Spinal cord- 2 Descending tracts

Skeletal muscle Skeletal muscle Upper motor neurons in primary motor cortex Somatic motor nuclei of brain stem Skeletal muscle Lower motor neurons Somatic motor nuclei of spinal cord Skeletal muscle In the somatic nervous system (SNS), an upper motor neuron in the CNS controls a lower-motor neuron in the brain stem or spinal cord. The axon of the lower-motor neuron has direct control over skeletal muscle fibers. Stimulation of the lower- motor neuron always has an excitatory effect on the skeletal muscle fibers.

Visceral motor nuclei in hypothalamus Preganglionic neuron Visceral effectors Autonomic nuclei in brain stem Ganglionic neurons Autonomic nuclei in spinal cord Preganglionic neuron In the autonomic nervous system (ANS), the axon of a preganglionic neuron in the CNS controls ganglionic neurons in the periphery. Stimulation of the ganglionic neurons may lead to excitation or inhibition of the visceral effector innervated

First order neuron: has its cell body in the cerebral cortex Second order neuron: an interneuron situated in the anterior gray column Third order neuron: the lower motor neuron in the anterior gray column and Innervates the skeletal muscle through the anterior root of spinal nerve In some instances the axon of the first order neuron terminates directly on the third order neuron

Motor tracts •There are two major descending tracts •Corticospinal tract (pyramidal) : Conscious control of skeletal muscles •Subconscious tract (extrapyramidal): Subconscious regulation of balance, muscle tone, eye, hand, and upper limb position

•Corticobulbar tracts: conscious control over eye, jaw, and face muscles •Lateral corticospinal tracts: conscious control over skeletal muscles (mostly distal muscles) •Anterior corticospinal tracts: conscious control over skeletal muscles (mostly proximal muscles) UMN LMN To skeletal muscles Corticobulbar tract Motor nuclei of cranial nerves Lateral corticospinal tract MESENCEPHALON To skeletal muscles Motor nuclei of cranial nerves MEDULLA OBLONGATA Pyramids Lateral corticospinal tract Anterior corticospinal tract To skeletal muscles Anterior corticospinal tract

Corticospinal tracts The corticospinal tracts are often called the pyramidal tracts because they form pyramid-shaped enlargements on the anterior surface of the medulla concerned with controlling skilled movements of the distal extremities (facilitation of alpha and gamma motor neurons which innervate the distal flexor muscles ) The upper motor neurons of these tracts originate in the precentral gyrus of the cerebral cortex In medulla oblongata they form the medullary pyramids Most of the fibers (85 percent) cross over (decussate) to the opposite side in the pyramidal decussation, where they continue to descend in the lateral funiculus of the spinal cord as the lateral corticospinal tract (LCST). The tract descends all the way of spinal cord with fibers continually leaving it in order to synapse on interneurons in the anterior gray horn. ( Some even synapse directly on alpha and gamma motor neurons) Those corticospinal fibers which do not decussate in the medulla continue descending on the same (ipsilateral) side of the cord and become the anterior corticospinal tract (ACST). This tract does not extend below the midthoracic level. Fibers leave the tract at various levels to cross over in the anterior white commissure to synapse on interneurons in the anterior gray horn.

The anterior corticospinal tract acts on the proximal muscles of upper limb (shoulder muscle) of the ipsilateral and contralateral sides

corticospinal tract for fine skilled movements

The Corticobulbar Tract This tract is composed of fibers originating in the precentral gyrus of the lower quarter of the motor cortex. The descending fibers terminate in the motor nuclei of cranial nerves III and IV in the midbrain; V, VI. and VII in the pons; and IX, X, XI, and XII in the medulla. The corticobulbar fibers from one side of the brain project to the motor nuclei on both sides of the brainstem (bilateral input) The corticobulbar input is bilateral to : 1- 5th 2- Part of 7th ( which supplies UPPER facial muscles) 3- 9th,10th,11th 4- 12th !!!!

These motor pathways are complex and multisynaptic, and regulate: The Subconscious Motor Tracts •Consists of four tracts involved in monitoring the subconscious motor control •Vestibulospinal tracts •Tectospinal tracts •Reticulospinal tracts •Rubrospinal tracts Extrapyramidal tracts arise in the brainstem, but are under the influence of the cerebral cortex These motor pathways are complex and multisynaptic, and regulate: Axial muscles that maintain balance and posture Muscles controlling coarse movements of the proximal portions of limbs Head, neck, and eye movement

the anterior white column Vestibulospinal tracts Vestibular nuclei are situated in the pons and medulla beneath the floor of 4th ventricle •Send information from the inner ear to monitor position of the head • balance by facilitate the activity of the extensor muscles The tract descends uncrossed through medulla and through spinal cord in the anterior white column Terminates by synapsing with interneurons of anterior gray column The inner ear and the cerebellum facilitate the activity of extensor muscles and inhibit the activity of flexor muscles in association with the maintenance of balance

( balance by facilitate the activity of the extensor muscles ) Vestibulospinal tract nerve cells in vestibular nucleus (in the pons and medulla oblongata received afferents from inner ear and cerebellum axons descend uncrossed through medulla and through the length of spinal cord synapse with neuron in the anterior gray column of the spinal cord ( balance by facilitate the activity of the extensor muscles )

by synapsing with interneurons Tectospinal tracts Concerned with reflex postural movements in response to visual stimuli •Send information to the head, neck, and upper limbs in response to bright and sudden movements and loud noises •The tectum area consists of superior and inferior colliculi •Superior colliculi: receives visual information •Inferior colliculi: receives auditory information Arise from nerve cells in the superior colliculus Most of the fibers cross the midline soon after their origin The tract descends in the anterior white column close to Anterior median fissure Majority of fibers terminate in the anterior gray horn in the upper cervical segments by synapsing with interneurons

Tectospinal tract nerve cells in superior colliculus of the midbrain nerve fibres/ axons cross the mid line terminate in the anterior gray column of upper cervical segments of spinal cord ( responsible for reflex movement of head & neck in response to visual stimuli )

Reticulospinal tracts • influence voluntary movements and reflex activity by facilitating or inhibiting the activity of alpha and gamma motor neurons Reticular formation (RF): group of scattered nerve cells in the brain stem From pons: axons of RF neurons descend uncrossed into the spinal cord ( pontine Reticulospinal tracts ) descend in the anterior white column as the medial reticulospinal tract (MRST) activate the axial and proximal limb extensors From medulla : axons of RF neurons descend crossed and uncrossed into the spinal cord ( medullary Reticulospinal tracts ) descend in the lateral white column as the lateral reticulospinal tract (LRST). inhibit the axial and proximal limb extensors (and to a lesser degree it also excites axial and proximal limb flexors) The reticulospinal tracts exert both somatic and autonomic control Has also descending autonomic fibers ( providing a pathway by which the hypothalamus can control the sympathetic and sacral parasympathetic outflow) Most of these fibers are derived from the lateral reticulospinal tract

Rubrospinal tracts • facilitate the activity of flexors and inhibit the activity of extensors The axons of neurons in the red nucleus cross over near their point of origin and descend contralaterally in the lateral funiculus of the cord adjacent to the lateral corticospinal tract Terminates by synapsing with interneurons of anterior gray column It has connections with cerebral cortex and cerebellum

( facilitate the activity of flexor muscles ) Rubrospinal tract nerve cells in red nucleus ( tegmentum of midbrain at the level of superior colliculus ) nerve fibres / axons cross the mid line descend as rubrospinal tract through pons and medulla oblongata terminate anterior gray column of spinal cord ( facilitate the activity of flexor muscles )

Summary of somatic motor control •Cerebral cortex initiates voluntary movement •Information goes to the basal nuclei and cerebellum •These structures modify and coordinate the movements so they are performed in a smooth manner Information goes from the basal nuclei and cerebellum back to the cerebral cortex to constantly monitor position and muscle tone

Somatic Motor Control Motor Association areas Decision in frontal lobes Basal nuclei Cerebellum The planning stage: When a conscious decision is made to perform a specific movement, information is relayed from the frontal lobes to motor association areas. These areas in turn relay the information to the cerebellum and basal nuclei.

Primary motor cortex Motor Association areas Movement: As the movement begins, the motor association areas send instructions to the primary motor cortex. Feedback from the basal nuclei and cerebellum modifies those commands, and output along the conscious and subconscious pathways directs involuntary adjustments in position and muscle tone Basal nuclei Cerebellum Other nuclei of the medial and lateral pathways Corticospinal pathway Lower motor neurons Motor activity