Friday, 6 February 2015

Human brain


    Human brain is one of the most complex and magnificent organs in the human body. Our brain gives us self consciousness and consciousness of our environment, processing a constant stream of sensory data.  The brain’s neurons record the memory of every event in our lives.It is the organ which coordinates several muscular moments, analyzes the streams data from sensory organs, controls secretions of glands,memorizes the past experiences, controls the emotions and thoughts with touch of judgement and intelligence.Every creative thought, feeling, and plan is developed by our brain.

Brain is dome shaped mass of soft nervous tissue protected by bony case called Cranium.It is command center of human nervous system. It receives information in the form of electrical and impulses, analyzes them and generates appropriate out put signal to the muscles.Human brain is largest in size in proportion to body  when compared to other animals.It weighs about 1.5kgs consuming 20% of total oxygen and glucose in body.

The outer surface of brain is dull grey in color. The deeper layers of brain is white in color.  A fluid called Cerebro-spinal fluid (CSF) flows in between the outer and middle membranes. This fluid flows from brain to spinal cord and back to brain. It protects brain and spinal cord from injuries, and provides nutrients to the nervous tissue.Brain nerve cells are two types namely neurons and neuroglia. Neurons are responsible for communication and processing. Neuroglia act as helper cells for the brain. They are four types namely astrocytes, oligodendrocytes, microglia, and ependymal cells.Astrocytes protect neurons by filtering nutrients out of the blood and preventing chemicals and pathogens from leaving the capillaries of the brain.Oligodendrocytes wrap the axons of neurons in the brain to produce the insulation known as myelin. Myelinated axons transmit nerve signals much faster than unmyelinated axons, so oligodendrocytes accelerate the communication speed of the brain.Microglia act much like white blood cells by attacking and destroying pathogens that invade the brain.Ependymal cells line the capillaries of the choroid plexuses and filter blood plasma to produce cerebrospinal fluid

   Brain is divided into three parts namely Fore brain( prosencephalon), mid brain(mesencephalon) and hind brain(rhombencephalon).The hind brain continues as spinal cord.

Fore brain (prosencephelon)
Fore brain is the largest part of the brain and is called Cerebrum. A deep groove in the middle of the cerebrum divides it into two equal halves called cerebral hemispheres. Right and left cerebral hemispheres are connected to each other by a bundle of axons. The outer portion of cerebral hemispheres is grey in color and is called Cerebral cortex.  There are several ridges and grooves on cerebrum. The ridges are called Gyri and grooves are called sulci. These folded pattern on cerebrum greatly increases surface area for accommodation of nerve cells.Three deep grooves present across each cerebral hemisphere divides  into four lobes namely Frontal lobes, Parietal lobes, Temporal lobes and Occipital lobes.

Cerebrum is the highest centre for controlling several functions of body. The interesting fact is that the right hemisphere control the left side of body and left hemisphere controls the right side of body.In cerebral hemispheres there are centers to receive and analyze various information such as visual, auditory, olfactory, gustatory etc.,The part of the brain below the cerebrum are together known as Dienchephalon.This portion of brain controls the emotions such as anger, pain, pleasure. This part connects fore brain with mid brain.

Dienchephalon

The thalamus, hypothalamus, and pineal glands make up the major regions of the diencephalon.Thalamus acts like the telephone exchange  of the brain by routing sensory inputs to the correct regions of the cerebral cortex.The thalamus relays sensory and motor signals to the cortex and is involved in regulating consciousness, sleep and alertness.The pineal gland produces the hormone melatonin. Light striking the retina of the eyes sends signals to inhibit the function of the pineal gland. In the dark, the pineal gland secretes melatonin, which has a sedative effect on the brain and helps to induce sleep. This function of the pineal gland helps to explain why darkness is sleep-inducing and light tends to disturb sleep. Hypothalamus present in this region secretes hormones. Pitutary gland is attached to the hypothalamus by a stalk. . The hypothalamus connects the nervous system to the endocrine system where hormones are produced via the pituitary gland.Hypothalamus has centers for regulating body temperature, hunger, thirst and emotions.




Mid brain 

The mid brain has groups of nerve cells and large number of myelinated axons of the nerve cells present in other regions of the brain. Some of the nerve cells in this region control the body muscles and their movements.Midbrain can be further subdivided into two main regions: the tectum and the cerebral peduncles.
The tectum is the posterior region of the midbrain, containing relays for reflexes that involve auditory and visual information. The pupillary reflex (adjustment for light intensity), accommodation reflex (focus on near or far away objects), and startle reflexes are among the many reflexes relayed through this region.the cerebral peduncles contain many nerve tracts.


Hind brain


It has two parts namely Cerebellum and brain stem. Cerebellum lies behind the cerebrum. It controls the equilibrium and posture of the body. Brain stem has two parts Pons voroli and Medulla oblongata. Medulla oblongata continues as the spinal cord.It  controls several vital functions of the body such as respiration, heart beat, blood pressure, temperature and secretion of salivary glands etc.



Many reflexes in the body are integrated in the brain, including the pupillary light reflex, coughing, and sneezing. Many reflexes protect the body from harmful stimuli such as a thorn pricking the foot and in couple of other  instances like coughing and sneezing clear the airways of the lungs. Other reflexes help the body respond to stimuli, such as adjusting the pupils to bright or dim light.These reflex actions are controlled mostly by spinal cord and mid brain. Reflex actions are fast responses to dangerous stimuli.

Meninges

Three layers of tissue protects the brain from almost all mechanical injuries, collectively known as the meninges, surround and protect the brain and spinal cord.The dura mater forms the leathery, outermost layer of the meninges. Dense irregular connective tissue made of tough collagen fibers gives the dura mater its strength. The dura mater forms a pocket around the brain and spinal cord to hold the cerebrospinal fluid and prevent mechanical damage to the soft nervous tissue. The name dura mater comes from the Latin for “tough mother,” due to its protective nature.

The arachnoid mater is found lining the inside of the dura mater. Much thinner and more delicate than the dura mater, it contains many thin fibers that connect the dura mater and pia mater. The name arachnoid mater comes from the Latin for “spider-like mother”, as its fibers resemble a spider web. Beneath the arachnoid mater is a fluid-filled region known as the subarachnoid space.

 The pia mater as the innermost of the meningeal layers, rests directly on the surface of the brain and spinal cord. The pia mater’s many blood vessels provide nutrients and oxygen to the nervous tissue of the brain. The pia mater also helps to regulate the flow of materials from the bloodstream and cerebrospinal fluid into nervous tissue.

Prepared by Kiran.B



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