Human Nervous System
Complete Notes & Free Quiz

Coordination means all parts of the body work together in a controlled and organized way. The nervous system coordinates all body activities so they function properly. It is a network of billions of nerve cells (neurons) that carries signals throughout the body to control and coordinate all body activities.

The Central Nervous System (CNS) is made up of the Brain and the Spinal Cord. These two organs coordinate all activities of the body. The CNS receives information from all over the body, processes it, and sends commands back to control all body functions and movements. It is called the control center because all decision-making happens here.

The Peripheral Nervous System (PNS) is the network of nerves that connects the CNS to the receptors and effectors elsewhere in the body.

CNS (brain + spinal cord) = the control center.
PNS = the communication network that carries messages to and from the CNS.

Together, the brain, spinal cord, nerves, receptors, and effectors form the complete nervous system.

The spinal cord is a long bundle of nerve fibers that runs from the medulla (brainstem) down through the backbone (vertebral column). It acts as the main communication highway between the brain and the rest of the body, carrying signals up to the brain and commands down to the body. Together with the brain, it forms the CNS.

The human brain has three basic parts: (1) Forebrain, (2) Midbrain, and (3) Hindbrain.

The brain is located inside the skull (cranium), protected by the hard skull bone and cerebrospinal fluid. It is a wrinkled, greyish organ — the wrinkles increase its surface area to allow more brain cells to fit inside.

The forebrain is the largest and most highly developed part of the human brain. It consists of three parts: Cerebrum, Thalamus, and Hypothalamus.

The cerebrum is the largest part of the human brain and the thinking part. It controls: conscious thoughts, reasoning and decision-making, memory, emotions, and voluntary body movements through voluntary muscles.

The cerebrum is divided into two halves called cerebral hemispheres (left and right). The left hemisphere controls the right side of the body and handles language, speech, mathematics and logical thinking. The right hemisphere controls the left side of the body and handles creativity, artistic ability, music and visual-spatial skills.

The thalamus is a small structure just above the brainstem in the forebrain. It acts as a relay station: it receives sensory information from the sense organs (eyes, ears, skin) and forwards it to the correct part of the cerebrum for processing. It also helps in controlling body movement and coordination.

The hypothalamus is a small part of the forebrain at the base of the brain, just above the pituitary gland. It maintains a stable body temperature of about 37°C (98.6°F).

If the body becomes too hot → hypothalamus triggers sweating to cool down.
If the body becomes too cold → hypothalamus triggers shivering to generate heat.

The midbrain is the smallest and most central part of the brain. It acts as a bridge between the forebrain and hindbrain, transmitting signals in both directions:

Upward: From hindbrain to forebrain, carrying sensory information.
Downward: From forebrain to hindbrain, carrying motor commands for movement.

The hindbrain is the lower part of the brain. It consists of three parts: Cerebellum, Medulla (brainstem), and Pons.

The cerebellum is found at the back and bottom of the brain. It controls: balance of the body and coordinated (smooth) body movements. It also allows us to perform learned movements almost automatically, without thinking.

After repeated practice (e.g. riding a bicycle, swimming), the cerebellum stores the pattern and executes these movements automatically without conscious effort.

The medulla (brainstem) is located where the brain connects to the spinal cord. It controls involuntary (automatic) actions essential for life: breathing, heartbeat, and peristalsis (wave-like movement of food through the digestive system).

Peristalsis is the rhythmic contraction and relaxation of muscles in the digestive tract that pushes food forward. It is involuntary and controlled by the autonomic nervous system.

If the medulla stops working, breathing stops, the heart ceases to beat — it is instantly life-threatening.

The pituitary gland is a small gland attached to the base of the brain, just below the hypothalamus. It produces hormones (chemical messengers) that regulate: the amount of water in the body, blood sugar levels, and the growth rate of the body.

It is called the master gland because it controls other glands in the body.

A neuron (nerve cell) is the basic working unit of the nervous system. Neurons are thin and can be up to one metre in length. A neuron consists of:

1. Cell body: Contains the nucleus; controls all cell activities.
2. Dendrites: Short, branching extensions that receive signals TOWARDS the cell body.
3. Axon: Long, thin extension that carries signals AWAY from the cell body.
4. Myelin sheath: Fatty insulation wrapped around the axon (like plastic around a wire) — speeds up transmission and prevents signal leakage.
5. Axon terminals: End branches where signals pass to the next neuron.

Sensory neuron: Carries impulses FROM sense receptors (skin, eyes, ears) TOWARDS the CNS. It INFORMS the brain about the environment.

Motor neuron: Carries impulses FROM the CNS TOWARDS muscles and glands (effectors), causing them to respond. It COMMANDS the body to act.

Receptors are specialized cells or structures that detect stimuli (changes) inside and outside the body. They convert the detected stimulus into a signal sent to the brain. Types include: single-cell receptors (e.g. pain receptors in skin) and complex sense organs (eye, ear, nose).

Effectors are the body parts that conduct the response to a stimulus. In multicellular organisms, the effectors are muscles — they either contract or relax to produce movement.

Simply: Receptors DETECT; Effectors ACT.

Voluntary actions are actions that require conscious thought — we deliberately decide to do them. They are controlled by the cerebrum.

Examples: writing, speaking, kicking a ball, picking up a glass.

Reflex actions happen automatically and very quickly without conscious thought. They do not need a decision from the thinking brain.

Example: pulling your hand away from a hot object instantly.

They are faster than voluntary actions because the signal does NOT travel to the brain for processing — the spinal cord coordinates the response directly. This speed is critical in emergencies to protect the body from injury.

A reflex arc is the nerve pathway of a reflex action:

The spinal cord coordinates the reflex response. However, a signal also travels to the brain AFTER the reflex — which is why we feel pain a split second later. The brain gets information for awareness but is NOT needed for the actual reflex.

Flash of bright light → Pupil becomes smaller (to protect the retina).
Insect touches eyelid → Eye blinks.
Food enters windpipe → Coughing (to clear the airway).
Hand touches electric wire → Hand pulled away instantly.
Smell of food when hungry → Saliva produced.
Body gets too hot → Sweating begins.
Body gets too cold → Shivering begins.

A synapse is a tiny gap (junction) between the axon terminal of one neuron and the dendrite of the next neuron. Nerve impulses cannot jump across this gap electrically.

When a signal reaches the axon terminal, it triggers the release of neurotransmitters (chemical messengers) into the synapse. These chemicals cross the gap and bind to receptors on the next neuron, restarting the electrical signal.

Nerve damage can cause: loss of sensation, paralysis (loss of movement), loss of reflex responses, and inability to control muscles.

To maintain a healthy brain: Regular exercise (improves blood flow to the brain), a balanced diet (provides nutrients for nerve function), adequate sleep (allows the brain to repair and consolidate memory), avoiding alcohol and drugs (which damage neurons), and mental stimulation through reading and learning.