Nervous System

The nervous system is the body's control and communication system, built from a network of nerves and specialised cells. Everything we sense, decide and do passes through it.

• It is the control and communication system of our body.
• It helps in thinking, memory, sensation, movement and coordination.
• It works through a network of nerves and cells.
• Based on location, the nervous system has two parts — the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

The whole system is built on a simple two-part plan based on where the parts lie, and almost every exam question starts here. The CNS is the control centre; the PNS is the communication network that links the body to it.

• The CNS is the control centre of the body.
• The PNS is composed of the Somatic and Autonomic Nervous Systems.
• The PNS is the communication network that connects the body to the CNS.
• CNS diagram labels: Brain, Spinal cord.
• PNS diagram labels: Cranial nerves, Spinal nerves, Ganglia.

Beyond location, the system is also classified by the kind of work it does — voluntary actions we control, and involuntary ones the body runs on its own.

• Autonomic Nervous System (ANS): responsible for involuntary body movements, usually to maintain body homeostasis. Example: sweating when the body is hot to cool it down.
• Enteric Nervous System (ENS): responsible for controlling smooth muscle and glandular tissue in the digestive system.
• SNS is voluntary, while ANS and ENS are involuntary.

The whole network is built from a single kind of cell, the neuron, the basic structural and functional unit of the nervous system.

• The neuron is the longest cell in the human body and is shaped to carry electrical messages over long distances.
• Each neuron has a cell body (cyton) with the nucleus, branching dendrites that receive incoming signals, and one long axon that carries the signal away; the axon is often wrapped in a myelin sheath that insulates it and speeds up the impulse.
• Neurons are classified into three types — Sensory, Motor, and Association/Connecting neurons.
• The main function of the CNS (via these neurons) is to keep control over the entire body and the nervous system.

A message runs along a single neuron as an electrical impulse, but where one neuron meets the next there is a tiny gap, and the signal cannot simply jump it.

• At the end of the axon the impulse releases chemical messengers called neurotransmitters, which cross the synapse and start a fresh impulse in the next neuron; acetylcholine is a well-known one.
• Because each junction passes the message in only one direction, impulses always flow the correct way — in through sensory neurons, out through motor neurons.

Not every response needs the brain to think it through; some are automatic and protective.

• It is controlled by the spinal cord, not the brain, which is why it is so fast.
• The body protects itself before we are even consciously aware of the pain.

The CNS — the brain and spinal cord — is the processing hub, and because it is delicate it is heavily protected.

• These two parts are covered by three membranes called meninges, with cerebrospinal fluid (CSF) cushioning them.

• Dura mater — the outer layer / membrane.
• Arachnoid membrane — the middle layer.
• Pia mater — the innermost membrane.

The brain is covered here only briefly, as the Human Brain has its own dedicated note.

• The brain is divided into three parts — Forebrain, Midbrain and Hindbrain.
• Forebrain: Cerebrum, Thalamus, Hypothalamus — the Cerebrum handles memory, wisdom, knowledge, will power and thinking; the Thalamus regulates cold, heat and pain; the Hypothalamus controls sweat, blood pressure, hormone secretion, thirst and hunger.
• Midbrain: manages the main functions of hearing and vision.
• Hindbrain: Cerebellum, Pons and Medulla — the Cerebellum manages body posture; the Pons acts as a bridge between the spinal cord and brain; the Medulla regulates respiration, heart rate, sneezing and coughing.
• Brain diagram labels: Forebrain (cerebrum), Midbrain, Hindbrain (cerebellum), Pons, Medulla oblongata, Spinal cord.

The spinal cord is the long cable of nervous tissue running down the backbone, acting both as a relay line and an independent reflex centre.

• There are 31 pairs of spinal nerves that arise from the sides of the spinal cord.
• Its function is to control all reflexes that take place below the neck.
• It conducts sensory impulses from the muscles and skin to the brain.
• The vertebrae protect the spinal cord from injury.
• It also conducts motor responses from the brain to the muscles of the limbs and trunk.
• It begins from the medulla oblongata and ends near the L1–L2 vertebra (in adults).
• Spinal cord diagram labels: Medulla oblongata, Spinal nerves, Spinal cord, Vertebrae.

The PNS is made of all the nerves branching out from the brain and spinal cord, and their exact counts are among the most asked facts in the topic.

• The human body has 31 pairs of spinal nerves and 12 pairs of cranial nerves.
• The PNS is divided into the Somatic Nervous System and the Autonomic Nervous System.
• Somatic Nervous System: includes the peripheral nerve fibres that convey sensory information to the motor nerve fibres and the CNS.
• Autonomic Nervous System: consists of three sections — sympathetic, parasympathetic and enteric; it regulates the smooth muscle of the viscera (internal organs) and glands.
• Memory hook: 12 + 31 = 43 pairs of nerves leave the CNS in all.

The nervous system's core job is signalling — sending fast, precise messages from one point to another.

• These messages are sent very fast as electrical impulses.
• Signalling is done point to point, so the nervous system is more specific, accurate and speedy (unlike hormones released into the circulatory system).
• The body reacts to a stimulus because the system extracts external signals via sensory receptors; the CNS processes them, decides a response, and sends an output signal to the right body part to activate it.
• In short: the nervous system uses electrical signals → fast, specific, accurate and speedy.

The body has two great coordinating systems, and examiners like to contrast them.

• The hormonal (endocrine) system sends chemical signals (hormones) through the blood — slower, widespread and longer-lasting.

A quick lookup table of the numbers and facts examiners ask most.

The full-body diagram of the nervous system is labelled with the central organs and the major nerves and plexuses; these names occasionally appear in exams.

• Brachial plexus, Musculocutaneous nerve, Radial nerve, Median nerve.
• Iliohypogastric nerve, Genitofemoral nerve, Obturator nerve, Ulnar nerve.
• Common peroneal nerve, Deep peroneal nerve, Superficial peroneal nerve.
• Intercostal nerves, Subcostal nerve, Lumbar plexus, Sacral plexus.
• Femoral nerve, Pudendal nerve, Sciatic nerve, Muscular branches of femoral nerve.
• Saphenous nerve, Tibial nerve.

Disorders of the nervous system arise from genetic defects, traumatic experiences, infections or ageing, and several appear regularly in general-awareness papers.

• Diseases of the peripheral nervous system are mainly due to stoppage in nerve conduction.
• Vaccination is the best way to prevent many nervous-system diseases.
• Early detection and timely treatment reduce complications.
• Maintain a healthy lifestyle — balanced diet, exercise and proper sleep.
• Avoid alcohol, drugs and risky behaviour to keep the nervous system healthy.
• Avoid stress, alcohol, smoking and drugs.
• Healthy food, regular exercise and enough sleep keep the nervous system healthy.
• Early diagnosis and proper care can improve quality of life.