Week 6 Flashcards
Reflex
Protective eg limb withdrawal
Motor patterns generated in the spinal cord
‘Closed loop’ system regulates itself without the need for external input
Rhythmic motor patterns
Eg chewing, walking, breathing
A combination of reflex and voluntary
A reflex is performed without conscious thoughts as an automatic response to a stimulus
Voluntary movement
Purposeful, goal directed
Command originates from higher centres
‘Open loop’ system regulates itself via external input
Lower motor neurons
The alpha motor neuron a-MN:
-large, multipolar lower motor neurons of the brainstem and spinal cord
-are located in the ventral horn of the spinal cord. Their axons which are efferent fibres travel to the muscles via spinal nerves
-innervate skeletal muscle and cause the muscle contractions that generate movement
A-MNs alone directly control muscle contraction
‘The final common pathway’ of motor control
Spinal motoneuron activity
Sensory input is integrated with motor commands at several levels of the motor system; spinal, subcortical and cortical
Is governed by inputs from:
Sensory input- local feedback control (via dorsal roots)
Spinal interneurons- circuitry generating motor programmes
Upper motor neurons- initiation and control
Spinal cord circuits can generate movement in isolation
Even when descending influences are severed, coordinated movements can occur
“See the headless chickens running”
Central pattern generators:
-circuits within the spinal cord are responsible
Descending input from upper motorneurons
Sophisticated, adaptable, patterns of movement (voluntary or otherwise)
Involves input descending from the brain
(Super) imposed upon the intrinsic circuitry of the spinal cord
The musculature: definitions and roles
Fine motor: fine motor skill (or dexterity) is the coordination of small muscles in movement with the eyes, hands and fingers. Distal (hands, feet, digits) sites located away from a specific area most often the centre of body . Innervated by= LATERAL motoneurones
Innervated by MEDIAL motorneurones:
-proximal (elbow, knee) nearer to centre or to point of attachment to body. Axial (trunk muscles) running or extending in the direction of the length of an object. Toward the middle of center opposite of lateral
Summary of descending pathways
Proximal/axial muscles extensors. POSTURE
-a muscle whose contraction extends or straightens a limb or other part of the body
-the ventromedial pathway conveys information from the diffuse areas of the cortex, midbrain, and cerebellum
Distal muscles flexors. VOLUNTARY
-a muscle whose contraction bends a limb or other part of the body
-the lateral pathways contains upper motor neuronal axons that synapse on the dorsal lateral lower motor neurones which are involved in distal limb control
Descending tracts of the spinal cord
Lateral pathways:
-Corticospinal tract/ pyramidal tract connects cortex to spinal cord for voluntary movement by distal effectors
-Rubrospinal tracts: origin red nucleus
Ventromedial pathways ;
-medullary reticulospinal tract: arises from the nuclei of reticular formation located in the medulla of the brainstem
-pontine reticulospinal tract: arises from the medial zone of the pontine reticular formation it synapses at all levels of the spinal cord
-tectospinal tract: arises in the superior colliculus of the midbrain; coordinates head and eye movements in response to visual input
-vestibulospinal tract: axons of the vestibular nuclei (within the brainstem) and terminates by synapsing with interneurons in the anterior gray column of the spinal cord
Lateral descending motor pathways
The lateral motor system includes:
-lateral corticospinal tract; responsible for voluntary movement of the limbs
-Rubrospinal tract; augments the activity of the flexor muscles and inhibits the action of the extensor (antigravity) muscles
Corticospinal: carry motor info in efferent nerves from upper motor neurons of cortical structures like the cerebellum and cerebrum
Rubrospinal: begins as axons of neurons present in the red nucleus and terminates by synapsing with the interneurons in the spinal cord
Lateral pathways the corticospinal (pyramidal) tract
A “direct line” contralateral projection from cortex to lateral spinal motor neurones
-monosynaptic contact with a motor neurons (aMNs)
-Majority of axons from neurons with cell bodies in the motor cortex (areas 4 and 6)
-Innervate aMNs (and to a lesser extent interneurons) controlling distal muscles and particularly flexors
Lateral pathways the Rubrospinal tract
Contralateral projections from red nucleus running down the lateral column of the spinal cord
Similar role to corticospinal tract
Much smaller component of the lateral pathway
Lesion of lateral pathways
Severing lateral ascending spinal sensory pathways (sometimes used for relief of intractable pain)
Can-> motor deficit -voluntary movements slower
Posture unaffected (no damage to ventromedial tracts)
In selective corticospinal tract lesion:
-Rubrospinal can compensate almost entirely for the loss of corticospinal (except fine digit control)
-re routing of cortical output via the Rubrospinal tract
Ventromedial motor pathways extrapyramidal tracts
All originate from brain stem nuclei
Both contra and ipsilateral descending projections
-control of motor output to proximal and axial muscles
-control of body position and posture
NB. The extrapyramidal side effects of dopaminergic drugs affect trunk muscles
-the mechanisms by which this occurs is largely unknown. It’s generally believed that antagonistic binding of dopaminergic D2 receptors in the mesolimbic and mesocortical regions of the brain plays a major role in
