09 Control and Coordination - part 07 - The Human brain

 

09 Control and Coordination - part 07 - The Human brain

The Human brain :

• Study of all aspects of the brain is called encephalology. 
• Brain can be divided into three main parts - 

1. Forebrain
2. Midbrain and
3. Hindbrain.

a. Forebrain : 

• Forebrain consists of - 

1. Olfactory lobes
2. Cerebrum and 
3. Diencephalon.

i. Olfactory lobes :

• Highly reduced in human brain .
• Covered by cerebrum from all sides except ventral. 
• Each lobe consists of a olfactory peduncle and olfactory bulb.

ii. Cerebrum :

• Largest part of the brain,
• Making up about 85 % of total brain. 
• Divided into right and left cerebral hemisphere by means of a deep median, long fissure. 
• Two hemispheres internally connected to each other by a thick band of nerve fibres called corpus callosum.
• Outer surface of cerebrum is called cerebral cortex 
• Deep inner part is cerebral medulla. 
• Cerebral cortex has outer thin region composed of grey matter and inner medulla composed of white matter.
• Surface of each cerebral hemisphere is greatly folded by many convolutions or gyri and grooves called sulci. 
• These greately increase total surface area for accomodation of the vast number of nerve cells.
• Each cerebral hemisphere is further divided into four main lobes by three deep sulci. These are -

I. Centre sulcus  : 

• Which demarcates frontal lobe from the parietal lobe.

II. Parieto-occipital sulcus  :

• Separates the parietal from occipital lobe.

III. The lateral or sylvian sulcus :

• Demarcates the temporal lobe from the frontal and parietal lobes

• Since these three sulci are not complete, the lobes are not clearly demarcated from each other. 
• A fifth median lobe called insula or insular cortex is folded deep within the lateral sulcus.
• Grey matter of cerebral cortex mainly consists of cell bodies of billions of neurons along with non-medulated fibres and dendrons. 
• White matter mainly has axons of myelinated nerves.
  

Do you know ?
Corpus callosum  Typically seen in mammalian brain.

• Largest commissure of the brain. 
• Has an anterior and posterior fold called genu and splenium respectively.

Functional areas of cerebrum :

1. Frontal lobes : 

• Have motor area which controls voluntary motor activities or movements of muscles. 
• Premotor area is higher centre for involuntary movements and autonomus nervous system. 
• Association area is for coordination between sensation and movements. 
• Broca’s area/motor speech area. It translates the thoughts into speech.
• Expression of emotions, intelligence, willpower, memory, personality areas are located in the frontal lobe.

2. Parietal lobes : 

• Mainly for somaesthetic sensation of pain, pressure, temperature, taste (gustatoreceptor).

3. Temporal lobes : 

• Contains centres for smell (olfactory), hearing (auditory), speech and emotions.

4. Occipital lobes : 

• Have visual are mainly for sense of vision.

Wernicke’s area :

• Area of contact between temporal, parietal and occipital lobes is centre for Wernicke’s area or intelligence centre. 
• It helps in the understanding of spoken and written words.
• The cerebrum, thus shows all three types of areas sensory, motor and association area.

Basal nuclei or basal ganglia

• Are grey masses present within the white matter or lying on the lateral sides of thalamus. 
• Receive neurotransmitters from various parts. 
• Help the cortex in the execution of activities at the subconscious level e.g. writing slow or rapid typing. 

Corpus striatum :  at the floor of cerebrum is the largest basal nucleus.

iii. Diencephalon :

• Part of the forebrain that contains the epithalamus, thalamus and hypothalamus.
• Lies below the corpus callosum and above the midbrain. 
• Encloses a single cavity termed third ventricle/Diocoel which communicates with the two lateral ventricles of cerebrum through a narrow opening called foramen of Monro.

Epithalamus :

• Is the thin non nervous roof of the diencephalon. 
• Anterioly fused with the piamater to form the anterior choroid plexus and from its dorsal wall it is connected to pineal gland through a pineal stalk. 
• Earlier this gland was though to be vestigeal, but later it has been found to produce to hormone melatonin (sleep inducing hormone; also related to reproductive behaviour). 
• The lateral thick walls of diencephalon form the thalami. They mainly contain grey matter. 
• The habenculor commissure connects two thalami. 
• Different parts of the brain are interconnected by the RAS (Reticular Activating System) through the thalami. It is called relay centre .
• Transmitts all sensory impulses except those of olfactory (smell) to the cerebrum (gatekeeper of cerebrum connecting the anterior lateral ventricle to the iter posteriorly. 
• The narrow cavity of diencephalon is called IIIrd ventricle or diocoel. 
• It connects anteriorly to the two lateral ventricles by a single opening called Foramen of Monroe and posteriorly to the IVth ventricle or metacoel through a narrow duct of Sylvius or iter.

Hypothalamus : 

• Forms a floor of the diencephalon. 
• Richly supplied with blood vessels (Hypothalamo-hypophyseal portal vein) helps in feed back mechanism for hormonal control). 
• Maintains homeostasis, internal equilibrium of the body and involuntary behaviour control. 
• Also contains hypothalamic nuclei in its white matter with neuro-secretory cells involved in the production of hormones oxytocin and vasopressin.
• Is a link between the nervous and the endocrine system.
• Has higher centres for endocrine system. 
• Regulates heart rate, respiration, blood pressure (B.P.), body temperature, water and electrolyte balance. 
• Has centres for hunger, thirst, sleep, fatique, satiety centre, secretion of glands of stomach and intestine. 
• Produces neurohormones that stimulate the pituitary gland. 
• A complex neuronal circuit called the limbic system is formed by the hypothalamus amygdala, parts of epithalamus and thalamus, hippocampus and other areas. 
• It appears to be responsible for emotional reactions, motivational drives and memory. 
• Floor of the hypothalamus continues as a downward projection called hypopyseal stalk or infundibulum 
• Infundibulum connects it to the hypophysis (pituitary gland) both physically and functionally by secretion of neurotransmitters. 
• The inferior surface Also bears the optic chiasma (crossing of the two optic nerves) and a pair of mammillary bodies (unique to mammmalian brain and responsible for recollective memory).

b. Mid brain :

• Located between diencephalon and the pons varolli. 
• Contains the cerebral aqueduct or iter that connects the third and fourth ventricles. 
• Corpora quadrigemina are four rounded elevations on the dorsal surface of the mid brain. 
• Two superior colliculi are involved in visual reflexes and the two inferior colliculi are relay centres for auditory reflexes that operate when it is necessary to move the head to hear sounds more distinctly.
• Also contains on its inferior surface two thick fibrous tracks called cerebral peduncles or crura cerebri. 
• These tracts of ascending and descending nerve fibres from RAS and connect the cerebrum mid brain. 
• Near the centre of the mid brain is a mass of grey matter scattered within the white matter. It is called the red nucleus. 
• It plays an important role in -  

1. controlling posture and muscle tone
2. modifying some motor activities and 
3. motor coordination.

Always Remember
Wernicke's area :

• Sensory speech area responsible for understanding and formulating written and spoken language.

Broca's area :

• Motor speech area and translates thoughts into speech .
• Controls movement of tongue, lips and vocal cords.

c. Hind brain :

• Posterior region of the brain. \
• Consists of -  

1. Pons varolli
2. Cerebellum and
3. Medulla oblongata. 

Pons varolli :

• Appears as a rounded bulge on the underside of the brain stem .
• Brain stem consist of -  

1. mid brain
2. pons and medulla and 
3. Continues upto spinal cord.

• Contains a cross band of nerve fibres connecting cerebrum, cerebellar lobes, medulla oblongata and spinal cord. 
• Also contains several nuclei. 

Cerebellum :

• Second largest part of the brain.
• Consists of - 

1. Two lateral hemispheres and 
2. A central vermis.

• Composed of white matter with a thin layer of grey matter, the cortex. 
• White matter intermixes with the grey matter and shows a tree-like pattern called arbor vitae.
• Surface of cerebellum shows convolutions (gyri and sulci) a number of nuclei lie deep within each lateral or cerebellar hemisphere. 
• Over 30 million neurons lie in the cortex. 
• Three pairs of myelinated nerve bundles called cerebullar penduncles connect cerebellum to the other parts of CNS. 
• It is an important centre which maintains - 

1. equilibrium of body
2. posture
3. balancing orientation
4. moderation of voluntary movements
5. maintainance of muscle tone. 

• Is a regulatory centre for neuromuscular activities.
• Controls the rapid activities like walking, running, speaking etc. 
• All activitie of cerebellum are involuntanry (though may involve learning in early stages). 

Medulla oblongata :

• Posterior conical part of the brain.
• Continues as the spinal cord.
• Has inner grey matter and outer white matter.
• Controls involuntary vital functions like heart beat, respiration, vasomotor activities and peristalsis. 
• Also controls non vital reflex activities like coughing, sneezing, swallowing, vomitting, yawning etc. 
• Cavity of medulla is called IVth ventricle or metacoel. 
• It’s roof has the posterior choroid plexes for secretion of CSF. 
• The posterior choroid plexes also shows 3 openings - 

1. A pair of lateral foramen of Luschka and 
2. A median foramen of Magendie.