Hypothalamus - Neuroanatomy Transcription

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:

1. Optic chiasma
2. tuber cinereum and the infundibulum
3. 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:

1. Part of the preoptic nucleus
2. Anterior nucleus (merges with the preoptic nucleus) 
3. Part of the suprachiasmatic nucleus
4. Paraventricular nucleus
5. dorsomedial nucleus
6. ventromedial nucleus
7. Infundibular (arcuate) nucleus
8. Posterior nucleus

Lateral Zone

From anterior to posterior:

1. Part of the preoptic nucleus
2. Part of the suprachiasmatic nucleus
3. supraoptic nucleus
4. lateral nucleus
5. Tuberomamillary nucleus
6. 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:

1. Nervous connections
2. Bloodstream
3. 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:

1. Somatic and visceral afferents
1. 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
2. Visual Afferents
1. Leave the optic chiasma and pass to the suprachiasmatic nucleus
3. Olfaction
1. Travels through the medial forebrain bundle
4. Auditory afferents
1. Not been identified
5. Corticohypothalamic fibers
1. Arise from the frontal lobe of the cerebral cortex and pass directly to the hypothalamus
6. Hippocampohypothalamic fibers
1. Pass from hippocampus through the fornix to the mammillary body
2. Hypothalamus is the main output pathway of the limbic system
7. Amygdalohypothalamic fibers
1. Pass from the amygdaloid complex to the hypothalamus through the stria terminalis and by a route that passes inferior to the lentiform nucleus

8. Thalamohypothalamic fibers
1. Arise from the dorsomedial and midline thalamic nuclei
9. Tegmental fibers- arise from the midbrain

EFFERENT NERVOUS CONNECTIONS OF THE HYPOTHALAMUS

Main pathways:

1. Descending fibers to the brainstem and spinal cord
1. Influence the peripheral neurons of the ANS
2. Descend through a series of neurons in the reticular formation
3. The hypothalamus is connected to the parasympathetic nuclei of the oculomotor, facial, glossopharyngeal, and vagus nerves in the brainstem
4. 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
2. mammillothalamic tract
1. Arises in the mammillary body
2. Terminates in the anterior nucleus of the thalamus
3. Here the pathway is relayed to the cingulated gyrus
3. mammillotegmental tract
1. Arises from the mammillary body and terminates in the cells of the reticular formation in the tegmentum of the midbrain
4. 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

Thirst Center

-          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

Brainstem - Neuroanatomy Transcription

Brainstem is made up of:

1. Medulla Oblongata
2. Pons
3. Midbrain

Occupies the: Posterior Cranial Fossa of the Skull

* Stalklike in shape

* Connects the narrow spinal cord with the expanded forebrain

Broad Functions:

1.       Serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to the different parts of the higher centers in the forebrain

2.       Contains important reflex centers associated with the control of respiration and the cardiovascular system; also associated with the control of consciousness 

3.       Contains important nuclei of cranial nerves III through XII

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GROSS APPEARANCE OF THE MEDULLA OBLONGATA

* Medulla Oblongata connects the pons superiorly with the spinal cord interiorly

* Junction of the medulla and spinal cord

            ° located at the origin of the anterior and posterior roots of the first cervical spinal nerve corresponding to the level of foramen magnum

* Conical in shape

* Broad extremity being directed superiorly

* Central Canal

            ° Continues upward into the lower half of the medulla

° In upper half it expands as the cavity of the 4^th ventricle

* Anterior Median Fissure  

            °  Located at the anterior surface if the medulla

            ° Continuous inferiorly with the anterior median fissure of the spinal cord

* Pyramid

            °  Swelling located on each side of the median fissure

            ° Composed of bundles of nerve fibers, corticospinal fibers, which originate in large nerve cells in the precentral gyrus of the cerebral cortex

°         Taper inferiorly

°         It is here that majority of the descending fibers cross over the opposite side, forming the decussation of the pyramids

* Anterior external arcuate fibers

° Few nerve fibers that emerge from anterior medial fissure above the decussation and pass laterally over the surface of the medulla oblongata to enter the cerebellum

* Olives

            ° Posterolateral to the pyramids

° Oval elevations produced by the underlying inferior olivary nuclei

* Rootlets of the hypoglossal nerve

            ° Emerge in the groove between the pyramid and the olive

* Inferior Cerebellar Peduncles

            ° Posterior to the olives

° Connect the medulla to the cerebellum

* Roots of the glossopharyngeal and vagus nerves and cranial roots of accessory nerve

            ° Emerge in the groove between the olive and the inferior cerebellar peduncle

* Floor of the fourth ventricle

            ° Lower part is formed by the posterior surface of the superior half of the medulla oblongata

* Posterior Medial Sulcus

            ° Possessed by the posterior surface of the inferior half of the medulla which is continuous with the posterior aspect of the spinal cord

* Gracile Tubercle

            ° Elongated swelling on each side of median sulcus

            ° Produced by the underlying gracile nucleus

* Cuneate Tubercle

            ° Lateral to the gracile tubercle

            ° Similar swelling as the gracile tubercle

            ° Produced by the underlying cuneate nucleus

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INTERNAL STRUCTURE

Medulla Oblongata consists of white and gray matter but have been extensively rearranged

Explanation:

1. This rearrangement is due to the expansion of the neural tube to form the hindbrain vesicle
2. This hindbrain vesicle becomes the 4^th ventricle
3. The extensive lateral spread of the 4^th ventricle results in alteration in the position of the derivatives of the alar and basal plates of the embryo 

To assist in understanding this concept:

Remember!

                                    In the Spinal cord                              In Medulla Oblongata

derivatives                   Posterior to sulcus limitans                 lateral to sulcus limitans

of alar plates

derivatives                   anterior to sulcus limitans                   medial to sulcus limitans

of basal plates

4 Levels of the internal structure of the medulla oblongata

1. Level of decussation of pyramids
2. Level of decussation of lemnisci
3. Level of the olives
4. Level just inferior to the pons

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LEVEL OF DECUSSATION OF PYRAMIDS

* Great Motor Decussation

* Where a transverse section through the inferior half of the medulla oblongata passes through

* In superior part of the medulla: corticospinal fibers occupy and form the pyramid

* Inferiorly: about ¾ of the fibers cross the medial plane and continue down the spinal cord in the lateral white column as the lateral corticospinal tract

            ° As these fibers cross the midline, they sever the continuity between the anterior column of the gray matter of the spinal cord and the gray matter that surrounds the central canal

* Fasciculus gracilis and the fasciculus cuneatus

            ° Continue to ascend superiorly posterior to the central gray matter

* Nucleus gracilis and nucleus cuneatus

            °Appear as posterior extensions of the central gray matter

* Substantia gelatinosa in posterior gray column of the spinal cord

            ° Becomes continuous with the inferior end of the nucleus of the spinal tract of the trigeminal nerve

* The fibers of the tract of the nucleus

° Situated between the nucleus and the surface of the medulla oblongata

° The lateral and anterior white columns of the spinal cord are easily identified in these sections and their fiber arrangement is unchanged

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LEVEL OF DECUSSATION OF LEMNISCI

* The Great Sensory Decussation

* Where a transverse section through the inferior half of the medulla oblongata, a short distance above the level of the decussation of the pyramids passes through

* Takes place anterior to the central gray matter and posterior to the pyramids

* The lemnisci have been formed from the internal arcuate fibers

* Internal arcuate fibers

            ° Emerged from the anterior aspects of the nucleus gracilis and nucleus cuneatus

            ° First travel anteriorly and laterally around the central gray matter

            ° They then curve medially toward the midline, where they decussate with the corresponding fibers of the opposite side.

* Nucleus of the spinal tract of the trigeminal nerve

            ° Lies lateral to the internal arcuate fibers

            ° Spinal tract of the trigeminal nerve lies lateral to the nucleus

* Lateral and Spinothalamic tracts and Spinotectal tracts

            ° Occupy an area lateral to the decussation of the lemnisci

            ° They are very close to one another and collectively known as the spinal lemniscus

* Situated in the anterolateral region of medulla oblongata:

            ° Spinocerebellar, vestibulospinal, rubrospinal tracts

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LEVEL OF OLIVES

* A transverse section through the olives passes across the inferior part of the 4^th ventricle

* The amount of gray matter has increased owing to the presence of the olivary nuclear complex:

            ° Nuclei of vestibulocochlear

            ° Glossopharyngeal

            ° Vagus

            ° Accessory

            ° Hypoglossal nerves

            ° Arcuate nuclei

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OLIVARY NUCLEAR COMPLEX

Inferior Olivary nucleus – Largest nucleus of this complex

* Gray matter – shaped like a crumpled bag with its mouth directed medially

-          Responsible for the elevation on the surface of the medulla called, olive

* Smaller dorsal and medial accessory olivary nuclei are also present  

* Cells of the inferior olivary nucleus send fibers medially across the midline to enter the cerebellum through the inferior cerebellar peduncle

* Afferent fibers reach the inferior olivary nuclei from the spinal cord (spino-olivary tracts) and from the cerebellum and cerebral cortex

* Function of the olivary nuclei: Associated with voluntary muscle movement

 

VESTIBULOCOCHLEAR NUCLEI

Made up of the following nuclei:

1. Medial vestibular nucleus
2. Inferior vestibular nucleus
3. Lateral vestibular nucleus
4. Superior vestibular nucleus

Cochlear nuclei          - two in number

Anterior Cochlear nucleus               - situated on the anterolateral aspect of the inferior cerebellar peduncle

Posterior Cochlear nucleus              - situated on the posterior aspect of the peduncle lateral to the floor of the 4^th ventricle

NUCLEUS AMBIGUUS

* Consists of large motor neurons

* Situated deep within the reticular formation

* The emerging nerve fibers join the glossopharyngeal vagus, and cranial part of the accessory nerve and are distributed to voluntary skeletal muscle

* Deeply placed within the reticular formation

 

CENTRAL GRAY MATTER

* Lies beneath the floor of the 4^th ventricle

* Passes from medial to lateral

* The following important structures may be recognized:

1.      Hypoglossal nucleus

2.      Dorsal nucleus of the vagus

3.      Nucleus of the tractus solitarius

4.       Medial and inferior vestibular nuclei

* Arcuate nuclei

            ° Thought to be inferiorly displaced pontine nuclei

            ° Situated on the anterior surface of the pyramids

            ° They receive nerve fibers from the cerebral cortex

            ° Send efferent fibers to the cerebellum through the anterior external arcuate fibers

* Pyramids containing the corticospinal and some corticosnuclear fibers

            ° Situated in the anterior part of the medulla

            ° Separated by the anterior median fissure

            ° Corticospinal fibers descend to the spinal cord

            ° Corticonuclear fibers are distributed to the motor nuclei of the cranial nerves situated within the medulla

* Medial Lemniscus

            ° Forms a flattened tract on each side of the midline posterior the pyramid

            ° These fibers emerge from the decussation of the lemnisci and convey sensory information to the thalamus

* Medial Longitudinal Fasciculus

            ° Forms a small tract of nerve fibers situated on each side of the midline posterior to medial lemniscus and anterior to hypoglossal nucleus

            ° Consists of ascending and descending fibers

* Inferior Cerebellar Peduncle

            ° Situated in posterolateral corner of the section on lateral side of 4^th ventricle

* Spinal tract of trigeminal nerve and its nucleus

            ° Situated on the anteromedial aspect of the inferior cerebellar peduncle

* Anterior Spinocerebllar Tract

            ° Near the surface in the interval between the inferior olivary nucleus and the nucleus of the spinal tract of the trigeminal nerve

* Spinal Lemniscus

            ° Deeply placed

            ° Consists of:

                        Anterior Spinothalamic

                        Lateral Spinothalamic

                        Spinotectal tracts

Reticular formation

            ° Consists of a diffuse mixture of nerve fibers and small groups of nerve cells

            ° Deeply placed posterior to the olivary nucleus

            ° Represents, at this level, only a small part of this system, which is also present in the pons and midbrain

Glossopharyngeal

Vagus                                                                       can be seen running forward and

Cranial part of the accessor nerves                 laterally through the reticular formation

* The nerve fibers emerge between the olives and the inferior cerebellar peduncles

 

Hypoglossal nerves

            ° Run anteriorly and laterally through the reticular formation and emerge between the pyramids and the olives

LEVEL JUST INFERIOR TO THE PONS

° No major changes

° Lateral vestibular nucleus has replaced the inferior vestibular nucleus

° Cochlear nuclei now are visible on the anterior and posterior surfaces of the inferior cerebellar peduncle

GROSS APPEARANCE OF THE PONS

* Pons is anterior to the cerebellum

* Connects the medulla oblongata to the midbrain

* About 1 inch (2.5 cm) long

* Owes its name to the appearance presented on the anterior surface, which is that of a bridge connecting the right and left cerebellar hemispheres

* Anterior surface

° Convex from side to side

° Shows many transverse fibers that converge on each side to form the middle cerebellar peduncle

* Basilar groove

° Shallow groove in the midline

° Lodges the basilar artery

* Anterolateral surface of the pons

° trigeminal nerve emerges on each side

° Each nerve consist of:

Smaller, medial part – the motor root

Larger, lateral part – sensory root

* In the groove between the pons and medulla oblongata

° There emerge from medial to lateral:

abducent, facial, and vestibulocochlear nerves

 * the Posterior surface of the pons

° hidden from view

° Forms the upper half of the floor of the 4^th ventricle

° Triangular in shape

° Limited laterally by the superior cerebellar peduncles

° Divided into symmetrical halves by a median sulcus

° Lateral to this sulcus is an elongated elevation, the medial eminence

° Medial eminence is bounded laterally by the sulcus limitans and its inferior end is slightly expanded to form the facial colliculus

° Facial colliculus  is produced by the root of the facial nerve winding around the nucleus of the abducent nerve

° The floor of the superior part of the sulcus limitans is bluish gray in color and is called the substantia ferruginea (it owes its color to a group of deeply pigmented nerve cells)

° Lateral to the sulcus limitans is the area vestibuli produced by the underlying vestibular nuclei

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INTERNAL STRUCTURE OF THE PONS

Divisions of the pons:

1. Tegmentum - posterior part

2. Basal part – anterior part by the transversely running fibers of the trapezoid body

The structure of the pons may be studied at two levels:

1. Transverse section through the caudal part
1. Passing through the facial colliculus 
2. Transverse section through the cranial part
1. Passing through the trigeminal nuclei

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TRANSVERSE SECTION THROUGH THE CAUDAL PART

Medial lemniscus

• Rotates as it passes from the medulla into the pons
• Situated in the most anterior part of the tegmentum with its long axis running transversely
• Accompanied by the spinal and lateral lemnisci

Facial nucleus

• Lies posterior to the lateral part of the medial lemniscus
• The fibers of the facial nerve wind around the nucleus of the abducent nerve, producing the facial colliculus
• Fibers of the facial nerve pass anteriorly between the facial nucleus and the superior end of the nucleus of the spinal tract of the trigeminal nerve

Medial longitudinal fasciculus 

• Situated beneath the floor of the 4^th ventricle on either side of the midline
• Main pathway that connects the vestibular and cochlear nuclei with the nuclei controlling the extraocular muscles (oculomotor, trochlear, and abducent nuclei)

Medial Vestibular nucleus

• Situated lateral to the abducent nucleus
• In close relationship to the inferior cerebellar peduncle
• Where the superior part of the lateral and the inferior part of the superior vestibular nucleus are located
• Posterior and anterior cochlear nuclei are also found at this level

Spinal Nucleus of the trigeminal nerve

• Lie on the anteromedial aspect of the inferior cerebellar peduncle

Trapezoid body

• Made up of fibers derived from the cochlear nuclei and the nuclei of the trapezoid body
• Run transversely in the anterior part of the tegmentum

Pontine Nuclei

• Small masses of nerve cells contained in the basilar part of the pons

Corticopontine fibers of the crus cerebri of the midbrain 

• Terminate in the pontine nuclei
• Axons of these cells gives origin to the transverse fibers of the pons

Transverse fibers of the pons

• Cross the midline and intersect the corticospinal and corticonuclear tracts, breaking them up into small bundles
• Enter the middle cerebellar peduncle
• Distributed to the cerebellar hemisphere
• This connection forms the main pathway linking the cerebral cortex to the cerebellum

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TRANSVERSE SECTION THROUGH THE CRANIAL PART

* Internal structure is similar to that seen at the caudal levels

* But it now contains the motor and principal sensory nuclei of the trigeminal nerve

Motor nucleus of the trigeminal nerve

• Situated beneath the lateral part of the 4^th ventricle within the reticular formation
• Emerging motor fibers travel anteriorly through the substance of the pons and exit on its anterior surface

Principal sensory nucleus of the trigeminal nerve

• Situated on the lateral side of the motor nucleus
• Continuous inferiorly with the nucleus of the spinal tract
• Entering sensory fibers travel through the substance of the pons and lie lateral to the motor fibers

Superior Cerebellar peduncle

• Situated posterolateral to the motor nucleus of the trigeminal nerve
• Joined by the anterior spinocerebellar tract

Trapezoid body  and medial lemniscus

• Situated in the same position as they were in the previous section

Lateral and spinal lemnisci

• Lies at the lateral extremity of the medial lemniscus

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GROSS APPEARANCE OF THE MIDBRAIN

* 0.8 inch (2 cm) in length

* Connects the pons an dcerebellum with the forebrain

* Its long axis inclines anteriorly as it ascends through the opening in the tentorium cerebelli

Cerebral Aqueduct

• narrow channel traversing the midbrain
• filled with CSF

Four colliculi

• Corpora quadrigemina
• Found on the posterior surface
• Rounded eminences that are divided into superior and inferior pairs by a vertical and a transverse groove

Superior Colliculi                                                      Inferior  Colliculi

• Centers for visual reflexes                                          Lower auditory centers

Trochlear nerves

• Emerging from the midline below the inferior colliculi
• Small diameter nerves that wind around the lateral aspect of the midbrain to enter the lateral wall of the cavernous sinus

* The superior and inferior brachia ascend in an anterolateral direction on the lateral aspect of the midbrain

Superior brachium

• Passes from the superior colliculus to the lateral geniculate body and the optic tract

Inferior brachium

• Connects the inferior colliculus to the medial geniculate body

* On anterior aspect of midbrain 

• There’s a deep depression in the midline the interpeduncular fossa
• interpeduncular fossa
• Bounded on either side by the crus cerebri
• Floors are perforated by many blood vessels and this region is termed the posterior perforated substance
• Oculomotor nerve
• Emerges from a groove on the medial side of the crus cerebri
• Passes forward in the lateral wall of the cavernous sinus

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INTERNAL STRUCTURE OF THE MIDBRAIN

2 lateral halves of midbrain: Cerebral peduncles

Each divided into

-          an anterior part: the Crus cerebri

-          posterior part: Tegmentum

Division is made possible by  substantia nigra, pigmented band of gray matter

Cerebral aqueduct

• Narrow cavity of the midbrain
• Connects the 3^rd and 4^th ventricle

Tectum

• Part of the midbrain posterior to the cerebral aqueduct
• Has four small surface swellings referred to previously
• 2 superior and 2 inferior colliculi

Central gray matter

• Surrounds the cerebral aqueduct (lined by ependyma)

Interpeduncular fossa on transverse sections of the midbrain

• Can be seen to separate the crura cerebri
• Whereas the tegmentum is continuous across the median plane

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TRANSVERSE SECTION OF THE MIDBRAIN AT THE LEVEL OF THE INFERIOR COLLICULI

Inferior colliculus

• Consists of a large nucleus of gray matter
• Lies beneath the corresponding surface elevation
• Forms part of the auditory pathway
• Receives many of the terminal fibers of the lateral lemniscus
• Pathway then continues through the inferior brachium to the medial genicualte body

Trochlear nucleus

• Situated in the central gray matter close to the median plane
• Posterior to the medial longitudinal fasciculus

1. Emerging fibers pass laterally and posteriorly around the central gray matter
2. Leave the midbrain just below the inferior colliculi
3. Fibers of the trochlear nerve now decussate completely in the superior medullary velum

* Mesencephalic nuclei of the trigeminal nerve

            ° Lateral to the cerebral aqueduct

* The Decussation of the superior cerebellar peduncles

            ° Occupies the central part of the tegmentum anterior to the cerebral aqueduct

*Reticular formation

° Smaller than that of the pons

° Situated lateral to the decussation

Medial lemniscus – ascends posterior to the substantia nigra

Spinal and trigeminal lemnisci – situated lateral to the medial lemniscus

Lateral Lemniscus – located posterior to the trigeminal lemniscus

Substantia Nigra

° Large motor nucleus

° Situated between the tegmentum and the crus cerebri

° Found throughout the midbrain

° Nucleus is composed of medium sized multipolar neurons that possess inclusion granules of melanin pigment within their cytoplasm

° Concerned with muscle tone

° Connected to the cerebral cortex, spinal cord, hypothalamus and basal nuclei

Crus cerebri

° Contains important descending tracts

° Separated from the tegmentum by the substantia nigra

° Middle 2/3 is occupied by the corticospinal and corticonulcear fibers

° Medial part is occupied by the frontopontine fibers

° Lateral part occupied by the temporopontine fibers 

* These descending tracts connect the cerebral cortex to the anterior gray column cells of the spinal cord, cranial nerve nuclei, pons, and cerebellum

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TRANSVERSE SECTION OF THE MIDBRAIN AT THE LEVEL OF THE SUPERIOR COLLICULI

Superior colliculus

• Large nucleus of gray matter that lies beneath the corresponding surface elevation
• Forms part of  the visual reflexes
• Connected to the lateral geniculate body by the superior brachium
• It receives afferent fibers from the optic nerve, the visual cortex and the spinotectal tract 
• Efferent fibers
• Form the tectospinal and tectobulbar tracts, 
• Responsible for the reflex movements of the eyes, head, and neck in response to visual stimuli
• Afferent pathway for the light reflex ends in the pretectal nucleus 
• This is small group of neurons situated close to the lateral part of the superior colliculus
• After relaying in the pretectal nucleus, the fibers pass to the parasympathetic nucleus of the oculomotor nerve (Edinger-Westphal nucleus)

• The emerging fibers then pass to the oculomotor nerve.

Oculomotor nucleus

• Situated in the central gray matter close to the median plane, just posterior to the medial longitudinal fasciculus
• The fibers pass anteriorly through the red nucleus to emerge on the medial side of the crus cerebri in the interpeduncular fossa
• Nucleus of the oculomotor nerve is divisible into a number of cells group

* Medial, spinal, and trigeminal lemnisci

• Form a curved band posterior to the substantia nigra

* Lateral lemniscus

• Does not extend superiorly to this level

Red nucleus

• Rounded mass of gray matter
• Situated between the cerebral aqueduct and the substantia nigra
• Reddish hue seen in fresh specimen
• Due to its vascularity and the presence of an iron-containing pigment in the cytoplasm of many its neurons

•  Afferent fibers reach the red nucleus from

1. Cerebral cortex through the corticospinal fibers
2. Cerebellum through the superior cerebellar peduncle
3. Lentiform nucleus, subthalamic and hypothalamic nuclei, substantia nigra, and spinal cord

• Efferent fibers leave the red nucleus and pass to

1. Spinal cord through the rubrospinal tract (as this tract descends, it decussates)
2. Reticular formation through the rubroreticular tract
3. Thalamus
4. Substantia nigra

Reticular formation

• Situated in the tegmentum lateral and posterior to the red nucleus

Crus cerebri

• Contains the identical important descending tracts,
• The corticospinal, corticonuclear and corticopontine fibers
• Present at the level of the inferior colliculus