The nervous system is organized into two main divisions that work together to sense, integrate, and respond to information. The central nervous system (CNS) consists of the brain and spinal cord and serves as the integrative center. The peripheral nervous system (PNS) extends outside the CNS and includes sensory neurons that bring information toward the CNS, motor neurons that carry signals outward to effectors, and a special subdivision called the autonomic nervous system (ANS) that controls involuntary functions. The ANS itself splits into the sympathetic division, with thoracolumbar outflow that drives fight-or-flight responses, and the parasympathetic division, with craniosacral outflow that promotes rest-and-digest activities, along with an enteric subsystem governing the gastrointestinal tract.
The brain is shielded from injury by several protective layers. The bony skull forms the outermost rigid barrier, while three membranous meninges (the tough dura mater, the web-like arachnoid mater, and the delicate pia mater that hugs the brain surface) envelope the brain. Between the arachnoid and pia lies the subarachnoid space filled with cerebrospinal fluid, which is also contained within the ventricles inside the brain; this cushioning fluid absorbs shock and circulates nutrients. At the cellular level, the blood-brain barrier, formed by tight junctions between capillary endothelial cells reinforced by astrocyte foot processes and a continuous basement membrane, selectively filters substances reaching neural tissue. Within the cranial cavity, the cerebrum dominates the brain and is divided by a central longitudinal fissure into two hemispheres, each containing four lobes: the frontal lobe for motor control and executive function, the parietal lobe for sensory integration, the temporal lobe for auditory processing and memory, and the occipital lobe for vision.
The hypothalamus regulates homeostasis by controlling hunger, thirst, body temperature, autonomic output, and hormonal release through its connection to the pituitary gland via the pituitary stalk. The cerebellum, located in the posterior cranial fossa behind the brainstem, coordinates voluntary movement, balance, and posture, displaying folia on its surface and a characteristic tree-like arbor vitae of white matter internally. The spinal cord is a cylindrical extension of the CNS running from the medulla oblongata down to roughly the L1-L2 vertebral level. In cross section it shows an H-shaped core of gray matter containing neuron cell bodies surrounded by white matter of myelinated tracts, and it is organized into 31 segments that give rise to 31 pairs of spinal nerves. Communication between the body and the brain is mediated by twelve pairs of cranial nerves with specialized functions: olfactory for smell, optic for vision, oculomotor, trochlear, and abducens for eye movement, trigeminal for facial sensation, facial for facial expression, vestibulocochlear for hearing and balance, glossopharyngeal for swallowing and taste, vagus for parasympathetic innervation of the viscera, accessory for shoulder muscles, and hypoglossal for tongue movement.