Spinal cord- 2

Descending tracts

Upper motor neurons in primary motor cortex

Somatic motor nuclei of brain stem

Skeletal muscle

Skeletal muscle

Somatic motor nuclei of spinal cord

Lower motor neurons

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

Preganglionic neuron

Ganglionic neurons

Autonomic nuclei in brain stem

Autonomic nuclei in spinal cord

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

Visceral effectors

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 tract To skeletal muscles

To skeletal muscles

Lateral corticospinal tract

To skeletal muscles

Anterior corticospinal tract

Pyramids

MEDULLA OBLONGATA

MESENCEPHALON Lateral corticospinal tract

Anterior corticospinal tract

Motor nuclei of cranial nerves

Motor nuclei of cranial nerves

UMN

LMN

•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)

The corticospinal tracts are often called the pyramidal tracts because they form pyramid-shaped enlargements on the anterior surface of the medullaconcerned 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 cortexIn medulla oblongata they form the medullary pyramidsMost 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.

Corticospinal tracts

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 !!!!

The Subconscious Motor Tracts •Consists of four tracts involved in monitoring the subconscious motor control •Vestibulospinal tracts •Tectospinal tracts •Reticulospinal tracts •Rubrospinal tracts

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

Extrapyramidal tracts arise in the brainstem, but are under the influence of the cerebral cortex

•Send information from the inner ear to monitor position of the head • balance by facilitate the activity of the extensor muscles

Vestibular nuclei are situated in the pons and medulla beneath the floor of 4th ventricle

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

Vestibulospinal tracts

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 )

http://upload.wikimedia.org/wikipedia/commons/b/b2/Spinal_cord_tracts_-_English.svg

•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

Concerned with reflex postural movements in response to visual stimuli

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 tracts

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 ofhead & neck in response to visual stimuli )

• 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)

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

The reticulospinal tracts exert both somatic and autonomic control

Reticulospinal tracts

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

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

Motor Association areas

Basal nuclei

Cerebellum

Decision in frontal lobes

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.

Somatic Motor Control

Motor Associationareas

Primary motor cortex

Basal nuclei

Cerebellum

Lower motor neurons

Motor activity

Other nuclei of the medial and lateral pathways

Corticospinal pathway

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