Diencephalon Physiology

DIENCEPHALON PHYSIOLOGY

HYPOTHALAMUS, THALAMUS & EPITHALAMUS

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OUTLINE

• Introduction

• Hypothalamus

• Nuclei

• Endocrine & Vegetative functions

• Emotion and other behaviours

• Thalamus

• Nuclei

• Thalamus and Sensation

• Thalamus and Motor function

• Thalamic syndrome

• Epithalamus

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INTRODUCTION TO DIENCEPHALON

• Literally means “between-brain” (or b/n brain stem and cerebrum)

• The third ventricle is a narrow midline cavity within the diencephalon.

• The diencephalon, together with the telencephalon (cerebrum), constitutes the forebrain

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HYPOTHALAMUS

• Most inferior portion.

• Located below the thalamus.

• Less than 1% of total brain volume.

• Center for homeostasis.

• Centers for behavioral drives.

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HYPOTHALAMUS: NUCLEI

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HYPOTHALAMUS: ENDOCRINE AND VEGETATIVE FUNCTIONS

• Regulation of the Pituitary Gland

• Pituitary gland immediately inferior to the hypothalamus.

• Releasing and inhibiting hormones.

• Supraoptic: produce ADH.

• Paraventricular: Oxytocin.

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• Regulation of Circadian Rhythms

• Suprachiasmatic nuclei (SCN) is the major region.

• Electrical activity that oscillates automatically.

• SCN influenced by way of tracts from the retina.

• Synchronized to the day/night cycles.

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• Cardiovascular Regulation

• Different areas of the hypothalamus can cause many neurogenic effects.

• Posterior and lateral hypothalamus: inc AP and HR.

• Preoptic area: dec AP and HR.

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• Body Temperature Regulation

• Body’s “thermostat” is located.

• Preoptic-anterior hypothalamus.

• Correct deviations in a negative feedback fashion.

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• Body Water Regulation

• Creates sensation of thirst.

• Thirst center: lateral hypothalamus.

• Control excretion of water in the urine.

• Supraoptic nuclei: secretes ADH.

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• Gastrointestinal and Feeding Regulation

• Lateral hypothalamic area: hunger.

• Damage causes the animal to lose desire for food.

• Ventromedial nuclei: satiety center.

• If damaged the animal can’t be satiated obesity.

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HYPOTHALAMUS: EMOTION AND OTHER BEHAVIORS

• Interacts with limbic system to influence behavior and emotions.

• Stimulation of visceral responses during emotional situations.

• A major part of the limbic system.

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HYPOTHALAMUS: EMOTION AND OTHER BEHAVIORS

• Sends signals in three directions:

• Backward and downward to the brain stem.

• Upward toward many higher areas of the diencephalon and cerebrum.

• Into the hypothalamic infundibulum.

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THALAMUS

• Bilateral ovoid mass of gray matter

• 80% of diencephalon.

• Intermediate mass or interthalamic adhesion.

• Hypothalamic sulcus

• Thalamus (except reticular nucleus) --------Cerebral cortex

• All part of Cerebral Cortex -------- Thalamus

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THALAMIC NUCLEI ANATOMIC CLASSIFICATION:

Lateral Group of nuclei Medial Group of Nuclei

Anterior Group of Nuclei

Other Nuclei

Ventral Group Dorsal Group

Centromedian N.

Reticular Nucleus

Ventral Anterior/Lateral N.

Pulvinar N Midline Nucleus

Intralaminar Nuclei

Ventral Posterior N(VPL, VPM)

Lateral Posterior N.

Dorsomedial N.

Medial Geniculate body

Lateral Dorsal N.

Lateral Geniculate Body

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THALAMIC NUCLEI ANATOMIC CLASSIFICATION:

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FUNCTIONAL DIVISION OF THALAMIC NUCLEI

• Specific Relay Nuclei: project to specific parts of Cerebral cortex

Specific relay nuclei Cortical Projection sites

LGB Visual Cortex

MGB Auditory Cortex

VPL Somatosensory Cortex (trunk & limbs)

VPM Somatosensory Cortex (head)

Ventral Lateral (afferent from basal ganglia & Cerebellum)

Motor cortex

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• Association Nuclei: project to association cortices

• Afferent: from cerebral cortex

• Efferent: only to cortical association areas

Association Nuclei Cortical Projection sites

Lateral Dorsal Cingulate gyrus & Precuneus

Lateral Posterior Parietal Association cortex

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• Non-specific Nuclei: project to wide areas of the cortex.

• Afferent from Ascending reticular activating system

• Efferent to: Hypothalamus, Limbic system & Cerebral Cortex

• Function:- They increase cortical excitability and responsiveness

• Subcortical Nuclei: these nuclei don’t project to the cortex. E.g. Reticular nucleus (projects to other thalamic nuclei)

Nonspecific Nuclei Projection sites

Intralaminar Diffuse cortical areas

Anterior Nuclei Limbic structures

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FUNCTIONS OF THALAMUS

• Sensory functions

• Center for sexual sensation (perception of sexual sensations)

• Role in arousal & alertness reactions (because of its connections with nuclei of reticular formation)

• Center for reflex activity

• Center for integration of motor activity

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THALAMUS AND SENSATIONS

• The nervous pathways of somatic sensations:

• Dorsal Column pathway

• Spinothalamic Pathway

• Trigeminal pathway

• Other pathways related to thalamus:

• Visual pathway through LGN of thalamus

• Auditory system through MGN of thalamus

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THALAMUS AND SENSATIONS

• Trigeminal pathway:

• Sensation of head’s pain, temperature, tactile & proprioception.

• 1st order neurons: primarily from trigeminal ganglion to trigeminal nuclei.

• 2nd order neurons: from the nuclei to both ipsilateral and contralateral VPM of thalamus (primarily contralateral)

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THALAMUS AND SENSATION: SENSORY FUNCTIONS

• Relay center for sensations:

• Almost all sensations reach thalamic nuclei (particularly VPM, VPL) to be processed before reaching cerebral cortex

• Center for processing of sensory information and functional gateway for cerebral cortex

• All the peripheral sensory impulses reaching thalamus are integrated and modified before being sent to specific areas of cerebral cortex

• Determining the Affective nature of sensations

• Affective nature is the capacity to determine whether a sensation is pleasant or unpleasant and agreeable or disagreeable

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THALAMUS AND MOTOR FUNCTION

• Through the connections with cerebellum and basal ganglia, thalamus serves as a center for integration of motor functions.

• Role in arousal/wakefulness & alertness reactions (because of its connections with nuclei of reticular formation)

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THALAMIC OR DEJERINE-ROUSSY SYNDROME

• Cause blockage (due to thrombosis) in thalamogeniculate branch of posterior cerebral artery leading to infarction of posteroventral part of thalamus.

• Medial and Intralaminar nuclei are not affected in this syndrome.

• Symptoms are:

• Loss of all sensations (anesthesia) initially but weeks later pain sensibility regained due to facilitation of medial and intralaminar nuclei

• Astereognosis

• Sensory Ataxia (loss of kinesthetic sensation)

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THALAMIC OR DEJERINE-ROUSSY SYNDROME

• Symptoms continued….

• Thalamic Phantom limb: the illusion of limb loss

• Anosognosia

• Lack of awareness or denial of existance of a neurological defect or general illness or any disability

• Involuntary Movements like intention tremor

• Thalamic/Athetoid Hand

• Abnormal attitude of hand w/c wrist moderately flexes and all fingers hyperextend

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EPITHALAMUS

• Dorsal segment of the diencephalon.

• Choroid plexus over the third ventricle.

• Contains the pineal gland

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EPITHALAMUS: PINEAL GLAND

• Secretes the hormone melatonin.

• Helps regulate circadian rhythms.

• Lacks direct nervous connections to the rest of the brain.

• Highly innervated by SNS.

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• SCN of the hypothalamus regulates pineal secretion.

• Through hypothalamic control of SNS.

• Activity of the SCN + secretion of melatonin:

• Increase with darkness.

• Decrease with light.

• Neural pathways from retina depress activity of SCN.

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• Different functions of Melatonin:

• Powerful free-radical scavenger and wide spectrum antioxidant.

• Have positive immunological effect.

• Involved in the initiation of puberty.

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REFERENCE:

• John Mendoza, Anne Foundas; Clinical Neuroanatomy: A Neurobehavioral Approach

• K Sembulingam, Prema Sembulingam; Essentials of Medical Physiology 6th edition, 2012

• Martini, Timmons, Tallitsch; Human Anatomy 7th edition, 2012

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Diencephalon Physiology