Many multicellular organisms have complex nervous systems that function to receive and process internal and external information and to control body responses. These systems are able to receive and interpret sensory signals that come from external stimuli. Once received, these signals are transmitted through the body so that the organism can respond. All body activities are performed through a coordinated action of the nervous system with other body systems. The nervous system is divided into two broad categories: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of nerves that travel to and from the CNS to the rest of the body.A neuron is a cell in the nervous tissue that transmits electrical and chemical signals throughout the body. A neuron has three main parts: a cell body, dendrites, and a single axon. The neuron's cell body contains the nucleus and other major organelles. The dendrite is an extension from the neuronal cell body that receives input from other cells. Another extension, the axon, extends from the neuronal cell body that transmits the signal to receiving cells. Information moves in one direction, starting from the dendrite to the cell body and ending at the axon. To help with the transmission of an electrical signal, the nerve impulse travels quickly through neurons, as well as the myelin sheath—a lipid-rich material that wraps around the axon and present in some nerve cells. The myelin sheath helps to speed up impulses traveling along neurons. A gap between myelin sheaths where the axon is uncovered is called a node of Ranvier. The propagation of nerve impulses from one node of Ranvier to another, across myelinated axons, is called saltatory conduction, a process that allows impulses to move quickly without degrading over the long distance of the axon. The end of an axon is the axon terminal, where nerve impulses are released to the next neuron via chemical transmission to the synaptic cleft.
Types of Neurons
The nervous system consists of different types of neurons that are classified according to where they transmit signals. An afferent neuron, commonly called a sensory neuron, carries sensory information from sensory organs (e.g., eyes, ears, and skin) to the central nervous system (CNS). An efferent neuron, commonly called a motor neuron, carries motor information, such as when the body should move, from the CNS to effector organs, including muscles and glands. An effector organ responds to a stimulus from a nerve. An interneuron relays information between an afferent (sensory) neuron and an efferent (motor) neuron. Interneurons are located in the brain and spinal cord to help process sensory information and coordinate motor activities. For example, if a person's hand touches a hot stove, a signal is first sent through an afferent neuron to an interneuron, which interprets the information. The interneuron transmits this signal to an efferent neuron, which stimulates the muscles in the hand to move. The interneurons are also involved in reflexes.
Individual neurons also vary in structure and are named based on how dendrites and axons are arranged around the cell body. A unipolar neuron is a sensory neuron that has a single, long axon extending from the cell body. It is found in the spine and cranial nerve ganglia and is also common in insects. A bipolar neuron consists of a cell body with one dendrite and one axon extending off of the cell body in opposite directions. Bipolar neurons are found in the retina, the inner ear, and the nasal cavity. Neurons are most commonly multipolar. A multipolar neuron has a single axon and many dendrites that extend from the cell body. These neurons are typically motor neurons found in the CNS. An anaxonic neuron has multiple dendrites and often no axons. These cells are found in the brain and retina.
Neuroglia, also known as glial cells or glia, are cells that support and protect neurons. Unlike neurons, neuroglia cells do not carry neural impulses. Different types of neuroglia are found in the central nervous system (CNS) and peripheral nervous system (PNS). There are several glial cells in the CNS:
- An oligodendrocyte is a glial cell that generates myelin, which wraps around axons in the CNS.
- An ependymal cell lines the spinal cord and ventricles of the brain. It produces and secretes cerebrospinal fluid (CSF).
- A microglia cell mediates immune responses in the CNS. It can transform into a special type of macrophage that can clear up neuronal debris via phagocytosis, which is a process by which cells such as macrophages engulf and digest pathogens and other material.
- An astrocyte is a star-shaped glial cell in the CNS that supports neurons by connecting them to nutrient supplies and repairing nervous tissue after injury.