D.: are the pathways by which motor signals are sent from brain to lower motor neurons.
The lower motor neurons then directly innervate muscles to produce movement.
They functionally divided into two major groups: 1.Pyramidal tracts : Tract starts from cerebral cortex carrying motor fibers to brain stem and spinal cord . They are responsible for the voluntary control of the musculature of the body and face. Their name derived from the medullary pyramids of the medulla oblongata, which they pass through. 2. Extrapyramidal tracts: These tracts originate in the brain stem, carrying motor fibers to the spinal cord.
They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion There are no synapses within descending pathways. At the termination of descending tracts, the neurons synapse with a lower motor neuron.
Thus, all neurons within descending motor system are classed as upper motor neurons and their cell bodies are found in cerebral cortex or brain stem, with their axons remaining within CNS.
Functionally, these tracts can be subdivided into two:
1.Corticospinal tracts – supplies musculature of body.
2.Corticobulbar tracts – supplies musculature of head and neck. Cortico spinal Tracts They are responsible for the voluntary control of the musculature of the body and face.
Motor fibers arised from:
.Primary motor cortex
.Premotor cortex
.Supplementary motor area Cerebral cortex internal capsule (a white matter pathway, located between the thalamus and the basal ganglia) neurons pass through crus cerebri of midbrain, pons & medulla., the tract divides into two: 1.Lateral corticospinal tract They then descend into the spinal cord, terminating in the ventral horn then neurons go on to supply muscles of body. 2. Anterior corticospinal tract remains ipsilateral, descending into spinal cord. It terminate in the ventral horn of the cervical and upper thoracic segmental levels.
Corticobulbar Tracts: They receive same inputs as corticospinal tracts. The fibers converge & pass through internal capsule to brainstem but neurons terminate on motor nuclei of cranial nerves. Here, they synapse with lower motor neurons which carry motor signals to muscles of face and neck. Many of these fibers innervate motor neurons bilaterally except : .Upper motor neurons for the facial nerve.
.Upper motor neurons for the hypoglossal nerve.
These above two provided a contralateral innervations. Extrapyramidal tracts: There are four tracts : 1.Vestibulospinal tract 2.Reticulospinal tract These tracts do not decussate----- providing ipsilateral innervation. 3.Rubrospinal tract 4. Tectospinal tract These tracts are decussating ------provide contralateral innervation Vestibulospinal Tract Medial Vestibulospinal Tract Lateral Vestibulospinal Tract.
input from the organs of balance-- ---vestibular nuclei----tracts convey this balance information to the spinal cord, where it remains ipsilateral.
Fibers in this pathway control balance & posture by innervating ‘anti-gravity’ muscles (flexors of the arm, and extensors of the leg).
Reticulospinal Tract: 1. Medial reticulospinal tract: It arises from Pons. It facilitates voluntary movements, and increases muscle tone.
2.Lateral reticulospinal tract: arises from medulla. It inhibits voluntary movements, and reduces muscle tone. Rubrospinal Tract: It originates from red nucleus (midbrain ).
As fibers emerge, they decussate & descend into spinal cord. Thus, they have a contralateral innervation.
Its exact function is unclear, but it is thought to play a role in the fine control of hand movements
Tectospinal Tract: This pathway begins at superior colliculus of mid brain. The superior colliculus is a structure that receives input from optic nerves. The neurons then quickly decussate & enter spinal cord. They terminate at cervical levels of t spinal cord. It coordinates movement of head in relation to vision stimuli.