Central Nervous System: Brain and Spinal Cord Functions

The central nervous system (CNS) consists of the brain and spinal cord. It receives sensory information, processes it, stores memory, and sends commands to muscles and glands through the peripheral nervous system.

Understanding the CNS matters because it controls consciousness, movement, sensation and vital functions such as breathing. Injuries and diseases here, from stroke to spinal trauma, can have body-wide effects.

Key Concepts

• Brain: Enlarged anterior CNS protected by skull and meninges, weighing about 1.3 to 1.4 kilograms in adults.
• Cerebrum: Largest part, divided into hemispheres and lobes, responsible for thought, memory, language and voluntary movement.
• Cerebellum: Posterior part coordinating balance, posture and fine movement.
• Brainstem: Midbrain, pons and medulla controlling breathing, heart rate, swallowing and consciousness pathways.
• Spinal cord: Cord from brainstem to about the first lumbar vertebra in adults, carrying tracts and mediating reflexes.
• Cerebrospinal fluid (CSF): Fluid cushioning the CNS, about 150 millilitres in adults, also removing wastes.
• Meninges: Three protective membranes: dura, arachnoid and pia mater.

How It Works

1. Sensory receptors send impulses through peripheral nerves into the spinal cord and up ascending tracts to the brain.
2. The thalamus relays most sensory input to the cerebral cortex, where perception, planning and decision occur.
3. Motor commands from the cortex descend through the brainstem and spinal cord to motor neurons that activate muscles.
4. The cerebellum compares intended and actual movement and corrects errors, while the brainstem maintains vital automatic functions and arousal.
5. The spinal cord also completes reflex arcs locally, such as withdrawing a hand from heat before conscious perception fully registers.

Worked Example

CSF volume is about 150 millilitres and production is about 500 millilitres per day. Turnover = 500 ÷ 150 ≈ 3.3 times per day, meaning the fluid is replaced roughly every 7 to 8 hours. If drainage is blocked, pressure can rise inside the skull because production continues, illustrating why hydrocephalus requires medical treatment.

Common Misconceptions

• We use only 10 percent of the brain: Wrong. Brain imaging shows virtually all parts are active over time, though not all at once.
• The spinal cord is just a cable: Wrong. It also integrates reflexes and modifies signals.
• Left brain is only logic and right brain only creativity: Oversimplified. Most tasks use networks in both hemispheres, with some specialisation.

Real-World Applications

MRI and CT imaging guide diagnosis of stroke, tumours and cord compression. Lumbar puncture samples CSF to diagnose infection. Rehabilitation after brain injury uses neuroplasticity, the ability of remaining networks to reorganise.

In conclusion, the brain commands and interprets while the spinal cord connects and reflexively protects. Protected by bone, meninges and fluid, the CNS is the control centre of the entire body.