In our article on neurons, we saw how these cells inside the human brain functioned and communicated with each other as well as other body cells. Thus, to understand the neuronal function, it was mandatory to know the structure of a neuron.

Similarly, to understand the functioning of the human brain, it is important to understand the structure and working of the Nervous System. The Nervous System performs the function of:

• receiving sensory input,
• transmitting information to the processing areas of the brain and spine
• integrating information in brain and spine
• sending information back to muscles, glands and organs for them to respond appropriately

Now, neurons, glial cells, and synapses help in gathering information known as the sensory input. The Nervous System consists of billions of nerve cells called neurons and synapses that form between two or more neurons. These synapses, in turn, connect the neurons with each other or other body cells.

Further, the neurons pass on signals from the sensory receptors to the brain and spinal cord. Then, the processing of such information takes place inside different parts of the brain. Once the information gets processed, the nerve impulses are then transmitted from the brain and spinal cord to muscles and glands.

The resulting nerve signals is known as the motor output. Thus, this entire functioning of the Nervous System can be understood only when one knows the components that make the Nervous System of the human brain.

The nervous System is a complex network of neurons that controls the functions of our body and is responsible for everything we experience consciously including seeing, feeling, thinking, reasoning, imagining, etc.

Although, the Nervous System as a whole function in a unified manner, it consists of two portions:

• Central Nervous System

This further consists of the Brain and Spinal Cord. The Brain is the control center of the body. It has various other centers that are responsible for receiving sensory input, processing of information and giving motor output.

• Peripheral Nervous System

This comprises of spinal and cranial nerves that connect with the brain and spinal cord. Furthermore, this part of the Nervous System is divided into the Somatic and Autonomic Nervous System. While Somatic Nervous System connects CNS to voluntary muscles like bones, joints, skin etc.

The Autonomic Nervous System connects CNS to organs over which we have little voluntary control like cardiac muscles, digestive system etc. This division does not end here. The Autonomic Nervous System is further subdivided into Sympathetic and Parasympathetic Systems. We will discuss these further in the upcoming sections.

As mentioned earlier, CNS is the control center of the body. As a control center, it regulates the functioning of organs, thought processes as well as body movements. The CNS consists of the brain and spinal cord. Let’s discuss them one by one.

The structure of the Central Nervous System comprises of the Brain and the Spinal Cord.

1. Brain

The most complex part is the Brain, which is the central control module of the human body. It coordinates the entire activity of the body, right from physical motion to release of hormones, development of memories and sense of emotions. To perform these functions, each part of the brain has defined roles to play. Thus, brain is subdivided into following main parts:

(i) Brain Stem

Brain Stem is the portion of the brain that starts right above the Spinal Cord and continues into the centre of the brain. It consists of structures that control all the vital processes of the human body which include:

• Medulla

This system inside the brain is responsible for controlling vital functions of the human body. These include heart rate, breathing, blood pressure etc.

• Pons

This portion of the brain is located between the midbrain and medulla through which sensory and motor information passes.

• Reticular Activating System

This structure inside the brain is responsible for sleep, arousal and control of muscle tone and cardiac reflexes.

(ii) Cerebellum

This part of the brain is located behind medulla and pons. It primarily coordinates motor activities in a synchronized way.

(iii) Midbrain

Midbrain is a structure that is located above medulla and pons near the end of the brain stem. It is the extension of the reticular activating system and consists of basic centers responsible for hearing and vision.

(iv) Hypothalamus

Hypothalamus is a small part in the brain that plays an important role in controlling autonomic nervous system and several aspects of our motivated behaviour such as eating and aggression.

(v) Thalamus

Thalamus is a structure that is located deep inside the brain that is associated with the changes in emotional reactions. This part of the brain receives sensory inputs from other parts of the nervous system, analyze the same and then transmits it to the other parts of the nervous system

(vi) Limbic System

These are the set of structures that play a key role in our emotional reactions and behaviour like escaping from danger, fighting, mating etc. Two of its structures includes Hippocampus and Amygdala. Hippocampus is the structure of the limbic system that plays an important role in creating memories. Whereas Amygdala is another limbic system structure that is responsible for various aspects of emotional control and creation of emotional memories.

(vii) Cerebral Cortex

This part of the brain is responsible for an individual’s capability to plan, reason, remember and imagine. The Cerebral Cortex is the outer covering of the brain that is divided into cerebral hemispheres that is divided into two cerebral hemispheres – the left and the right cerebral hemispheres.

The right hemisphere is responsible for the left side of the body. Whereas, the left hemisphere is responsible for right side of the body. Both hemispheres of the brain communicate with each other through a band of nerve fibres called the Corpus Callosum.

Each of the cerebral hemispheres performs specialized set of functions. Furthermore, these hemispheres are divided into separate lobes or distinct regions on the basis of the size of the fissures that it is folded into. These lobes include:

• Frontal Lobe – Portion of the cerebral cortex that lies near to the face and is bounded by the Central Fissure.
• Parietal Lobe – Portion of the cerebral cortex that lies behind the Central Fissure. It contains somatosensory cortex. This part of the cortex receives information from the skin senses like touch, pressure and temperature.
• Occipital Lobe – Portion of the cerebral cortex located near the back of the cortex. The primary function of this part of cerebral cortex is to help in visual processing. This region of the cerebral cortex contains a sensory area that receives input from the eyes.
• Temporal Lobe – Portion of the cerebral cortex that is located along the side of each cerebral hemisphere. This part of the cerebral cortex plays a major role in hearing and consists of a sensory area that receives information from the ears

2. Spinal Cord

Spinal Cord runs through the middle of the bony structure known as vertebrae. It has two major functions:

Firstly, spinal cord carries sensory information from the receptors throughout the body to the brain via afferent (sensory) nerve fibres. Further, the spinal cord also helps in carrying information from the brain to muscles and glands via efferent nerve fibres (motor nerve fibres).

The second major function of the spinal cord lies in reflexes. These reflexes are automatic actions induced by specific stimuli. In other words, these reflexes encompass neural pathways through which information received from various receptors is transmitted to the spinal cord. Here, these neural impulses stimulate other neurons known as interneurons. These inter neurons pass information to muscle cells, producing reflex actions in return. Therefore, the reflexes taking place at the spinal cord level allow us to respond to dangers much more quickly as compared to the case where information first had to be carried all the way to the brain.

Key References

Psychology Robert A Baron and Girishwar Misra (Fifth Edition) Pearson Education India

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