CHAPTER 13- the hypothalamus and its connections
Hypothalamus
- Small (about 0.3% of the total brain)
- Controls the autonomic nervous system and the endocrine system
- Indirectly controls body homeostasis
- Lies in the center of the limbic system
- Site of numerous converging and diverging neuronal pathways and through its adequate blood supply it is able to sample the blood chemistry
- Makes appropriate controlling responses following the integration of its nervous and chemical inputs
- Part of the diencephalon that extends from the region of the optic chiasma to the caudal border of the mammillary bodies
- Lies below the thalamus and forms the floor and the inferior part of the lateral walls of the third ventricle
- Caudally, the hypothalamus merges into the tegmentum of the midbrain
- Lateral boundary is formed by the internal capsule
Preoptic area of the hypothalamus
- Area anterior to the hypothalamus
- Extends forward from the optic chiasma to the lamina terminalis and the anterior commissure
When observed from below, the hypothalamus is seen to be related to the following structures form anterior to posterior:
- Optic chiasma
- tuber cinereum and the infundibulum
- mammillary bodies
HYPOTHALAMIC NUCLEI
Microscopically:
- the hypothalamus is composed of small nerve cells that are arranged in groups or nuclei, many of which are not clearly segregated from one another
Preoptic area
- Included as part of the hypothalamus for functional reasons
- The nuclei are divided by an imaginary plane into medial and lateral zones
- Fornix and mammillothalamic tract- Lies within the plane and serves as markers
Medial Zone
The following hypothalamic nuclei are recognized from anterior to posterior:
- Part of the preoptic nucleus
- Anterior nucleus (merges with the preoptic nucleus)
- Part of the suprachiasmatic nucleus
- Paraventricular nucleus
- dorsomedial nucleus
- ventromedial nucleus
- Infundibular (arcuate) nucleus
- Posterior nucleus
Lateral Zone
From anterior to posterior:
- Part of the preoptic nucleus
- Part of the suprachiasmatic nucleus
- supraoptic nucleus
- lateral nucleus
- Tuberomamillary nucleus
- The lateral tuberal nuclei
Modern technology: Histochemical, immunochemical and anterograde and retrograde tracer studies—precisely defines groups of neurons and their connections
- Most of the hypothalamic nuclei have ill-defined boundaries
Hypothalamic Lines of Communication
Hypothalamus receives information from the rest of the body through:
- Nervous connections
- Bloodstream
- CSF
- The neurons of the hypothalamic nuclei respond and exert their control via the same route
- CSF may serve as a conduit between the neurosecretory cells of the hypothalamus and distant sites of the brain
AFFERENT NERVOUS CONNECTIONS OF THE HYPOTHALAMUS
Hypothalamus
- Lies in the center of the limbic system
- Receives many afferent fibers from:
o the viscera
o Olfactory mucous membrane
o Cerebral cortex
o Limbic system
- The afferent connections are numerous and complex
- Main pathways are as follows:
- Somatic and visceral afferents
- General somatic sensation, gustatory and visceral sensations, reach the hypothalamus through collateral branches of the lemniscal afferent fibers and the tractus solitarius and through the reticular formation
- Visual Afferents
- Leave the optic chiasma and pass to the suprachiasmatic nucleus
- Olfaction
- Travels through the medial forebrain bundle
- Auditory afferents
- Not been identified
- Corticohypothalamic fibers
- Arise from the frontal lobe of the cerebral cortex and pass directly to the hypothalamus
- Hippocampohypothalamic fibers
- Pass from hippocampus through the fornix to the mammillary body
- Hypothalamus is the main output pathway of the limbic system
- Amygdalohypothalamic fibers
- Pass from the amygdaloid complex to the hypothalamus through the stria terminalis and by a route that passes inferior to the lentiform nucleus
- Thalamohypothalamic fibers
- Arise from the dorsomedial and midline thalamic nuclei
- Tegmental fibers- arise from the midbrain
EFFERENT NERVOUS CONNECTIONS OF THE HYPOTHALAMUS
Main pathways:
- Descending fibers to the brainstem and spinal cord
- Influence the peripheral neurons of the ANS
- Descend through a series of neurons in the reticular formation
- The hypothalamus is connected to the parasympathetic nuclei of the oculomotor, facial, glossopharyngeal, and vagus nerves in the brainstem
- The reticulospinal fibers connect the hypothalamus with sympathetic cells of origin in the lateral gray horns of the first thoracic segment to the second lumbar segment of the spinal cord and the sacral parasympathetic outflow at the level of the second, third, and fourth sacral segments of the spinal cord
- mammillothalamic tract
- Arises in the mammillary body
- Terminates in the anterior nucleus of the thalamus
- Here the pathway is relayed to the cingulated gyrus
- mammillotegmental tract
- Arises from the mammillary body and terminates in the cells of the reticular formation in the tegmentum of the midbrain
- Limbic system (multiple pathways)
CONNECTIONS OF THE HYPOTHALAMUS WITH THE HYPOPHYSIS CEREBRI
- The hypothalamus is connected to the hypophysis cerebri (pituitary gland) by 2 pathways:
1. Nerve fibers that travel from the supraoptic and paraventricular nuclei to the posterior lobe of the hypophysis
2. Long and short portal blood vessels that connect sinusoids in the median eminence and infundibulum with capillary plexuses in the anterior lobe of the hypophysis
- these pathways enable the hypothalamus to influence the activities of the endocrine glands
HYPOTHALAMOHYPOPHYSEAL TRACT
Vasopressin and oxytocin
- Synthesized in the nerve cells of the supraoptic and paraventricular nuclei
- Passed along the axons together with carrier proteins called neurophysins, and are released at the axon terminals
· Here the hormones are absorbed into the bloodstream in fenestrated capillaries of the posterior lobe of the hypophysis
Vasopressin
- Antidiuretic hormone
- Produced mainly in the nerve cells of the supraoptic nucleus
- Function: causes vasoconstriction
- Also has an important antidiuretic function, causing increased absorption of water in the distal convoluted tubules and collecting tubules of the kidney
Oxytocin
- produced mainly in the paraventricular nucleus
- Stimulates the contraction of the smooth muscle of the uterus and causes contraction of the myoepithelial cells that surround the alveoli and ducts of the breast
- Produced in large amounts during the end of pregnancy
- Stimulates labor contractions of the uterus
- Stimulated by the suckling of the nipples by the babies promoting contraction of the myoepithelial cells and assists in the expression of the milk from the breasts
Supraoptic nucleus
- Produces vasopressin
- Act as an osmoreceptor
- Osmotic pressure increase vasopressin increase causing reabsorption of water
Hypophyseal Portal System
Releasing hormones and release inhibitory hormones
- produced by the neurosecretory cells situated mainly in the medial zone of the hypothalamus
- Hormones are packaged into granules and transported along the axons of these cells in the medial eminence and infundibulum
o Here the granules are released by exocytosis onto fenestrated capillaries at the upper end of the hypophyseal portal system
* HPS is formed from the superior hypophyseal arterty, which is a branch of the internal carotid artery
* The portal system carries the releasing hormones and the release-inhibitng hormones to the secretory cells of the anterior lobe of the hypophysis
Releasing hormones stimulate the production and release of
- ACTH
- FSH
- LH
-Thyrotropic hormone or thyroid stimulating hormone (TSH)
- Growth hormone (GH)
Release of inhibiting hormones inhibits the release of:
- melanocyte-stimulating hormone (MSH)
- Luteotropic hormone (LTH)
Luteotropic hormone (LTH)
- Also known as lactogenic hormone or prolactin
- Stimulates the corpus luteum to secrete progesterone and the mammary gland to produce milk
Somatostatin
- Inhibits the release of growth hormone
- Growth hormone inhibitory hormone
Neurons of the hypothalamus
- Influenced by the Afferent fibers passing to the hypothalamus
- responsible for the production of the releasing hormones and the release-inhibiting hormones
- Also influenced by the level of hormone produced by the target organ controlled by the hypophysis
- Should the level of thyroxine in the blood fall the releasing factor for the thyrotropic hormone would be produced in increased quantities
FUNCTIONS OF THE HYPOTHALAMUS
Autonomic Control
Hypothalamus
- Has a controlling influence on the autonomic nervous system
- Appears to integrate the autonomic and neuroendocrine systems
- Preserves body homeostasis
- Regarded as a higher nervous center for control of lower autonomic centers in the brainstem and spinal cord
- Anterior hypothalamic area and the preoptic area influence parasympathetic responses. This include:
o Lowering of the blood pressure
o Slowing of the heart rate
o Contraction of the bladder
o Increased motility of the GIT
o Increase acidity of the gastric juice
o Salivation
o Pupillary constriction
- Stimulation of the posterior and lateral nuclei causes sympathetic responses, which include:
o Elevation of blood pressure
o Acceleration of the heart rate
o Cessation of persitalsis in the GIT
o Papillary dilation
o Hyperglycemia
ENDOCRINE CONTROL
Nerve cells of the hypothalamic nuclei
- produces releasing factors or release-inhibiting factors
- Control the hormone production of the anterior lobe of the hypophysis (pituitary gland)
Anterior lobe hormones include:
- growth hormone
- prolaction (luteotropic hormone)
- adrenocorticotropic hormone
- thyroid-stimulating hormone
- luteinizing hormone
- follicle stimulating hormone
* Some of these hormones act directly on body tissues while others such as ACTH act through an endocrine organ which in turn produces additional hormones that influence the activities of general body tissues
* Each stage is controlled by negative and positive feedback mechanisms
TEMPERATURE REGULATION
Anterior portion of the hypothalamus
- controls those mechanisms that dissipate heat loss
- Stimulation of this area causes dilatation of skin blood vessels and sweating, which lower the body temperature
Posterior portion of the hypothalamus
- Stimulation results in vasoconstriction of the skin blood vessels and inhibition of sweating
- There is shivering, in which the skeletal muscles produce heat
· Normally, the hypothalamus sets the body temperature at 98.0-98.6f when measured orally and one degree higher when measured rectally
· The temperature can be altered in response to extremes in environmental temperatures or in infection
REGULATION OF WATER AND FOOD INTAKE
Lateral region of the hypothalamus
- Stimulation initiates the feeing of hunger and results in an increase in food intake
- Referred to as the “hunger center”
- Bilateral destruction results in anorexia with loss of body weight
Medial region
- Simulation inhibits eating and reduces food intake
- Referred to as the “Satiety center”
- Bilateral destruction produces an uncontrolled voracious appetite causing extreme obesity
- Area in the lateral region of the hypothalamus that causes an immediate increase in the desire to drink water
Vasopressin (ADH)
- Secretion is controlled by the hypothalamus controlling the osmolarity of the blood by the posterior lobe of the hypophysis
- Causes a great increase in the reabsorption of water in the distal convoluted tubules and the collecting tubules of the kidneys
EMOTION AND BEHAVIOR
- Function of the hypothalamus, limbic system, and the prefrontal cortex
- The hypothalamus is the integrator of afferent information received from other areas of the nervous system and brings about the physical expression of emotion
o Can produce an increase in the heart rate
o Elevate the blood pressure
o Cause dryness of the mouth
o Flushing or pallor of the skin
o Sweating
o Often produce a massive peristaltic activity of the GIT
CONTROL OF CIRCADIAN RHYTHMS
- Controlled by the hypothalamus
- Circadian rhythms include:
o Body temperature
o Adrenocortical activity
o Eosinophil count
o Renal secretion
- Sleeping, wakefulness, although dependent on the activities of the thalamus, limbic system, and the reticular activating system are also controlled by the hypothalamus
Lesions of the anterior part of the hypothalamus
- Seriously interfere with the rhythm of sleeping and waking
Suprachiasmatic nucleus
- receives afferent fibers from the retina
- Appears to play an important role in controlling biological rhythms
- Where nerve impulses generated in response to variations in the intensity of light are transmitted to influence the activities of many of the hypothalamic nuclei
CLINICAL NOTES
Clinical Disorders Associated with Hypothalamic Lesions
- The hypothalamus may be the site of inflammation, neoplasm, or vascular disorder because of its deep-seated position
- Can be pressed upon by tumors of the surrounding brain tissue or may be compressed as the result of the development of internal hydrocephalus
- A lesion will produce a large number of different syndromes
- An acute lesion is more likely to produce signs and symptoms than is a slowly growing tumor
Obesity and Wasting
Severe obesity
- Can occur as the result of hypothalamic lesions
- Associated with genital hypoplasia or atrophy
Wasting
- Less common
- Severe cachexia is suggestive of damage to the hypophysis (pituitary gland)
Sexual Disorders
- In children: there may be sexual retardation and rarely sexual precocity with hypothalamic lesions
- After puberty: Patient with hypothalamic disease may have impotence or amenorrhea
Hyperthermia and Hypothermia
Hyperthermia
- Can follow lesions of the hypothalamus caused by head injury or following surgical operations in the regions of the hypothalamus
- Otherwise normal and has no signs of malaise, which occurs with pyrexia secondary to infections
Hypothermia
- Can follow a lesion of the hypothalamus
Diabetes Insipidus
- Result from a lesion of the supraoptic nucleus or from the interruption of the nervous pathway to the posterior lobe of the hypophysis
- Patient passes large volumes of urine of low specific gravity
- Patient is extremely thirsty and drinks large quantities of fluids
- In diabetes mellitus there is glucosuria
Disturbances of Sleep
- frequent short periods of sleep during the waking hours or insomnia in patients with hypothalamic lesions
Emotional Disorders
- Attacks of unexplained weeping or laughter
- Uncontrollable rage, depressive reactions
- Maniacal outbursts
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1 comment:
tnx for taking notes from neuro! keep it up!
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