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The nervous system is the master controlling and communicating system of the body. It is responsible for gathering information about the internal and external environment, processing the information and deciding on responses, and activating appropriate organs to carry out the responses.

The structural classification of the nervous system is based on anatomical features. It divides the nervous system into the central nervous system (CNS) and peripheral nervous system (PNS).

The central nervous system (CNS) is the brain and the spinal cord. It is the integration and command center of the nervous system.

The peripheral nervous system (PNS) is the nerves that extend from the brain and spinal cord. It serves as a communication line connecting the CNS to the rest of the body.

The functional classification of the peripheral nervous system is based on the direction of information flow. It divides the PNS into the sensory (afferent) division and the motor (efferent) division.

The sensory (afferent) division carries impulses towards the central nervous system (CNS) from sensory receptors throughout the body.

The motor (efferent) division carries impulses from the central nervous system (CNS) to the effector organs (muscles and glands).

The somatic nervous system is a part of the motor (efferent) division that controls voluntary movements of skeletal muscles.

The autonomic nervous system is a part of the motor (efferent) division that controls involuntary actions of smooth muscle, cardiac muscle, and glands.

The sympathetic nervous system is a part of the autonomic nervous system that prepares the body for action in stressful situations ('fight-or-flight' response).

The parasympathetic nervous system is a part of the autonomic nervous system that conserves energy and promotes housekeeping activities during rest.

Neuroglia are the supporting cells of the central nervous system (CNS). They support, insulate, and protect the delicate neurons.

Astrocytes are star-shaped neuroglial cells that provide structural support for neurons, help form the blood-brain barrier, and regulate the chemical environment of the brain.

Microglia are spider-like phagocytes that remove cellular debris, waste products, and pathogens from the central nervous system (CNS).

Ependymal cells line the cavities of the brain and spinal cord, and they help circulate cerebrospinal fluid (CSF) using cilia.

Oligodendrocytes are neuroglial cells that produce myelin sheaths, which insulate axons in the central nervous system (CNS), increasing the speed of nerve impulses.

Neurons are specialized cells that transmit messages throughout the body. They have three main regions: the cell body, dendrites, and axon.

The cell body of a neuron contains the nucleus and other organelles that maintain the cell's metabolism.

Dendrites are branching processes that receive information from other neurons and transfer it to the cell body. They are often found in large numbers and are shorter than axons.

Axons are single, long processes that transmit impulses away from the cell body. They are usually covered by a myelin sheath that increases the speed of impulse transmission.

Sensory (afferent) neurons carry impulses from sensory receptors to the central nervous system (CNS).

Motor (efferent) neurons carry impulses from the central nervous system (CNS) to the effector organs (muscles and glands).

Interneurons (association neurons) connect sensory and motor neurons within the central nervous system (CNS). They are responsible for processing and interpreting incoming sensory information and deciding on appropriate motor responses.

A synapse is the junction between two neurons, where communication occurs through the release of neurotransmitters.

A reflex is an automatic, rapid, and predictable response to a stimulus. It occurs via a pathway called a reflex arc.

A reflex arc is the direct route for a reflex, involving a sensory neuron, an interneuron, and a motor neuron.

Somatic reflexes are reflexes that involve skeletal muscles, such as the knee-jerk reflex.

Autonomic reflexes are reflexes that involve smooth muscle, cardiac muscle, or glands, such as the regulation of blood pressure.

The cerebrum is the largest part of the brain, responsible for higher-level functions such as reasoning, memory, and language.

The diencephalon is a region of the brain that sits on top of the brainstem and is responsible for controlling important functions related to the autonomic nervous system, sensory relay, and endocrine regulation.

The brainstem is a stalk-like structure connecting the cerebrum to the spinal cord. It is responsible for basic life functions like breathing, heart rate, and blood pressure.

The cerebellum is a region of the brain located behind the brainstem. It coordinates voluntary movements, maintains balance, and helps control posture.

The spinal cord is a long column of nerve tissue extending from the brainstem to the lower back. It serves as a two-way conduction pathway for nerve impulses between the brain and the rest of the body.

Meninges are three protective membranes that surround the brain and spinal cord. They provide cushioning and support for these delicate structures.

Cerebrospinal fluid (CSF) is a clear, watery fluid that cushions and protects the brain and spinal cord. It circulates within the ventricles of the brain and the subarachnoid space.

The blood-brain barrier is a specialized barrier that restricts the passage of substances from the bloodstream into the central nervous system (CNS). It protects the delicate neurons from harmful toxins and pathogens.