sign in sign up
<<
Home , Cerebral aqueduct, Pyramidal tracts

Diencephalon, stem, , . Sensory and motor pathways.

Sándor Katz M.D.,Ph.D. Diencephalon - overview

• Located below the corpus callosum and above the

• Part of the telencephalon

• Forms the lateral wall of the Diencephalon Diencephalon - parts

• Epithalamus

•

• Subthalamus

• Hypothalamus

Pineal body Thalamus

• Almost all of the sensory pathways are relayed via the thalamus and project to the .

• Major descending motor tracts from the cerebral cortex generally bypass the thalamus. Thalamus Consequently, a lesion of the thalamus or its cortical projection fibers caused by a or other disease leads to sensory disturbances. Thalamic Nuclei

Specific nuclei: • Anterior nuclei (Recive afferent fibers from the mammillary body.)

• Medial nuclei (Recive afferent fibers from ventral and intralaminar nuclei, hypothalamus, mesencephalon and globus pallidus.)

• Ventral nuclei: VA, VL, VPL, VPM (Recive afferent fibers- position sense, vibration, pressure, touch, and temperature- from the trunk and limbs.)

• Dorsal nuclei: LD, LP, Pulvinar (Recive afferent fibers from other thalamic nuclei.)

They have direct connections with specific areas of the cerebral cortex.

Nonspecific nuclei: have no direct connections with the cerebral cortex. Part of the general arousal system, they are connected directly to the . Thalamic Nuclei - Metathalamus

• Lateral geniculate body (component of the visual pathway)

• Medial geniculate body (component of the auditory pathway)

They belong to the category of specific thalamic nuclei. Subthalamus

• Subthalamic nuclei

• Zona incerta

• Globus pallidus Hypothalamus - overview

• It is the lowest level of the diencephalon, situated below the thalamus.

• The hypothalamus is the command center for all autonomic functions in the body.

• Over 30 hypothalamic nuclei located in the lateral wall and floor of the third ventricle. Hypothalamus Hypothalamic Nuclei

Anterior (rostral) group: Hormone synthesis. • Preoptic nucleus • Paraventricular nucleus • Supraoptic nucleus

Middle (tuberal) group: Controls hormone release from the anterior lobe of the pituitary gland. • Dorsomedial nucleus • Ventromedal nucleus • Tuberal nuclei

Posterior (mammillary) group: Activates the sympathetic nervous system when stimulated. • Posterior nucleus • Mammillary nuclei of mammillary body Functions of the hypothalamus

Region or nucleus Function

• Anterior preoptic region • Maintain constant body temperature. Lesion: central hypothermia.

• Midanterior and posterior • Activate sympathetic nervous system regions

• Activate parasympathetic nervous system • Paraventricular and anterior regions • Oxitocin production - uterine • Supraoptic and paraventricular contractions. Vasopressin - regulation of water balance, lesion: diabetes insipidus. nuclei

• Regulate appetite and food intake. • Anterior nuclei Lesion: obesity or anorexia.

Brain stem – overview

It is divided macroscopically into three parts:

• Mesencephalon

•

• Brain stem Midbrain - mesencephalon

Tectum (quadrigeminal plate): • Superior colliculi (part of the visual pathway) • Inferior colliculi (part of the auditory pathway)

Cerebral aqueduct

Tegmentum (nuclei, ascending-, descending pathways)

Cerebral peduncles Midbrain Pons and medulla oblongata

Dorsal wall is formed by the .

Pons

• Respiratory and circulatory regulation • Swallowing center • Vomiting center • Reticular activating center (wake up function) • Effect on the Cerebellum - overview Cerebellum - overview

Cerebellum - peduncles Cerebellum - functions Cerebellum - histology Cerebellum - histology Cerebellum - histology Basal Ganglia

• Modulate and regulate specific cortical functions • Preparation, planning, execution of motor activity

• Related nuclei: • of diencephalon • of mesencephalon • VA, VL thalamic nuclei

Sensory pathways - receptors Sensory pathways - receptors

Meissner’s corpuscle

Pacinian corpuscle Sensory pathways (ascending tracts)- Fasciculus gracilis and cuneatus

• Both tracts convey fibers for position sense (conscious proprioception) and fine cutaneous sensation (touch, vibration, fine pressure sense, two-point discrimination) =EPICRITIC SENSATION.

• The fasciculus gracilis carries fibers only from the lower limbs, while the fasciculus cuneatus carries fibers only from the upper limbs, therefore it is not presented in the below the T3 level. Sensory pathways(ascending tracts)- Fasciculus gracilis and cuneatus

• Muscle-, tendon receptors and Vater-Pacini corpuscles for conscious proprioception. Receptors about the hair follicles and additional receptors mediate the fine touch sensation of the skin. • 1st neuron: pseudounipolar neurons in the . • The ascend ipsilaterally in the posterior funiculi to the gracile and cuneate nuclei (2nd neuron) in the medulla oblongata. • The axons cross in the to the VPL of thalamus (3rd neuron). • The axons of the third neurons terminate in the primary somatosensory cortex, located in the .

Sensory pathways(ascending tracts)- Spinothalamic tracts

• The anterior is the pathway for crude touch and pressure sensation, while the lateral spinothalamic tract conveys pain, temperature, tickle, itch and sexual sensation =PROTOPATHIC SENSATION. Sensory pathways(ascending tracts)- Lateral spinothalamic tract

• Free nerve endings in the skin function as receptors for pain and temperature sensation. • The cell bodies of these nerve endings are located in the dorsal root ganglion (1st neuron). • The central processes of first neurons pass through the dorsal root into the of the spinal cord (dorsolateral tract of Lissauer), then they terminate on funiculate neurons in the gray matter (2nd neuron). • The axons of second neurons cross in the anterior commissure in the corresponding spinal segment and ascend in the anterolateral funiculus on the opposite (contralateral)side. • They terminate in the VPL of thalamus (3rd neuron). • The axons of third neurons radiate to the primary somatosensory cortex in the postcentral gyrus. Sensory pathways(ascending tracts)- Anterior spinothalamic tract

• Impulses from tactile corpuscules and from receptors about the hair follicles. • The cell bodies of these axons are located in the dorsal root ganglion (1st neuron). • The axons pass through the dorsal root and enter the gray matter where terminate on funiculate neurons (2nd neuron). • The axons of second neurons cross in the anterior commissure and ascend in the opposite . • The axons terminate in the VPL of thalamus (3rd neuron). • The axons of third neurons radiate to the primary somatosensory cortex in the postcentral gyrus. Sensory pathways(ascending tracts)- Spinocerebellar tracts Sensory pathways(ascending tracts)- Posterior

• Muscle spindles and tendon receptors convey proprioceptive information from lower body part to pseudounipolar neurons in the dorsal root ganglion (1st neuron). • The axons terminate at the dorsal nucleus (Clark’s) of gray matter (2nd neuron), which spans spinal cord segments C8 to L3. • The axons of the second neurons ascend ipsilaterally to the cerebellum, entering through the inferior cerebellar peduncle. • The axons terminate with mossy fibers in the stratum granulosum of cerebellar cortex. Sensory pathways(ascending tracts)- Anterior spinocerebellar tract

• Muscle spindles and tendon receptors convey proprioceptive information from lower body part to pseudounipolar neurons in the dorsal root ganglion (1st neuron). • The axons terminate at the dorsal gray horn (2nd neuron), which spans spinal cord segments L4 to S3. • The axons of the second neurons cross in the anterior commissure and ascend contralaterally to the cerebellum, entering through the superior cerebellar peduncle. • The axons terminate with mossy fibers in the stratum granulosum of cerebellar cortex. Motor pathways(descending tracts)-

• The most important pathway for voluntary motor function. • Some of its axons, the corticonuclear fibers terminate at the cranial nerve nuclei. • Others, the corticospinal fibers terminate on the motor neurons in anterior horn of gray matter. • A third group, the corticoreticular fibers are distributed to nuclei of reticular formation. Motor pathways(descending tracts)- Pyramidal (corticospinal) tracts

• The tracts originate in the at the pyramidal cells (1st neuron). • The corticospinal fibers pass through the , continuing into the brainstem and spinal cord. • The fibers descend to the pyramidal decussation in the medulla oblongata, where approximately 80% of them cross to the opposite side. The fibers continue into the spinal cord where they form the lateral , which has somatotopic organization: the fibers for the sacral cord are the most lateral, while the fibers for the cervical cord are the most medial. Motor pathways(descending tracts)- Pyramidal (corticospinal) tracts

• The remaining 20% of corticospinal fibers continue to descend without crossing, forming the anterior corticospinal tract, which borders the ventral median fissure. • The anterior corticospinal tract is particularly well developed in the cervical cord, but is not present in the lower thoracic, lumbar or sacral cords. • The axons of the pyramidal cells terminate via intercalated cells on alpha and gamma motor neurons, Renshaw cells and (2nd neuron). • Most fibers of ant. corticospinal tract cross at the segmental level to terminate on the same motor neurons (3rd neuron) as the lateral corticospinal tract. Motor pathways(descending tracts)- Extrapyramidal tracts

• Unlike the pyramidal tract, which controls conscious, voluntary motor activities, the extrapyramidal motor system (cerebellum, basal ganglia and motor nuclei of the brainstem) is necessary for autonomic and learned motor processes (e.g., walking, running, cycling).

Some important tracts: • : coordinates head and eye movements. • : mediation of voluntary movements. Ascending and descending tracts- overview Thank you for your attention.

References: A.D.A.M. McGraw-Hill Company’s pictures WebPath Nature Reviews Pearson Education Sinauer Associate studyblue.com