Brain stem Midbrain D.Nimer D.Rania Gabr D.Safaa D.Elsherbiny

Objectives Identify the gross features of the brainstem (midbrain). Briefly describe the internal structure of the brainstem (midbrain) Describe the main connections of the sensory cranial nuclei. Describe the main connections of the motor cranial nuclei.

External features

Midbrain Shortest part of the brain stem, not more than 2cm in length, lies in the posterior cranial Fossa, at the level of the cerebral aqueduct. Divided into: Dorsal part: “Tectum” which consists of inferior and superior colliculi Ventral portion: Right and left Cerebral Peduncles.

Ventral surface of the midbrain 2 Cerebral peduncle: Crus cerebri : anterior. Substantia nigra: middle. Tegmentum: posterior. Interpedunclar fossa. Occulomotor nerve. 1 3

Cerebral peduncles Each cerebral peduncle divides further into : 1-Ventral crus cerebri (massive fibrous mass) and a 2-Dorsal Tegmentum by a pigmented lamina “ Substantia nigra”

Cerebral peduncles Contain: 1-Descending fibers that go to the cerebellum via the pons 2-Descending pyramidal tracts -Running through the midbrain is the hollow cerebral aqueduct which connects the 3rd and 4th ventricles of the brain.

MID BRAIN – DORSAL SURFACE Marked by 4 elevations: Two superior colliculi (visual): concerned with visual reflexes connected with LGB with superior brachium Two inferior colliculi (auditory): forms part of auditory pathway, connected with MGB by inferior brachium Nerve emerging from dorsal surface: Trochlear (4th): just caudal to inferior colliculus (The only cranial nerve emerging from dorsal surface of the whole brain stem).

Internal structures of midbrain The midbrain is divided by the cerebral aqueduct into two parts: 1. Ventral part (cerebral peduncle): divided into: Crus cerebri: anterior part. Substantia nigra: middle part, formed of: Pars compacta: posterior, secrete dopamine. Pars reticularis: anterior, secrete GABA. c) Tegmentum: posterior part. 2. Dorsal part (tectum): formed of two superior colliculi and two inferior colliculi.

Crus Cerebri: it is the most anterior part of the cerebral peduncle Contents: it contains the following fibers: Medial one-fifth: Frontopontine fibers. Lateral one-fifth: Nonfrontopontine fibers. Middle three-fifths: Corticospinal and corticobulbar fibers.

Substantia Nigra: It is an extrapyramidal motor nucleus It consists of pigmented, melanin containing neurones.It projects to the basal ganglia. It is divided into 2 parts: Pars compacta (posterior): the nerve cells contain melanin and secrete dopamine. Pars reticularis (anterior): the nerve cells secrete gamma aminobutyric acid (GABA).

Connections of the substantia nigra: It projects two types of inhibitory fibers: Nigrostriate fibers: They convey dopamine to the corpus striatum to inhibit the activity of the caudate nucleus. Nigrothalamic fibers: They convey GABA to the thalamus to inhibit the activity of the ventral anterior and ventral lateral thalamic nuclei. Degeneration of the substantia nigra: Parkinson’s disease

Tegmentum: contains tracts and nuclei Tracts: such as the 4 lemnisci and other tracts. Medial lemniscus: conveys kinesthesia, discriminative touch and vibration sense from the opposite side of the body. 2. Trigeminal lemniscus: conveys sensations from the opposite side of the face.

3. Spinal lemniscus: conveys pain, temperature, simple touch and pressure sensations from the opposite side of the body. 4. Lateral lemniscus: conveys auditory sensation to the inferior colliculus from the two ears, mainly the opposite ear.

Nuclei: Cranial nerve nuclei: Oculomotor nuclei: at the level of the superior colliculus. Trochlear nucleus: at the level of the inferior colliculus. Mesencephalic nucleus of the trigeminal nerve. Other nuclei: Red nucleus: at the level of the superior colliculus. Nuclei of the reticular formation. Locus ceruleus: it is a pigmented area in the upper part of the pons and the lower part of the midbrain. The cells secrete norepinephrine

Red nucleus: extrapyramidal motor nucleus It relays motor impulses from the cerebral cortex and cerebellum to the thalamus & the spinal cord (cortico-rubro-spinal and dentato-rubro-spinal). Input: Corticorubral tract: from the motor and premotor areas of the cerebral cortex. Dentatorubral tract: from the opposite dentate nucleus of the cerebellum. Output: Rubrospinal tract: to the spinal cord Rubrothalamic tract: to the motor nuclei of the thalamus (VA and VL nuclei). Lesion: results in signs of cerebellar damage (contralateral tremor and ataxia).