent to and from the brain and body cia the spinal cord. There are 31 pairs of spinal nerves: 8 thoracic, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. Each spinal nerve consists of a ventral (motor) and dorsal (sensory) root. Grey matter = ?/ white matter = myelinated
What tracts from the lateral and medial descending system?
-> Corticospinal and rubrospinal tracts from the lateral descending system. Reticulospinal and vestibulospinal tracts form the medial descending system.
What is the somatotopic arrangement?
-> Motor neurons exhibit ‘somatotopy’. Somatotopy is the point for point reference of a part of the body in the central nervous system. Proximal and distal muscles are represented in proximal and distal spinal cord.
Describe the corticospinal tract:
-> Largest descending tract. Known as the pyramidal tract. Responsible for voluntary control of movement. Contains direct and indirect connections to motor neurons. Direct pathways contain lateral and anterior/ventral aspects. 90% of corticospinal pathways are lateral (crossed). Pyramidal decussation is why the brain controls the contralateral side of the body. Indirect connections are known as corticobulbar tracts.
Describe the lateral corticospinal tract:
-> Projects from motor cortex (precentral gyrus) to spinal cord. Upper motor neuron descends through the cerebral peduncles in the midbrain. Decussates at the base of medullary pyramids. Descends through lateral corticospinal tract. (Alpha)-motor neuron (or lower motor neuron) innervates skeletal muscles in distal parts of limbs, such as: muscles of the hand and muscles of the lower leg.
Describe the anterior/ ventral corticospinal tract:
-> Projects from motor cortex (precentral gyrus) to spinal cord. Upper motor neuron descends through the cerebral peduncles in the midbrain. Does not decussate in medulla. (alpha) – motor neuron (or lower motor neuron) innervate skeletal muscles in the trunk and proximal parts of limbs, such as muscles of the upper arm and upper leg.
Describe the corticospinal tract:
-Differences between species and dexterity
-> The extent of direct connections to the spinal cord differs between species. The extent of different connections to the spinal cord differs between species. In ‘higher’ animals (e.g. chimps, humans), there are more direct connections to spinal motor neurons -> corticospinal tract is evolutionary and correlates with manual dexterity. Reliance upon direct connections means humans are less able to adapt to spinal cord injury verses other species.
What is the Babinski sign?
-> A diagnostic of a lesion to the corticospinal tract. Characterized by abnormal plantar response.
How do you stimulate the corticospinal tract?
-> The motor cortex can be stimulated using magnetic or electrical stimulation. Transcranial magnetic stimulation has many applications. Assess integrity of corticospinal tract. First demonstrated by Anthony Barker and colleagues in Sheffield.
Describe the corticobulbar tract:
-> Corticobulbar tract forms connections from the motor cortex to the brainstem. The corticobulbar tract is a relay to the brainstem. Terminates mainly on interneurons. Importance in volitional control of breathing. Upper motor neuron projects from cerebral cortex to brainstem (pons and medulla) via cerebral peduncles and innervates interneurons or cranial nerves. Cranial nerves are lower motor neurons and include: CN VII from pons (facial, facial expression) and CN XII from medulla (hypoglossal, tongue). Interneurons innervate the reticular formation – project to various sites involved in somatic, autonomic, sensory, neuromodulatory functions.
Describe the rubrospinal tract:
-> Originates in the red nucleus. The red nucleus is a spherical collection of cell bodies in the midbrain (rostral to pons) – highly vascularised and large input from cerebellum, and primary motor cortex. Primarily involved in activation of flexor motor neurons.
-> Axons originating from the red nucleus cross the midline of the ventral midbrain (ventral tegmental decussation). Fibres terminate on interneurons that project to the ventral horn. Primary function is voluntary control of muscle -> stimulation of red nucleus produces contralateral flexion and inhibition of extension.
Describe the rubrospinal tract
-Compensation
-> The rubrospinal tract can compensate for damage to corticospinal tract. Lawrence and Kuypers (1968) – surgically severed corticospinal tract in monkeys.
Describe the reticulospinal tract:
-> Originates in the brainstem reticular formation. Reticular formation is a complex network of brainstem nuclei and poorly anatomically defined. Reticular formation coordinates many body functions necessary for survival. Reticulospinal tracts include: lateral tracts (medullary) and medial tracts (pontine). Projects to somatic (e.g. respiratory muscle) and autonomic (e.g. heart and kidneys) neurons.
