Podcast
Questions and Answers
What is the primary function of the suprachiasmatic nucleus?
What is the primary function of the suprachiasmatic nucleus?
Which hormones are synthesized by the supraoptic and paraventricular nuclei respectively?
Which hormones are synthesized by the supraoptic and paraventricular nuclei respectively?
What is the role of the hypophysial portal system?
What is the role of the hypophysial portal system?
How does vasopressin primarily affect the kidneys?
How does vasopressin primarily affect the kidneys?
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What initiates the production of more oxytocin during breastfeeding?
What initiates the production of more oxytocin during breastfeeding?
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Which nucleus is responsible for the secretion of releasing hormones or inhibiting factors?
Which nucleus is responsible for the secretion of releasing hormones or inhibiting factors?
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What is the main vascular feature of the hypophysial portal system?
What is the main vascular feature of the hypophysial portal system?
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What does the nucleus ventromedialis control in the hypothalamus?
What does the nucleus ventromedialis control in the hypothalamus?
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Which structure does oxytocin primarily act upon during labor?
Which structure does oxytocin primarily act upon during labor?
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Which of the following roles does the anterior part of the hypothalamus perform?
Which of the following roles does the anterior part of the hypothalamus perform?
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What is the primary function of the anterior part of the thalamus?
What is the primary function of the anterior part of the thalamus?
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Which part of the thalamus is responsible for the integration of sensory information?
Which part of the thalamus is responsible for the integration of sensory information?
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What role does the pulvinar play within the thalamus?
What role does the pulvinar play within the thalamus?
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What might result from damage to the VPL and VPM nuclei?
What might result from damage to the VPL and VPM nuclei?
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Which nucleus in the thalamus is specifically associated with auditory pathways?
Which nucleus in the thalamus is specifically associated with auditory pathways?
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The internal medullary lamina of the thalamus is responsible for dividing it into which parts?
The internal medullary lamina of the thalamus is responsible for dividing it into which parts?
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What major function does the hypothalamus perform related to the endocrine system?
What major function does the hypothalamus perform related to the endocrine system?
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Which zones divide the hypothalamic nuclei?
Which zones divide the hypothalamic nuclei?
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How does the anterior hypothalamic nucleus respond to heat?
How does the anterior hypothalamic nucleus respond to heat?
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What effect can stimulation of the posterior hypothalamic nuclei have?
What effect can stimulation of the posterior hypothalamic nuclei have?
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Which component of the hypothalamus stimulates eating behavior?
Which component of the hypothalamus stimulates eating behavior?
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What is the significance of lesions in the thalamic nuclei?
What is the significance of lesions in the thalamic nuclei?
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The reticular nucleus of the thalamus is thought to influence which of the following?
The reticular nucleus of the thalamus is thought to influence which of the following?
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Study Notes
Thalamus
- Egg-shaped mass of grey matter forming the major part of the diencephalon
- Located on either side of the third ventricle
- Connected to each other by the interthalamic adhesion, which is not a commissural pathway
- Inferior end bordered by the tegmentum of the midbrain
- Anterior end extends up to the interventricular foramen
- Posterior part is called the pulvinar
- Medial surface forms part of the lateral wall of the third ventricle
- Hypothalamic sulcus determines the border between the thalamus and hypothalamus
- Superior part covered by a thin layer of white matter called the stratum zonale
- Lateral part has a layer of white matter called the external medullary lamina
- A Y-shaped vertical layer of white matter extends within the thalamus known as the internal medullary lamina
- Internal medullary lamina divides the thalamus into three parts:
- Anterior part
- Medial part
- Lateral part
- Anterior part:
- Contains the anterior thalamic nuclei
- Mamillothalamic tract terminates here
- Has reciprocal connections with the cingulate gyrus and hypothalamus
- Functions related to the limbic system and deal with emotions and recent memory
- Medial part:
- Contains the large dorsomedial nucleus
- Has reciprocal connections with the prefrontal cortex and hypothalamic nuclei
- Has connections with all other thalamic nuclei
- Responsible for integration of sensory information
- Lateral part:
- Subdivided into a dorsal tier and a ventral tier
- Dorsal tier:
- Comprise the pulvinar
- Connections and functions are not well identified
- Ventral tier:
- Ventral anterior nucleus:
- Influences the activity of the motor cortex
- Ventral lateral nucleus:
- Influences the activity of the motor cortex
- Ventral posterior nucleus (subdivided into VPL and VPM nuclei):
- VPM receives the trigeminal and gustatory fibers
- VPL receives the medial and spinal lemnisci
- Efferents of these nuclei form the thalamocortical projections
- Pass through the posterior limb of the internal capsule and corona radiata to reach to the primary somatic sensory areas
- Ventral anterior nucleus:
- Intralaminar nuclei:
- Small collections of nuclei within the internal medullary lamina
- Probably influence the levels of consciousness and alertness
- Midline nuclei:
- Consists of groups of neurons adjacent to the third ventricle and interthalamic adhesion
- Function is unknown
- Reticular nucleus:
- Lie between the external medullary lamina and the posterior limb of internal capsule
- Probably deals with the mechanisms through which the cerebral cortex regulates thalamic activity
- Medial geniculate body:
- Related with the auditory pathways
- Afferent fibers to this nucleus from the inferior colliculus form the brachium of inferior colliculus
- Efferent fibers form the auditory radiation
- Information from both ears is represented here, the contralateral being predominant
- Lateral geniculate body:
- Related with the visual pathway
- All fibers of the optic tract terminate here, except those passing to the pretectal nuclei
- Afferent fibers start from the ganglionic cell layer of the retina
- Contralateral parts of each visual field are represented here
- Efferent fibers form the optic radiation
Hypothalamus
- Lies below the thalamus and forms the floor and the inferior part of the lateral walls of the third ventricle
- Main function is to control the autonomic nervous system and the endocrine system
- The part of the hypothalamus at the base of the 3rd ventricle, between the optic chiasm and mamillary bodies is called the tuber cinerum
- The median eminence is the inferior protrusion of the tuber cinerum
- Median eminence inferiorly continues with the infundibulum which is continuous with the neurohypophysis
- Hypothalamic nuclei divided into medial and lateral zones
- Mamillary bodies are part of the hypothalamus and are functionally related with the limbic system
Medial zone
- Part of the preoptic nucleus
- Part of the suprachiasmatic nucleus
- Anterior nucleus
- Paraventricular nucleus
- Dorsomedial nucleus
- Ventromedial nucleus
- Infundibular (arcuate) nucleus
- Posterior nucleus
Lateral zone
- Part of the preoptic nucleus
- Part of the suprachiasmatic nucleus
- Supraoptic nucleus
- Lateral nucleus
- Tuberomamillary nucleus
- Lateral tuberal nuclei
Functions of the Hypothalamus
- Autonomic control:
- Anterior and preoptic nuclei are involved in parasympathetic function
- Posterior and lateral nuclei are involved in sympathetic function
- The autonomic impulses from the hypothalamus are distributed to the brainstem and the spinal cord through the descending autonomic fibers
- Endocrine control:
- Certain hypothalamic nuclei control the pituitary gland by secreting the releasing-inhibiting hormones:
- Growth hormone releasing hormone and inhibiting hormone
- Prolactin releasing hormone and inhibiting hormone
- Corticotropin releasing hormone
- Thyrotropin releasing hormone
- Luteinizing hormone releasing hormone, and follicule-stimulating releasing hormone
- Certain hypothalamic nuclei control the pituitary gland by secreting the releasing-inhibiting hormones:
- Neurosecretion:
- Supraoptic nucleus synthesizes antidiuretic hormone (ADH) (vasopressin)
- Paraventricular nucleus synthesizes oxytocin
- Temperature regulation:
- Anterior hypothalamic nuclei response to heat - stimulation induces mechanism to dissipate heat (vasodilatation of the skin vessels and sweating)
- Posterior hypothalamic nuclei response to cold - stimulation induces mechanism to preserve heat
- Regulation of food and water intake:
- Lateral hypothalamic nuclei initiate eating and increase food intake (hunger center)
- Bilateral damage to this center will cause anorexia with consequent loss of body weight
- Lateral hypothalamic nuclei also increase the water intake (thirst center)
- Medial nuclei inhibit eating and reduces food intake (satiety center)
- Bilateral damage to this center will cause insatiable appetite causing obesity
- Supraoptic nucleus synthesizes vasopressin (antidiuretic hormone-ADH)
- ADH causes increase in the reabsorption of water from kidneys
- Lesions of the suproptic nucleus cause diabetes insipidus
- Patient passes large volumes of urine, and therefore drinks large quantities of water
- Lateral hypothalamic nuclei initiate eating and increase food intake (hunger center)
- Emotion and behavior:
- Certain aspects of emotions are expressed by the hypothalamus
- Control of circadian (biological) rhythms:
- Certain rhythms are daily fluctuations of certain bodily functions such as sleeping, wakefulness, body temperature, hunger etc.
- Suprachiasmatic nucleus most probably plays an important role in regulation of the biological rhythms
Connections of the hypothalamus with the hypophysis
- Hypothalamus is connected to the hypophysis by two pathways:
- Hypothalamohypohysial tract
- Hypophysial portal system
Hypothalamohypohysial tract
- Supraoptic and paraventricular nuclei synthesize vasopressin and oxytocin respectively
- These hormones are carried to the posterior lobe of hypophysis with carrier proteins called the neurophysins along the axons forming the supraopticohypophyseal tract
- The hormones are absorbed into the bloodstream through the fenestrated capillaries of the posterior lobe of hypophysis
- Vasopressin causes increased absorption of water from the distal convoluted tubules and collecting tubules of the kidneys
- Oxytocin:
- Stimulates the contraction of the smooth muscle of the uterus during labor
- Causes contraction of the smooth muscles around the ducts of the mammary glands
- When baby suckles at the breast, a nervous reflex stimulates hypothalamus to produce more oxytocin
Hypophysial portal system
- Neurosecretory cells situated mainly in the medial zone (especially the infundibular - arcuate - nucleus) produce releasing hormones and release-inhibiting hormones
- These hormones are carried by their axons to the median eminence and infundibulum of hypophysis
- In the hypophysis, the hormones are released onto the the fenestrated capillaries of the hypophysial portal system
- Superior and inferior hypophyseal arteries supply the hypophysis
- Branches of the superior hypophyseal artery terminates as capillaries, which drain into the vascular sinusoids within the median eminence and infundibulum
- These sinusoids are drained by the hypophysial portal veins which in turn drains into the sinusoids of the adenohypophysis
- Vascular sinusoids within the median eminence and infundibulum, the hypophysial portal veins and the sinusoids of the adenohypophysis form the hypophyseal portal system
- Through this system, the releasing hormones and the release-inhibiting hormones are carried to the adenohypophysis where they either stimulate or inhibit the secretion of the adenohypophysis hormones
Clinical note
- Lesions of the thalamus, also known as thalamic syndrome, can be tricky to diagnose
- Thalamic nuclei are small so lesions producing highly specific effects are uncommon
- The thalamus is immediately bounded by the internal capsule on its lateral side and is in close proximity to the deep motor nuclei (basal ganglia) of the cerebral hemisphere (putamen, caudate and globus pallidus) so that thalamic lesions frequently are accompanied by symptoms from damage to these other structures
- If the damage includes VPL and VPM, contralateral hemianesthesia usually results
- Typically, all somatic sensory modalities are affected: light touch, conscious proprioception, 2-point discrimination & vibration, and pain & temperature
- This loss of all somatic sensory modalities is an important diagnostic sign for thalamic damage
- Sometimes, after a period of recovery from damage to VPL and VPM (days to months), hyperalgesia (exaggerated unpleasant or painful sensation resulting from mild cutaneous stimulation) or spontaneous pain with no apparent stimulation (causalgia) is seen
- Such pain can be severe and intractable
- Hyperalgesia and spontaneous pain do not occur with lesions confined to the cerebral hemispheres (cortex, internal capsule, or deep nuclei)
- If the damage extends into the VA/VL nuclei complex movement disorders can result
- The movement disorders can be reminiscent of cerebellar damage (ataxia and intention tremor) and/or basal ganglia damage (choreoathetoid movements)
- This reflects the fact that both the cerebellum and basal ganglia project to VA and VL
- All such problems occur contralateral to the side of the lesion
- The above signs are termed as thalamic syndrome and usually results from a lesion that is vascular in origin as can be seen in occlusion of the posterior cerebral or its thalamic branches
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