Diencephalon, brain stem, cerebellum, basal ganglia

Diencephalon, brain stem, cerebellum, basal ganglia.

Sensory and motor pathways.

Sándor Katz M.D.,Ph.D.

Diencephalon - overview

• Located below thecorpus callosum andabove the midbrain

• Part of thetelencephalon

• Forms the lateral wallof the third ventricle

Diencephalon

Diencephalon - parts

• Epithalamus

• Thalamus

• Subthalamus

• Hypothalamus

Pineal body

Thalamus

• Almost all of the sensorypathways are relayed via thethalamus and project to thecerebral cortex.

• Major descending motortracts from the cerebralcortex generally bypass thethalamus.

Thalamus

Consequently, a lesionof the thalamus or itscortical projectionfibers caused by astroke or other diseaseleads to sensorydisturbances.

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, pain andtemperature- from the trunk and limbs.)

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

They have direct connections with specificareas 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 brainstem.

Thalamic Nuclei - Metathalamus

• Lateral geniculate body(component of the visualpathway)

• Medial geniculate body(component of the auditorypathway)

They belong to the category ofspecific thalamic nuclei.

Subthalamus

• Subthalamic nuclei

• Zona incerta

• Globus pallidus

Hypothalamus - overview

• It is the lowest level of thediencephalon, situated belowthe thalamus.

• The hypothalamus is thecommand center for allautonomic functions in thebody.

• Over 30 hypothalamic nucleilocated in the lateral wall andfloor of the third ventricle.

Hypothalamus

Hypothalamic Nuclei

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

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

Posterior (mammillary) group: Activatesthe sympathetic nervous system whenstimulated.• Posterior nucleus• Mammillary nuclei of mammillary body

Functions of the hypothalamus

Region or nucleus

• Anterior preoptic region

• Midanterior and posteriorregions

• Paraventricular and anteriorregions

• Supraoptic and paraventricularnuclei

• Anterior nuclei

Function

• Maintain constant body temperature.Lesion: central hypothermia.

• Activate sympathetic nervous system

• Activate parasympathetic nervous system

• Oxitocin production - uterine contractions. Vasopressin - regulation of water balance, lesion: diabetes insipidus.

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

Brain stem – overview

It is divided macroscopicallyinto three parts:

• Mesencephalon

• Pons

• Medulla oblongata

Brain stem

Midbrain - mesencephalon

Tectum (quadrigeminal plate):

• Superior colliculi (part of thevisual pathway)

• Inferior colliculi (part of theauditory pathway)

Cerebral aqueduct

Tegmentum (nuclei, ascending-,descending pathways)

Cerebral peduncles

Midbrain

Pons and medulla oblongata

Dorsal wall is formedby the rhomboid fossa.

Pons

Reticular Formation

• Respiratory and circulatory regulation

• Swallowing center

• Vomiting center

• Reticular activating center (wake upfunction)

• Effect on the motor system

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:

• subthalamic nucleus of diencephalon

• substantia nigra 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 positionsense (conscious proprioception) andfine cutaneous sensation (touch,vibration, fine pressure sense, two-pointdiscrimination) =EPICRITICSENSATION.

• The fasciculus gracilis carries fibersonly from the lower limbs, while thefasciculus cuneatus carries fibers onlyfrom the upper limbs, therefore it is notpresented in the spinal cord below theT3 level.

Sensory pathways(ascending tracts)-Fasciculus gracilis and cuneatus

• Muscle-, tendon receptors and Vater-Pacinicorpuscles for conscious proprioception.Receptors about the hair follicles andadditional receptors mediate the fine touchsensation of the skin.

• 1st neuron: pseudounipolar neurons in thedorsal root ganglion.

• The axons ascend ipsilaterally in theposterior funiculi to the gracile and cuneatenuclei (2nd neuron) in the medullaoblongata.

• The axons cross in the medial lemniscus tothe VPL of thalamus (3rd neuron).

• The axons of the third neurons terminate inthe primary somatosensory cortex, located inthe postcentral gyrus.

Sensory pathways(ascending tracts)-Spinothalamic tracts

• The anteriorspinothalamic tract is thepathway for crude touchand pressure sensation,while the lateralspinothalamic tractconveys pain,temperature, tickle, itchand sexual sensation=PROTOPATHICSENSATION.

Sensory pathways(ascending tracts)-Lateral spinothalamic tract

• Free nerve endings in the skin function as receptorsfor pain and temperature sensation.

• The cell bodies of these nerve endings are located inthe dorsal root ganglion (1st neuron).

• The central processes of first neurons pass throughthe dorsal root into the white matter of the spinalcord (dorsolateral tract of Lissauer), then theyterminate on funiculate neurons in the gray matter(2nd neuron).

• The axons of second neurons cross in the anteriorcommissure in the corresponding spinal segment andascend in the anterolateral funiculus on the opposite(contralateral)side.

• They terminate in the VPL of thalamus (3rdneuron).

• The axons of third neurons radiate to the primarysomatosensory cortex in the postcentral gyrus.

Sensory pathways(ascending tracts)-Anterior spinothalamic tract

• Impulses from tactile corpuscules and fromreceptors about the hair follicles.

• The cell bodies of these axons are located inthe dorsal root ganglion (1st neuron).

• The axons pass through the dorsal root andenter the gray matter where terminate onfuniculate neurons (2nd neuron).

• The axons of second neurons cross in theanterior commissure and ascend in theopposite anterior funiculus.

• The axons terminate in the VPL of thalamus(3rd neuron).

• The axons of third neurons radiate to theprimary somatosensory cortex in thepostcentral gyrus.

Sensory pathways(ascending tracts)-Spinocerebellar tracts

Sensory pathways(ascending tracts)-Posterior spinocerebellar tract

• Muscle spindles and tendonreceptors convey proprioceptiveinformation from lower body partto pseudounipolar neurons in thedorsal root ganglion (1st neuron).

• The axons terminate at the dorsalnucleus (Clark’s) of gray matter(2nd neuron), which spans spinalcord segments C8 to L3.

• The axons of the second neuronsascend ipsilaterally to thecerebellum, entering through theinferior cerebellar peduncle.

• The axons terminate with mossyfibers in the stratum granulosum ofcerebellar cortex.

Sensory pathways(ascending tracts)-Anterior spinocerebellar tract

• Muscle spindles and tendonreceptors convey proprioceptiveinformation from lower body partto pseudounipolar neurons in thedorsal root ganglion (1st neuron).

• The axons terminate at the dorsalgray horn (2nd neuron), whichspans spinal cord segments L4 to S3.

• The axons of the second neuronscross in the anterior commissureand ascend contralaterally to thecerebellum, entering through thesuperior cerebellar peduncle.

• The axons terminate with mossyfibers in the stratum granulosum ofcerebellar cortex.

Motor pathways(descending tracts)-Pyramidal tracts

• The most important pathway forvoluntary motor function.

• Some of its axons, the corticonuclearfibers terminate at the cranial nervenuclei.

• Others, the corticospinal fibersterminate on the motor neurons inanterior horn of gray matter.

• A third group, the corticoreticularfibers are distributed to nuclei ofreticular formation.

Motor pathways(descending tracts)-Pyramidal (corticospinal) tracts

• The tracts originate in the motor cortexat the pyramidal cells (1st neuron).

• The corticospinal fibers pass through theinternal capsule, continuing into thebrainstem and spinal cord.

• The fibers descend to the pyramidaldecussation in the medulla oblongata,where approximately 80% of them crossto the opposite side. The fibers continueinto the spinal cord where they form thelateral corticospinal tract, which hassomatotopic organization: the fibers forthe sacral cord are the most lateral, whilethe fibers for the cervical cord are themost medial.

Motor pathways(descending tracts)-Pyramidal (corticospinal) tracts

• The remaining 20% of corticospinalfibers continue to descend withoutcrossing, forming the anteriorcorticospinal tract, which borders theventral median fissure.

• The anterior corticospinal tract isparticularly well developed in thecervical cord, but is not present in thelower thoracic, lumbar or sacral cords.

• The axons of the pyramidal cellsterminate via intercalated cells on alphaand gamma motor neurons, Renshawcells and interneurons (2nd neuron).

• Most fibers of ant. corticospinal tractcross at the segmental level to terminateon the same motor neurons (3rdneuron) as the lateral corticospinaltract.

Motor pathways(descending tracts)-Extrapyramidal tracts

• Unlike the pyramidal tract, whichcontrols conscious, voluntary motoractivities, the extrapyramidalmotor system (cerebellum, basalganglia and motor nuclei of thebrainstem) is necessary forautonomic and learned motorprocesses (e.g., walking, running,cycling).

Some important tracts:

• Tectospinal tract: coordinates headand eye movements.

• Rubrospinal tract: mediation ofvoluntary movements.

Ascending and descending tracts-overview

Thank you for your attention.References:A.D.A.M.McGraw-Hill Company’s picturesWebPathNature ReviewsPearson EducationSinauer Associatestudyblue.com