In complex multicellular organisms, the ability to respond to environmental stimuli and coordinate the functions of various organs and organ systems is paramount for survival. This intricate task is accomplished by two primary systems: the neural (or nervous) system and the endocrine system. The neural system provides a rapid, point-to-point network for transmitting information, enabling quick responses and precise control over bodily functions.

The fundamental unit of the neural system is the neuron, a highly specialized cell designed to detect, receive, and transmit different kinds of stimuli. A typical neuron is composed of three main parts: the cell body (cyton), which contains the nucleus; dendrites, which are short, branched extensions that receive signals from other neurons; and the axon, a long fiber that transmits impulses away from the cell body. The end of the axon branches into synaptic terminals, which form connections, or synapses, with other neurons or effector cells like muscles and glands.

The transmission of information along a neuron occurs in the form of an electrical signal called a nerve impulse or action potential. In its resting state, a neuron maintains a difference in electrical charge across its membrane, known as the resting potential. This is achieved by the active transport of ions, primarily by the sodium-potassium pump. When a neuron is stimulated, the permeability of its membrane to sodium ions changes, causing a rapid influx of sodium and a reversal of the membrane potential, a process called depolarization. This wave of depolarization propagates along the axon as an action potential.

When the nerve impulse reaches the axon terminal, it must be transmitted to the next neuron across the synapse. This synaptic transmission is typically a chemical process. The arrival of the action potential triggers the release of chemical messengers called neurotransmitters (such as acetylcholine) from the presynaptic terminal into the synaptic cleft. These neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane, exciting or inhibiting the next neuron and thereby continuing or stopping the signal.

The human neural system is a highly complex network divided into two main components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS, consisting of the brain and the spinal cord, is the primary site for information processing and control.

The brain is the command center of the body, protected within the skull. It is divided into three major regions: the forebrain, midbrain, and hindbrain. The forebrain is the largest part and includes the cerebrum, thalamus, and hypothalamus. The cerebrum is the seat of higher functions like thought, memory, intelligence, and voluntary actions. The midbrain acts as a relay center for auditory and visual information. The hindbrain consists of the pons, cerebellum, and medulla oblongata. The cerebellum is crucial for maintaining posture, balance, and coordinating voluntary movements, while the medulla controls vital involuntary functions like breathing, heart rate, and blood pressure. The spinal cord, extending from the medulla, serves as a major pathway for nerve impulses to and from the brain and is the center for controlling reflex actions.

The Peripheral Nervous System (PNS) comprises all the nerves that extend from the CNS to the rest of the body. It is further divided into the somatic and autonomic nervous systems. The somatic nervous system relays impulses from the CNS to skeletal muscles, controlling voluntary movements. The autonomic nervous system regulates involuntary functions of visceral organs and is subdivided into the sympathetic and parasympathetic divisions, which generally have opposing effects—the sympathetic system prepares the body for “fight-or-flight” responses, while the parasympathetic system promotes “rest-and-digest” activities.

A reflex action is a rapid, involuntary response to a stimulus, processed at the level of the spinal cord without conscious thought. The neural pathway involved is called the reflex arc.

To interact with the external world, the nervous system relies on sensory organs. The eye is the organ of sight. Light enters the eye and is focused by the lens onto the retina, which contains photoreceptor cells called rods and cones. These cells convert light energy into electrical signals, which are sent to the brain via the optic nerve for interpretation. The ear is responsible for both hearing and balance. Sound waves cause vibrations in the eardrum and the middle ear ossicles, which are transmitted to the fluid-filled cochlea in the inner ear. Here, hair cells in the organ of Corti are stimulated, generating nerve impulses that travel to the brain. The vestibular apparatus of the inner ear is responsible for maintaining the body’s equilibrium and balance.