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Term Paper on the Mid Brain

Term Paper # 1. Position of the Mid Brain:

It is a small part which connects the forebrain with hind brain.



Lies in posterior cranial fossa, superior to pons and it is found within the tentorial notch.

Length is about 2 cm.

Cerebral aqueduct is a canal which traverses the mid­brain, and connects third and fourth ventricles.

Term Paper # 2. Parts of the Mid Brain: 


A transverse section of mid brain shows following parts:

1. Tectum – dorsally situated.

2. Cerebral peduncles – ventrally situated.

Each cerebral peduncle is divided into crus cerebri and tegmentum by the substantia nigra.


Crus cerebri is the anterior part of the mid brain and forms posterior boundary of the inter peduncular fossa. Here it is related to the posterior perforated substance.

The crura are separated by a median sulcus, through this sulcus the oculomotor nerve enters the inter peduncular fossa. The lateral surface of the crus is crossed by the trochlear nerve.

Colliculi (tectum) are situated on the dorsal surface of the tegmentum. Colliculi are four in number, arranged into a pair of superior colliculi and a pair of inferior colliculi. They are separated from each other by a cruciform shaped sulcus.

Superior colliculi are larger and darker than the inferior colliculi and are associated with the reflex activities of the visual pathway. It is made up of collection of neurons present in different layers.


Superior colliculus is connected to the lateral geniculate body by a rope like structure called superior brachium.

Inferior colliculi are smaller, associated with reflex activities of auditory pathway and is connected with medial geniculate body by the inferior brachium.

Term Paper # 3. Structure of the Mid Brain:

I. Transverse section of mid-brain at the level of superior colliculus.


Structures can be identified are:

1. Crus cerebri – contains fibres – Lateral 1/5 – temporo pontine fibres, Medial 1/5 – fronto pontine fibres and Middle 3/5 – cortico spinal tract.

2. Substantia nigra – Band of grey matter situated in between the tegmentum and crus cerebri.

3. Red nucleus is situated anterior to the peri aqueductal grey matter.


4. Oculomotor nucleus is a nuclear complex situated within the anterior part of peri aqueductal grey at the level of superior colliculus.

It has many components namely:

(a) Dorsal nucleus

(b) Ventral nucleus

(c) Ventrimedial nucleus

(d) Intermediate nucleus

(e) Caudal central nucleus

(f) Edinger Westphal nucleus.

Oculomotor nerve arises from this nuclear complex. Edinger Westphal nucleus is parasympathetic part of the oculomotor nucleus. It supplies sphincter pupillae and ciliaris muscles.

5. Inter peduncular nucleus

6. Pre tectal nucleus

7. Superior colliculus

8. Medial lemniscus

9. Trigeminal lemniscus

10. Dorsal tegmental decussation

11. Ventral tegmental decussation.

II. Transverse section of mid brain at the level of the inferior colliculus.

Following structures can be identified:

1. Crus cerebri

2. Lateral lemniscus

3. Medial lemniscus

4. Spinal lemniscus

5. Trigeminal leminscus

6. Superior cerebellar penduncles decussate medial to the medial lemniscus. It connects the mid­brain with cerebellum.

7. Rubrospinal tract.

8. Nucleus of trochlear nerve – lies ventral to the cerebral aqueduct but within the peri aqueductal grey matter.

The fibres of the trochlear nerve pass dorsally and decussate with the nerve of the opposite side. This is the only cranial nerve emerging from the dorsal surface of the brain. Inferior brachium connects the inferior colliculus with the medial geniculate body.

Substantia Nigra:

Grey matter between crus cerebri and tegmentum. It is divided into thickly arranged dorsal portion and sparsely arranged ventral portion. The ventral portion contains nerve fibres and neurons.


Afferents are received from:

1. Motor cortex

2. Sensory cortex

3. Red nucleus

4. Lentiform nucleus

5. Reticular formation.

Efferents are received from:

1. Superior colliculus

2. Amygloid body

3. Ventro lateral thalamic nucleus

4. Caudate nucleus

5. Lentiform nucleus

6. Cingulate gyrus.

Functions of Substantia Nigra:

The neuro transmitter substance found within the substantia nigra is called dopamine. It forms a part of extra pyramidal system. Lesions – cause – Parkinso­nian’s disease.

Term Paper # 4. Connections of the Mid Brain:

1. It is connected to the cerebellum via – superior cerebellar peduncle.

2. Red nucleus, afferents are – dendato rubro- thalmic fibres. Efferents are rubro spinal tract, reticular formation, cerebellum, thalamus and inferior olivary nucleus.

3. Substantia-nigra – It is connected with red nucleus.

4. Superior colliculus.

5. Inferior colliculus.