The midbrain
The midbrain
-The midbrain, mesencephalon, connects the hindbrain with the forebrain.
-Its cavity is known as the cerebral aqueduct (aqueduct of Sylvius), aqueductus cerebri: connects the third ventricle with the fourth.
midbrain connects the pons and diencephalon
conveys motor impulses from the cerebrum to the cerebellum and spinal cord,
sends sensory impulses from the spinal cord to the thalamus
regulates auditory and visual reflexes.
It also contains the nuclei of origin for III and IV cranial nerves.
External structure
posterior (inferior) part of the midbrain is formed by cerebral peduncles, pedunculi cerebri
The anterior (superior) part is the tectum with lámina quadrigémina.
it is made of the superior and inferior colliculi, colliculi superiores et inferiors
Each of the colliculus is connected to the appropriate geniculate body of the metathalamus (the diencephalon).
superior colliculus is connected to the lateral geniculate body, corpus geniculatum laterale, by the superior brachium.
inferior colliculus is connected to the medial geniculate body, corpus geniculatum mediale, by the inferior brachium
geniculate bodies and the colliculi of the midbrain are acting as the primary (subcortical) visual and auditory centers.
Each cerebral peduncle is subdivided into the
01.crus cerebri anteriorly;
02.the substancia nigra in the middle and
03.the tegmentum, posteriorly.
Between the peduncles there is the interpeduncular fossa, fossa interpeduncularis
floor of the fossa is perforated by vessels
That is why it has the name of posterior perforated substance, substantia perforata posterior.
Internal structure
transverse section of the midbrain they distinguish 3 parts:
01.lamina tecti,
02. tegmentum and
03. basis pedunculi cerebri
Between the lamina tecti and tegmentum there is the aqueductus cerebri,
tegmentum is separated from the crus cerebri by the black substance,
Substancia nigra is a lamina of grey matter made up of deeply pigmented nerve cells.
It is concerned with the muscle tone (extrapyramidal system).
inferior colliculus receives afferent stimuli from the lateral lemniscus and gives them to the medial geniculate body.
It is considered as the centre for auditory reflexes and helps in localizing the source of sounds.
The superior colliculus receive afferent stimuli from retina (visual) and gives them to the lateral geniculate body.
It is considered as the centre for visual reflexes and helps in the eyes, head and neck movements in response to visual stimuli.
colliculi give efferents to the spinal cord and form tractus tectospinalis, it controls reflex movements of the eyes, head and neck in response to visual and auditory stimuli.
The tract crosses the midbrain under the aqueductus cerebri and forms the dorsal decussation of Meynert.
The central (periaqueductal) grey matter contains the nucleus of the trochlear nerve (IV pair) and the oculomotor nerve (III pair).
IV pair, n. trochlearis, has one motor nucleus situated at the level of the inferior colliculi.
III pair, n. oculomotorius, has two nuclei situated at the level of the superior colliculi: motor nucleus and vegetative nucleus (parasympathetic) – nucleus accessorius, for the smooth eye muscle innervation.
tegmentum has gray matter (formatio reticularis, nucleus ruber) and white matter (the tracts).
The red nucleus, nucleus ruber has an inhibitory influence on muscle tone, this is the central nucleus for the extrapyramidal system (here starts the descending rubrospinal tract).
The rubrospinal tract, tractus rubrospinalis, forms the ventral tegmental Forel’s decussation.
tegmentum contains also ascending tracts as lemniscus medialis, lemniscus lateralis (up to the level of the inferior colliculi) and fasciculus longitudinalis medialis.
base of the cerebral peduncles is formed by the descending tracts coming from the brain cortex:
01. fibrae corticopontinae (to the pontine nuclei),
02. tractus corticonuclearis (to the motor nuclei of the cranial nerves) and
03. tractus corticospinalis (to the motor nuclei of the anterior horns of the spinal cord).
Forebrain. The diencephalon
diencephalon consists of two parts:
01. thalamencephalon – that is the highest subcortical sensory center and serves as a relay station for all sensory impulses to the cerebral cortex;
02. hypothalamus – phylogenetically is older than the thalamencephalon. It acts as the highest center for autonomic (vegetative) functions regulation.
cavity of the diencephalon is the third ventricle
Thalamencephalon consists of three parts:
01.thalamus,
02.epithalamus and
03.metathalamus.
thalamus
thalamus is a paired oval mass of gray matter
anterior end is narrow with anterior tubercule
posterior end is expanded and is known as the pulvinar
which contains subcortical visual center
medial surfaces of the thalami are interconnected by an interthalamic adhesion.
gray matter of the thalamus is divided to form several groups of nuclei that can be specific, non-specific and associative.
01.anterior – subcortical centers of olfaction;
02. posterior – subcortical visual centers;
03.lateral – serve as subcortical centers of general sensitiveness (touch, pain, temperature etc.); the lemniscus medialis ends in this group of nuclei;
04.medial – are the nuclei of formation reticularis; and
05.central group is connected with extrapyramidal system.
epithalamus
The epithalamus consists of habenular nuclei, each situated beneath the floor of the corresponding trigonum habenulae, habenula and pineal gland, corpus pineale (epiphysis).
Between the habenulae is the comissura habenulárum.
The habenular nuclei form a part of the limbic system.
Epiphysis or pineal gland
endocrine gland suspended between the superior collicules of the midbrain.
gland is covered by a capsule formed by the pia mater and consists of neuroglia and secretory cells called pinealocytes.
One hormone secreted by the pineal gland is melatonin – the regulator of the body’s internal clock; promoter of sleep and a coordinator of the hormones involved in fertility.
Pineal gland modifies the activity of other internal glands (hypophysis, thyroid, parathyroid, suprarenal glands and gonads).
action is mostly inhibitory
produces a large number of neuromediators including serotonin.
metathalamus
metathalamus consists of two oval structures which are situated on both sides of the midbrain under the thalamus.
These are medial and lateral geniculate bodies, corpus geniculatum mediale et laterale
inferior and superior brachia connect the geniculate bodies with superior and inferior collicules of the midbrain.
outer surface of the geniculate body is covered by white mater and internal structure contains nuclei.
Nuclei of the medial geniculate body (together with nuclei of the inferior collicules of the midbrain) are the subcortical hearing centers.
Here terminate the afferent acoustic fibers of the lemniscus lateralis that bring the impulses from cochlear nuclei of the VIII cranial nerve.
Nuclei of the lateral geniculate body (together with nuclei of the superior collicules of the midbrain) are the subcortical visual centers.
Here terminates the lateral part of afferent visual fibers of the tractus opticus.
hypothalamus
The hypothalamus is located inferior to the thalamus. It is divided in several regions
Anterior region (párs óptica) forms the floor of the IIIrd ventricle and contains optic tracts, tractus opticus and chiasma optica, formed by the visual fibers;
the tuber cinereum
the infundibulum, which attaches the hypophysis to the hypothalamus
terminal medullary plate and the subthalamic area, formed by the subthalamic nuclei.
The túber cineréum is an elevated thin mass of grey matter situated between the chiasma optica anteriorly and mamillary bodies posteriorly
Nuclei of the tuber cinereum are vegetative and connected with the emotional cortical areas
tuber cinereum contains the center of thermoregulation and metabolism
thin elongation of the anterior wall of the tuber cinereum upwards and backwards forms terminal medullary plate (lámina terminális), that serves as the anterior wall of the IIIrd ventricle.
Ventrally the tuber cinereum is attached to the infundibulum that is connected with the hypophysis.
Anterior (optic) part of the hypothalamus contains nucl. paraventriculáris, nucl. supraópticus and nucl. preópticus.
Neurons of nucl. paraventriculáris and nucl. supraópticus produce neurosecret that by the axons is transported to the posterior lobe of the hypophysis (neurohypophysis), where it is released in two neurohormones
01. oxytocine (involved in labor and milk secretion, contraction of smooth musculature) and
02. (ADH) or vasopressin antidiuretic hormone (decreases urine volume and raises blood pressure).
- anterior hypothalamic nuclei causes hyposecretion of ADH and lead to the diabetes insipidus
- his disorder is characterized by excretion of large amounts of urine (10 liters per day), dehydratation and thirst.
In nucl. preópticus neurons produce one of the releasing hormones – luliberine, which stimulates luteinizing hormone of adenohypophysis production.
middle group of the hypothalamic nuclei contains nucl. dorsomedialis and nucl. ventromedialis, nuclei of the tuber cinereum and infundibulum.
- Middle group nuclei controls water, lipid and carbohydrates balance, influences blood sugar level, ionic balance, permeability of vessels and cell membranes.
- These nuclei regulate the food intake through two centers
01.The feeding (hunger) center is responsible for hunger sensations
02.When sufficient food has been ingested the satiety center is stimulated and sends impulses to inhibit the feeding center
- It also contains a thirst center
Lesions of the ventromedial nuclei cause the excess food intake (hyperphagia) and obesity;
lesions of the tuber cinereum nucleus cause decrease of appetite and leanness (cachexia).
Neurons of these nuclei produce several releasing hormones (somatostatin, somatoliberine, luliberine, folliberine, prolactoliberine, thyreoliberine etc.)
help of hypothalamo-hypophyseal system these hormones influence the processes of growth, physical and sexual development, and metabolism.
Nuclei of the middle group regulate food and sexual behavior, the activity of other endocrine glands.
Some areas in this hypothalamic zone are connected with emotions control such as fear, rage, aversion, pleasure and reward.
Posterior region (pars olfactoria) is between the tuber cinereum and substantia perforate posterior
It consists of the right and left mamillary bodies, corpóra mammilária (nipple-shaped structures).
mamillary body has nucleus that serves as relay stations in reflexes related to the sense of smell (subcortical olfactory center).
The mammillothalamic fasciculus (bundle of Vicq d’Azyr) arises from cells in both nuclei of the mammillary body and terminates in the anterior nuclei of the thalamus.
Posterior area contains nucl. hypothalámicus postérior. Together with the nuclei of the anterior group, nucl. hypothalámicus postérior participates in temperature regulation, and contains centers for sympathetic part of the autonomic nervous system activity regulation.
- Stimulation of nucl. hypothalamicus posterior causes expansion of the pupil, increase of the heart rate and blood pressure, acceleration of respiration and the weakening of peristalsis.
- Lesions of the posterior hypothalamic region causes fatigue, drowsiness and temperature decrease.
Pituitary gland (hypóphysis)
endocrine gland that has two separate portions.
01. anterior lobe (adenohypophysis) secretes hormones that regulate a wide range of bodily activities from growth to reproduction.
- Release of the anterior pituitary gland hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus
- This part consists of glandular cells that secrete tropins
(somatotropin, thyrotropins, gonadotropins, prolactin, corticotropin).
02.posterior lobe (neurohypophysis) does not synthesize
hormones; it stores and releases two hormones (oxytocin and vasopressin).
- It is continuation of the infundibulum and consists of neuroglia cells and axons of secretory neurons of hypothalamic neurosecretory cells.
hypothalamo-hypophyseal system (or hypothalamohypophyseal portal system) is the system of blood vessels that links the hypothalamus and the anterior pituitary.
It allows endocrine communication between the two structures.
- The anterior pituitary receives releasing and inhibitory hormones of the hypothalamus from the hypophyseal portal vessels.
With the help of these hormones the anterior pituitary is able to fulfill its function of regulating the other endocrine glands to maintain homeostasis.
