3.2. Topic content.

Midbrain lies between pons and diencephalon. It consists of two parts:

A. Tectum - comprising superior and inferior colliculi

B. Cerebral peduncles - consisting of basis pedunculi, substantia nigra and tegmentum, which includes red nucleus.

Superior Colliculus

It is a small structure and is an important center for reflexes. Through the tectospinal tract, the superior colliculus controls the movements of eyes, head, trunk and limbs in response to visual impulses. The efferent fibers from superior colliculus going to the nucleus of III cranial (oculomotor) nerve cause constriction of pupil during light reflex. Thus, it forms the center for light reflex. Superior colliculus also receives afferents from optic tract, which helps in the integration of optical and postural reflexes.

Inferior Colliculus

It consists of single layer of neurons to which the lateral lemniscus (auditory fibers) synapses. The inferior colliculus is the center for auditory reflexes. Stimulation of this also produces reflex vocalization.

Cerebral peduncles

Basis Pedunculus

It consists of pyramidal tract fibers in the middle, temporopontine fibers laterally and frontopontine fibers medially.

Substantia Nigra

It is situated below the red nucleus. Substantia nigra is considered as one of the components of basal ganglia. Its pathology is known as parkinsonism. The distinguishing features are like at Parkinson’s disease. Reason is one: dophamine deficiency.

Tegmentum

This lies dorsal to substantia nigra and is actually the upward continuation of the reticular formation in pons.

Three decussations take place in tegmentum. It also contains red nucleus.

1. Superior cerebellar peduncle: This is formed by fibers between cerebellum and other parts of central nervous system. The fibers are predominantly efferent fibers from dentate nucleus ofcerebellum; and few fibers are from other cerebellar nuclei—nucleus globosus and nucleus emboliformis.

2. Forel's decussation: This is due to the crossing of rubrospinal tracts from either side.

3. Mynert's decussation: It is due to the crossing of medial longitudinal bundle formed by the efferents of 3rd, 4th and 6th cranial nerves.

Red Nucleus

Red nucleus is a large oval or round mass of gray matter extending between the superior colliculus and hypothalamus.

Red nucleus has two parts:

1) Nucleus magnocellularis, which is formed by large cells. Fibers from this form the rubrospinal and rubrobulbar tracts.

2) Nucleus parvocellularis, which is formed by smaller cells. The fibers from this form mainly the rubroreticular tract.

Connections of Red Nucleus

1) Afferent connections: Red nucleus receives fibers from:

a). Motor cortex (area 6) to nucleus parvocellularis—corticorubral fibers

b) The globus pallidus and subthalamic nuclei to nucleus magnocellularis—pallidorubral fibers.

c) Dentate nucleus (of opposite side) to nucleus magnocellularis - cerebellorubral or dentatorubral tract.

2) Efferent connections: Red nucleus sends efferent fibers to various parts of brain and spinal cord.

a) Rubrospinal tract to spinal cord

b) Rubrobulbar tract to medulla

c) Rubroreticular fibers to reticular formation

d) Rubrothalamic tract to lateral ventral nucleus of thalamus

e) Rubroolivary tract to inferior olivary nucleus

f) Fibers to nuclei of 3rd, 4th and 6th cranial nerves.

Functions of Red Nucleus

1) Control of muscle tone: Because of its connections with cerebellum, vestibular apparatus and skeletal muscle, the red nucleus plays an important role in maintaining the muscle tone.

2) Control of complex muscular movements: Red nucleus controls the complex muscular movements.

3) It plays an important role in the integration of various impulses received from many important areas of brain.

4). Control of skilled movements: Red nucleus plays an important role in controlling skilled muscular movements by its connections with spinal cord and cerebral cortex.

Decerebrative rigidity. Cutting at the level of Varolii pons anterior limb (rubro-spinal tract) causes in muscles extensors tone increasing in animals. Such animals do not save equilibrium and fall down. This phenomenon is known as decerebrative rigidity. It is considered that such a state is delt with cerebellum and red nuclei descendant influencings liquidating to bulbar centers.

Animals with brain cutting at anterior colliculi level are known as mesencephalic animals. Mesencephalic anaimals differ from spinal ones with muscular tone regulation and body position in a space. Decerebrative rigidity is absent in them. Red nuclei in midbrain causes inhibitory influencing on vestibular nucleus of Deuters to which afferentation from auricular labirinthiums and muscular proprioreceptors comes through vestibular-spinal tract. Red nuclei decomposition causes expressed decerebrative rigidity in animals. Red nuclei irritation leads to significant muscular tone decreasing – the animals become “like a sac”. Decerebrative rigidity is determined also by afferentation coming to Deuters’ nuclei from muscles proprioreceptors and vestibular apparatus. Spine posterior radici and vestibular nerves cutting also cancels decerebraive rigidity.

If red nuclei do not give inhibitory influencings at posterior brain level, than central excitements are increased and get diffused. Thus, extensors activity is increased selectively.

MIDBRAIN AND BASAL NUCLEI STRUCTURES PARTICIPATION IN MASTICATION AND SWALLOWING REGULATION:

They are black substance in midbrain (dophamine source regulating respiration, mastication and swallowing alternation) and caudate nucleus, shell (putamen) among basal ganglii because they inhibit substantia nigra (black substance) activity with inhibitory mediator gama-amino-oleic acid secretion through third basal ganglium - globulus pallidus.

Reflex		Center	Animal preparation to demonstrate
General static reflexes (Righting reflexes)	1.Labyrinthine righting reflexes acting upon the neck muscles	Midbrain	Thalamic or normal blind folded animal
	2.Neck righting reflexes acting upon the body
	3.Body righting reflexes acting upon the head
	4.Body righting reflexes acting upon body
	5. Optical righting reflexes	Occipital lobe	Labyrinthectomized animal
Local static reflexes	1. Positive supporting reflexes	Spinal cord	Decorticate animal
	2. Negative supporting reflexes
Segmental static reflexes	Crossed extensor reflex	Spinal cord	Spinal animals
Statotonic or attitudinal reflexes	1. Tonic labyrinthine and neck reflexes acting on the limbs	Medulla oblongata	Decerebrate animal
	2. Labyrinthine and neck reflexes acting upon the eyes

Cranial-cerebral nerves and their functions

Nerve	Nuclei location	Functions
I pair, olfactory nerve	Skull	Participates in olfaction (smelling)
II pair, optic	Skull	Participates in vision
III pair, oculo-motor nerve	Midbrain	Rises superior eyelid, turns eyeball up, down, inside and outside
IV pair, trochlear	Midbrain	Innervates muscles turning eyeball below and outside
V pair, trigeminal nerve	Pons	Sensory part innervates skin of face, frontal-parietal head parts, haired part, eyeball, nasal and oral mucosa, anterior 2/3 of tongue, facial skull bones periosteum, dura mater of anterior and middle cranial fossae, proprioreceptors of masseter muscle ocular and mimic muscles. Motor fibers innervate masseter, temporal, medial and lateral pterygoideal, mandibular-hyoideal muscle as well as muscle rising tympanal membrane
VI pair, abducent (abducens) nerve	Midbrain	Innervates eye muscles abducting eyeball outside
VII pair, facial	Pons	Innervates mimic and other muscles of face and head, platysmal and stylo-hyoideal muscles; sensory part provides innervation of 2/3 of tongue (for sweat, salty and bitter tastes)
VIII pair, vestibular-cochlear nerve	Medulla oblongata	2 parts: vestibular (provides static reflexes and gravitation overcoming id est regulates body orientation in space) and acoustic (comes to quadrigemina inferior collins in lateral lemnisc and thus provides orienting reflex to sound stimuli)
IX pair, glosso-pharyngeal	Medulla oblongata	Motor part innervates stylo-pharyngeal muscle rising pharynx; sensory part – posterior 1/3 of tongue (bitter taste), soft palate, pharynx, fauces, epiglottis anterior surface, acoustic tube and tympanal cavity; vegetative parasympathetic nuclei innervate parotid salivary gland (much liquid saliva releasing)
X pair, vagus	Medulla oblongata	Motor fibers innervate soft palate, pharynx, larynx, epiglottis, oesophagus superior part, vocal cords, smooth muscles in stomach, small intestine and large intestine superior part; sensory part innervates inner organs; provides voice and swallowing
XI pair, accessory	Medulla oblongata	Innervates sterno-cleido-mastoideal muscle (provides head turning in opposite side) and trapezial muscle (together with the first muscle, participates in respiration enforcement)
XII pair, hypoglossal	Medulla oblongata	Motor part provides tongue turnings

Materials for auditory self-work.

4.1.List of study practical tasks necessary to perform at the practical class.

Materials and methods: scale, rotating arm-chair, stall.

Investigation object: guinea pigs, the investigated person.

Task 1. To investigate nerves: oculo-motorius (III), trochlear (IV) and abducens (VI).

While examination of eyeballs the investigator should pay the attention to the:

• pupils’ width;

• orbital fissures width,

• pupil shape,

• eyeballs localization in orbit (falling back, protrusion),

• strabismus (cross-eyedness or squint-eyedness).

Pupillar reactions – the investigated person eye should be closed by scale or hand. After this it’s necessary to determine change of another pupil size. To ask to perform eyes movement on the right and on the left.

At III-rd or IV-th pairs (cranial-cerebral nerves) injury orbital fissure can be narrowed or closed – ptosis. At III-rd pair injury one can see pupil deformation, disturbance convergent eyes movement disturbance.