Nervous System and Spinal Cord

Neuron Structure

A neuron consists of three parts: a cell body that contains the nucleus, dendrites attached to the cell body that receives signals, and the axon that plays a role in the propagation of these signals.
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, 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 a myelin sheath, which is a lipid-rich material that wraps around the axon and helps impulses travel through neurons. The myelin sheath insulates the axon to prevent the electric current of the nerve impulse signal from leaving the axon and increases the speed of the signal. 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. This process 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 next neuron.
A neuron consists of a cell body that contains the nucleus, dendrites, and an axon. When information is received from an external or internal stimulus, it is propagated down the neuron's axon as a nerve impulse. Eventually this information terminates at the axon terminal, where it is chemically transmitted to another neuron.

Types of Neurons

Neurons differ based on function (afferent, efferent, interneurons) and structure (multipolar, bipolar, unipolar, anaxonic), which is defined by the number and type of projections extending from the cell body.

The nervous system consists of different types of neurons. These neurons are classified according to where they transmit information. An afferent neuron carries sensory information from sensory organs (e.g., eyes, ears, and skin) to the central nervous system (CNS). An efferent neuron carries motor information, such as when the body should move, from the CNS to effector organs including muscles and glands. The 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.

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. A bipolar neuron consists of a cell body with one dendrite and one axon extending off of it in opposite directions from each end. 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

Oligodendrocytes, ependymal cells, microglia, and astrocytes are neuroglia found in the central nervous system (CNS), while Schwann cells and satellite cells are neuroglia found in the peripheral nervous system (PNS).

Neuroglia, also known as glial cells, are cells that support and protect neurons. Unlike neurons, they do not carry neural impulses. Different types of neuroglia are found in the central nervous system (CNS) and peripheral nervous system (PNS). The following glial cells are found 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.
  • An astrocyte is a star-shaped glial cell that supports neurons by connecting them to nutrient supplies and repairing nervous tissue after injury.
Schwann cells and satellite cells are neuroglia found in the PNS. Like an oligodendrocyte in the CNS, the Schwann cell provides myelination to axons in the PNS. Each individual Schwann cell sheaths only one axon. It has phagocytic activity, which means it can engulf and clear cellular debris in order to facilitate the regrowth for PNS neurons. A satellite cell is a glial cell in the PNS that surrounds the cell bodies of neurons in sensory, sympathetic, and parasympathetic ganglia. A ganglion is a mass of tissues that contains several cell bodies. By functioning in this capacity, satellite cells provide support in a way that is similar to astrocytes in the CNS. Satellite cells also modulate the PNS following injury and inflammation.
There are different types of neuroglia in the central and peripheral nervous systems. Astrocytes and satellite cells provide support. Oligodendrocytes and Schwann cells provide insulation, microglia provide immune surveillance, and ependymal cells help create cerebrospinal fluid. These neuroglia have unique morphologies and distributions in the nervous system.