Structure and Function

Core Concepts

Divisions: CNS, PNS, and the two "P" splits

The CNS (brain and spinal cord) is the command center. The PNS consists of the cranial nerves (12 pairs emerging from the brain) and the spinal nerves (emerging from the cord). Signals flow in two directions: afferent (sensory — toward the CNS) and efferent (motor — away from the CNS). The efferent side splits again: the somatic division drives voluntary skeletal muscle, while the autonomic division runs involuntary systems — the sympathetic branch ("fight-or-flight," the accelerator) and the parasympathetic branch ("rest-and-digest," the brake). Most organs receive input from both branches, and their balance keeps the body stable.

The neuron: the network's wiring

The neuron is the nervous system's basic cell. Dendrites receive signals; the cell body processes them; the axon conducts the signal away; and a myelin sheath — a fatty insulation made by supporting cells — speeds conduction along the axon. Neurons communicate at synapses: an electrical signal reaches the end of the axon and triggers the release of chemical messengers (neurotransmitters) that diffuse across the tiny gap and excite or inhibit the next neuron, muscle, or gland. This is why many neurological medications work by influencing neurotransmitter activity — a topic for pharmacology courses — and why a neuron that loses its myelin (as in multiple sclerosis) conducts poorly.

The brain: regions and rough job assignments

The cerebrum is the largest part of the brain: two hemispheres covered by the cerebral cortex. Each hemisphere is loosely mapped into lobes, with rough job assignments:

• Frontal — planning, judgment, personality, initiating voluntary movement and speech
• Parietal — processing touch, temperature, pain, and body position
• Temporal — hearing, language comprehension, memory
• Occipital — vision

This is a simplified map — every lobe does many jobs, and functions overlap. The cerebellum, at the back and base, coordinates movement and balance. The brainstem (midbrain, pons, medulla) connects the cerebrum to the spinal cord and houses centers for vital functions such as breathing, heart rate, and consciousness — which is why brainstem injury or compression is immediately life-threatening.

Protection: skull, meninges, CSF, blood-brain barrier

The brain sits in the skull and the spinal cord in the vertebral canal. Three membranes — the meninges — surround them: the tough outer dura mater, the middle arachnoid mater, and the thin pia mater adhering to the surface. Cerebrospinal fluid (CSF) fills the spaces between and around these structures, cushioning the brain and spinal cord and carrying nutrients and wastes. A selective blood-brain barrier limits which substances pass from the blood into brain tissue, protecting the brain from many blood-borne agents while letting needed nutrients through.

The spinal cord: the expressway

The spinal cord carries ascending tracts (sensory information up to the brain) and descending tracts (motor commands down to muscles). It is also the site of reflexes: in a reflex arc, a sensory signal enters the cord and a motor response leaves it without waiting for the brain — the patellar (knee-jerk) response is the classic example. Reflexes are fast and protective; testing them tells the examiner whether the spinal cord and its nerves are intact.

Contralateral control and the cranial nerves

Most motor and sensory pathways cross over: the right side of the brain controls and senses the left side of the body, and vice versa. This is why a stroke on one side of the brain causes weakness on the opposite side of the body. The 12 pairs of cranial nerves emerge directly from the brain and serve the head, neck, and special senses; some are sensory (e.g., smell, vision), some motor (e.g., eye movement, tongue movement), and some mixed. They are examined one by one in the neurological assessment — pupils, facial movement, speech, and swallowing all reflect specific cranial nerves.

Common Confusions