Central nervous system

The central nervous system includes the brain and spinal cord. The brain and spinal cord are protected by bony structure, membranes and fluid . The brain is held in the cranial cavity of the skull and and it’s consists of the cerebrum,cerebellum and the brain stem. The nerves involved are cranial nerves and spinal nerves. The central nervous system (CNS) represents the largest part of the nervous system, including the brain and spinal cord. Together, with the peripheral nervous system (PNS),it has a fundamental role in the control of behaviour.

Central nervous system including the brain and spinal cord .

Brain (Encephalon)

Human brain is the only thing in the universe which is attempting to understand itself . The brain is found in the cranial cavity . Within it are found the higher nerve centers responsible for conducting the sensory and motor systems of the body(forebrain) . The brain stem house the lower nerve centers (consisting of midbrain,pons and medulla) .

Encephalology

The study of brain in all aspects is called encephalology (G. enkephalon- brain, logos- discourse).

Location 

The brain is situated in the cranial cavity of the skull . The cranial bones and protect it from mechanical injury.

What is size of brain ?

The brain forms about 98% of the weight of the whole CNS . The average human brain weighs from 1200 to 1400 grams.

How many number of neurons in brain ?

Brain has about 100 billion neurons.

Brain

Morphology of brain 

The brain is a soft , whitish, somewhat flattened organ. It is divisible into three regions : forebrain, midbrain and hindbrain. These regions are also called prosencephalon, mesencephalon and rhombencephalon respectively.

Forebrain (Prosencephalon)

It forms the greater part of the brain . It consists of three regions : olfactory lobes, cerebral hemispheres or cerebrum and diencephalon.

i. Olfactory lobes. The olfactory lobes are a pair of very small ,solid , club-shaped bodies widely separated from each other . They are fully covered by the cerebral hemispheres, and are visible only in the ventral view of the brain. Each olfactory lobe consists of anterior olfactory bulb and posterior olfactory tract. 

ii. Cerebral hemispheres (cerebrum) 

The cerebral hemispheres or cerebrum form the largest part of the brain, and constitute four-fifths of its weight. They cover all other parts. The cerebrum or top portion of the brain, is divided by a deep crevice, called the longitudinal sulcus. The longitudinal sulcus separates the cerebrum into the right and left hemispheres. In the hemispheres lies the cerebral cortex, basal ganglia and the limbic system. The two hemispheres are connected by a bundle of nerve fibres called the corpus callosum. The right hemisphere is responsible for the left side of the body while the opposite is true of the left hemisphere. Each of the two hemispheres are divided into four separated lobes the frontal in control of specialized motor control, learning planning and speech, parietal in control of somatic sensory functions, occipital in control of vision and temporal lobes which consists of hearing centers and some speech. Located deep to the temporal lobe of the cerebrum is the insula.

iii. Diencephalon

Diencephalon encloses a slit – like cavity, the third ventricle. The thin roof if this cavity is known as the epithalamus, the thick right and left sides as the thalami, and floor as the hypothalamus.

Hypothalamus – The hypothalamus is visible in the ventral view of the brain. It has optic chiasma, where some of the fibres of the optic nerves cross to opposite sides. Behind the optic chiasma is a greyish protuberance, the infundibulum. A rounded body, called hypophysis (pituitary) is attached to the infundibulum. The pituitary is the most important endocrine gland. It secretes a number of (nine) hormones. A pair of small, rounded eminences , the mammillary bodies, lie behind the infundibulum. The hypothalamus consists of many masses of grey matter called hypothalamus nuclei, scattered in the white matter. It is more than its size called hypothalamic nuclei, scattered in the white matter. It is more important than its size (less than 1% of the brain volume) suggests. 

Epithalamus. The epithalamus is not formed of nervous tissue. It consists of pia mater only. Hence, it is of relatively little significance as a nerve centre. Its anterior part is vascular and folded. It is called anterior choroid plexus. Behind this plexus, the epithalamus gives out a short stalk, the pineal stalk, which bears a small, rounded body, the pineal body, at its tip. 

Third Ventricle. The third ventricle of diencephalon communicates anteriorly with the first and second ventricles of the cerebral hemispheres by foramen of Monro, and posteriorly with the fourth ventricle of medulla oblongata by way of a narrow passage, the iter, present in the midbrain. 

(b) Midbrain (Mesencephalon)

The midbrain is significantly small. It consists of two heavy fibre tracts, called cerebral peduncles or crura cerebri, on the ventral side, and two swellings, termed superior and inferior colliculi on each side of the dorsal surface. The superior and inferior colliculi of each side are referred to as corpora bigemina and of both the sides as corpora quadrigemina. The cerebral peduncles connect the hindbrain with the forebrain.

 Iter. A very narrow cavity, the cerebral aqueduct or iter, extends through the midbrain, It connects the third and the fourth ventricles.

c) Hindbrain (Rhombencephalon)

The hindbrain consists of cerebellum, pons varolii and medulla oblongata. 

(i) Cerebellum (Little Brain). The cerebellum, like the cerebrum, is very large and well developed. It forms about one- eighth of the brain mass. It lies below the posterior portions of the cerebral hemispheres and above the medulla. It consists of a pair of large lateral parts, the cerebellar hemispheres, and a small median portion, the ver- mis. The cerebellum is solid. It has a branching, tree-like core of white matter, called arbor vitae- “the tree of life”, surrounded by a sheath of grey matter, the cerebellar cortex. The latter is greatly folded.

 (ii) Medulla Oblongata. The medulla is the lowermost part of the brain. It is conical in form, and encloses a cavity, the fourth ventricle. This ventricle, like the third ventricle, has a very thin, epithelial, nonnervous, folded roof. It is called the posterior choroid plexus. Below the plexus, the roof has three openings a pair of lateral apertures (foramina of Luschka) and a median aperture (foramen of Magendie). These apertures permit the exit of cerebrospinal fluid into spaces around the brain.In the medulla oblongata, most of the sensory and motor fibres cross from one side to the other. Thus, the left cerebral hemisphere controls the right side of the body and vice versa. The reason for this is not known. The lower end of medulla passes into the spinal cord. There is no demarcation between the two. The medulla is the control center for respiratory, cardiovascular and digestive functions.

(iii) Pons Varolii. An oval mass, called the pons varolii, lies above the medulla oblongata. It consists mainly of nerve fibres which interconnect the two cerebellar hemispheres and also join the medulla with higher brain centres, hence it’s name pons means bridge.  The pons houses the central centers for respiration inhibitory function. Here, it will interact with the cerebellum.

Brain stem: The medulla,pons, midbrain and diencephalon are collectively called the brain stem. All of them are hidden by cerebrum and cerebellum .

Cerebrospinal fluid : All the ventricles of the brain are continuos and lined by a columnar, ciliated epithelium, the ependyma . They contain lymph-like extracellular fluid called the cerebrospinal fluid . This fluid is secreted by the choroid plexuses by filtration of blood.

Meninges (protective coats)

The brain is surrounded by three protective coats of connective tissue besides the bony cranium. These are known as meanings (singular- meninx) .

Histology

The brain is composed of two types of nervous tissue : grey matter and white matter. They grey matter consists of nerve cell bodies along with their dendrites and proximal parts of axons. The white matter consists of tracts of medullated nerve fibres extending from and to the nerve cells.

Functions

The functions various parts of the brain are summed up below –

1. Olfactory Lobes – These receive information about smell. These are not well developed in man as he depends more on sight than on smell.

2. Cerebrum – It perceives the pulses coming from the sense organs and the seat of intelligence, memory, will, thinking, learning and emotions.

3. Diencephalon – Each thalamus acts (i) as a relay centre for sensory impulses, except those of smell ; (ii) as reflex centre for muscular and glandular activities ; and (iii) as a perceiving centre for crude sensation such as extremes of heat, cold, pain etc.

The hypothalamus contains centres for maintaining homeostasis by regulating body temperature, heart beat, water balance and blood pressure and controls hunger, thirst, sexual arousal and feeling of pain, pleasure, anger and fear, and hypophysis. The hypophysis functions as a master endocrine gland.

4. Corpora Quadrigemina Superior colliculi control reflexes of the iris and the eyelids. (i) Inferior colliculi analyse and relay sensory information coming from the ears to the cerebrum. In mammals, the role of vision has shifted from the midbrain to the cerebrum.

5. Crura Cerebri – These carry sensory impulses from the medulla oblongata and the spinal cord to the thalami.

6. Cerebellum – It maintains posture, equilibrium and muscle tone and coordinates the voluntary movements initiated by the cerebrum.

7. Pons Varolli– It transmits impulses from one side of the cerebellum to the other and from medulla to higher centres in the brain. It also controls some aspects of respiration.

8. Medulla Oblongata– (i) Its cardiac centre controls the rate of heart beat. (ii) Its respiratory centres control breathing movements and vasomotor centre controls expansion and contraction of blood vessels to regulate blood pressure. (iii) Its reflex centres control swallowing, coughing, sneezing, vomiting, urinating, defaecating and hiccoughing. (v) All messages entering and leaving the brain pass through the medulla.

Function ofBrain

Spinal cord (myelon)

Size

The spinal cord is about 45 cm.** long and weighs about 35 gm. in an average man. It is about 45cm (18 inch) long in men and around 43cm (17 inch) in women, oval-shaped and is enlarged in the cervical and lumbar region.

Location

The spinal cord is located in the vertebral canal of the vertebral column. 

Shape

The spinal cord has a shape that is compressed dorso-ventrally , giving it an elliptical shape. The cord has groove in the dorsal and ventral sides. The spinal cord is the main pathway for information connecting the brain and peripheral nervous system .

Spinal cord

Morphology of spinal cord

  • The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain (the medulla oblongata specifically) . The brain and spinal cord together make up the central nervous system.
  •  The spinal cord does not extend to the coccygeal region because during development the vertebral column elongates more rapidly than the spinal cord. The spinal cord is slightly flattened anteroposteriorly (dorsoventrally) and has 2 enlargements : upper cervical and lower sacral. These swellings have more nerve cell bodies than elsewhere, and send nerves to the arms and legs respectively. 
  • The spinal cord encloses a narrow longitudinal cavity, the central canal, lined by a simple ciliated columnar epithelium called ependyma. The central canal opens above into the fourth ventricle of the brain, but is closed below. It is filled with the cerebrospinal fluid.

 Histology

The spinal cord is formed of two types of nervous tissue : grey matter and white matter. The grey matter is internal, i.e., surrounds the central canal. It has the form of letter H in cross-section. The arms of H are called posterior and anterior grey columns. Portions of the grey matter present behind and in front of the central canal are called posterior and anterior grey commissures. At certain levels, the grey matter has lateral columns . The grey matter is composed of association neurons and cell bodies of motor neurons. The white matter is outside the grey matter. It consists of bundles of medullated nerve fibres. The white matter is divided by the columns of the grey matter into three columns or funiculi on either side : the posterior, lateral and anterior funiculi

Spinal Tracts

  In the white matter, the bundles of medullated nerve fibres form tracts of two types : 

the sensory ascending tracts carrying impulses to the brain from the peripheral receptors brought by spinal nerves; and the motor descending tracts bringing impulses from the brain for trans- mission to muscles and glands by spinal nerves. The sensory tracts are located mainly in the posterior part of the cord, the motor tracts are found in the anterior and lateral parts of the cord. The fibres of all the tracts cross over from one side to the other of the cord somewhere in their course. 

Meninges.

All the three meninges, namely, pia mater, arachnoid membrane and dura mater, covering the brain are continued over the spinal cord also. They likewise enclose the subarachnoid and sub- dural spaces. The subarachnoid space similarly contains the cerebrospinal fluid. This cavity contains thin-walled veins, adipose tissue and loose connective tissues. 

Functions

The spinal cord performs two important functions : 

(i) It conducts sensory and motor impulses to and from the brain via sensory ascending tracts and motor descending tracts. 

(ii) It acts as a centre for the spinal reflexes. Thus, it reduces brain’s work.

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