The coverings of the brain and the forebrain

The command and control centre

Your brain is working while you read this line. It keeps your heart going, holds your balance and turns marks on a page into meaning. The brain is the central information processing organ of the body, and it acts as the command and control system.

It controls:

• voluntary movements and the balance of the body;
• the working of vital involuntary organs such as the lungs, the heart and the kidneys;
• body temperature, hunger and thirst;
• the circadian rhythm, the 24 hour rhythm of the body;
• the activities of several endocrine glands, and human behaviour.

It is also the place where vision, hearing, speech, memory, intelligence, emotions and thoughts are processed.

The coverings of the brain

The brain is protected by the skull. Inside the skull it is covered by the cranial meninges. The meninges have three layers.

• The dura mater is the outer layer.
• The arachnoid is a very thin middle layer.
• The pia mater is the inner layer. It is in contact with the brain tissue.

A fluid called cerebrospinal fluid fills the space under the arachnoid and cushions the brain. NCERT does not describe it, so this is for A-Level and IB.

You should find that the layers come in the order skull, dura mater, arachnoid, pia mater and brain tissue. The pia mater is the one that touches the brain.

The three regions of the brain

The brain has three main regions.

• The forebrain: the cerebrum, the thalamus and the hypothalamus.
• The midbrain.
• The hindbrain: the pons, the cerebellum and the medulla.

The cerebrum

The cerebrum is the largest part of the human brain. A deep groove divides it lengthwise into two halves, the cerebral hemispheres. A tract of nerve fibres, the corpus callosum, joins the two hemispheres.

A layer of cells covers each hemisphere. It is the cerebral cortex, and it is thrown into many folds.

• The cortex looks grey, so it is called the grey matter. The cell bodies of the neurons are crowded here, and they give the colour.
• The cortex has motor areas, sensory areas and large regions that are neither clearly sensory nor motor. These are the association areas. They do complex jobs such as joining information from different senses, memory and communication.
• Under the cortex lie tracts of fibres with a myelin sheath. They look opaque and white, so this inner part is the white matter.

Each hemisphere is also divided into four lobes, named frontal, parietal, temporal and occipital. NCERT does not name the lobes. This part is not in the NEET syllabus, but A-Level and IB ask it. The motor area lies at the back of the frontal lobe, and the main sensory area for the body lies at the front of the parietal lobe.

The thalamus and the hypothalamus

The cerebrum wraps around the thalamus. The thalamus is a major coordinating centre for sensory and motor signals.

The hypothalamus lies at the base of the thalamus. It has:

• centres that control body temperature and the urge for eating and drinking;
• groups of neurosecretory cells, which make hormones called hypothalamic hormones.

The limbic system

The inner parts of the cerebral hemispheres and a group of deep structures, such as the amygdala and the hippocampus, form the limbic lobe, also called the limbic system. With the hypothalamus it controls sexual behaviour, emotional reactions such as excitement, pleasure, rage and fear, and motivation.

How big a job is it?

In an adult, the brain is only about 2 per cent of the body mass, but it uses about 20 per cent of the energy of the body at rest. These are standard values from reference texts. NCERT does not give them.

Try it in Python

The program checks the share of the body that the brain makes and the energy that it uses.

Practise with new numbers

Common mistakes

• Saying that the cortex is white matter. The cortex is the grey matter. The tracts under it are the white matter.
• Mixing up the layers of the meninges. From the skull inwards: dura mater, arachnoid, pia mater. The pia mater touches the brain.
• Saying that the corpus callosum joins the brain to the spinal cord. It joins the two cerebral hemispheres.
• Putting the hypothalamus above the thalamus. The hypothalamus lies at the base of the thalamus.
• Saying that the limbic system is a single organ. It is a group of structures, with the amygdala and the hippocampus.

Remember these

• Meninges: dura mater (outer), arachnoid (thin, middle), pia mater (inner, on the brain).
• Forebrain: cerebrum, thalamus, hypothalamus. Cerebrum: two hemispheres joined by the corpus callosum.
• Cortex: grey matter, with motor, sensory and association areas. Inner part: white matter.
• Thalamus: coordinating centre for sensory and motor signals. Hypothalamus: body temperature, hunger, thirst, hormones.
• Limbic system: amygdala, hippocampus and the hypothalamus. It controls emotion and motivation.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three layers of the cranial meninges from the outside inwards.

2. What joins the two cerebral hemispheres?

3. Why does the cerebral cortex look grey, and why does the tissue under it look white?

4. Name two centres that lie in the hypothalamus.

5. Which two structures of the limbic system does NCERT name, and what do they help to control?

References

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System, section 35.3, The Central Nervous System.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 13, Anatomy of the Nervous System.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.