Specialized Cells

Neurons

The nervous system relays information around the body using neurons that each consist of three parts: a cell body that contains the nucleus, dendrites attached to the cell body that receive signals, and the axon, which plays a role in the propagation of the signals.

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.

Neuron Structure

A neuron consists of a cell body that contains the nucleus, dendrites (appendages attached to the cell body that receive information from the internal and external environment), and an axon that propagates this information as an electrical signal to elicit a response by various body systems.
When neurons are not conducting impulses, these cells are at rest, or have a resting membrane potential. At rest, the inside of the neuron is negatively charged (typically –70 mV) compared to the extracellular conditions. A resting neuron also contains a higher concentration of potassium and a lower concentration of sodium than a stimulated neuron. Potassium and sodium ions pass through specific pathways, each called a voltage-gated channel, which is an ion pathway that opens and closes when an electrical signal is received and is activated by changes in membrane potential. These gateways are selective, working specifically to allow the passage of sodium, potassium, or other ions. Because of this concentration difference between both ions, a gradient is established. To maintain this gradient, Na+/K+ ATPase protein pumps sodium out of the cell and potassium into the cell. When a resting cell is stimulated, an action potential occurs. An action potential is a rapid change in membrane potential due to changes in the flux of potassium and sodium ions inside and outside the cell. First, a stimulus causes the sodium channels to open. Since there is more sodium outside than inside, sodium diffuses into the cell. This causes the membrane potential to become positive (+30 mV or higher), or depolarized. Once the sodium channels close, the potassium channels open, and potassium ions diffuse out. The inside of the nerve cell's membrane becomes negative again. Every action potential is followed by a refractory period. The period of time from the initiation of the action potential to just after the peak is called the absolute refractory period. During this time, another stimulus provided to the neuron, regardless of strength, cannot produce another action potential. This is because sodium channels are in a temporary inactive state and will not open. After the absolute refractory period, the sodium channels begin to recover from inactivation. It is during this time, called the relative refractory period, that another action potential can be elicited, albeit with a stronger-than-normal stimulus.

Action Potential

For an action potential to occur, a series of events must take place. The inside of a nerve cell's plasma membrane is typically negatively charged. But after a stimulus occurs, the inside of the membrane becomes positive due to an influx of sodium ions into the cell. Once a negative charge is reestablished inside the cell, the action potential ends, and the resting state, or membrane potential, of the cell is restored.

Types of Neurons

Neurons differ based on function, which includes afferent, efferent, and interneuron cells. Neurons also differ based on structure, which includes multipolar, bipolar, unipolar, and anaxonic cells. These cells are defined by the number and type of projections extending from the cell body.

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

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 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.
Schwann cells and satellite cells are neuroglia found in the PNS. Like an oligodendrocyte in the CNS, the Schwann cell is a glial cell in the PNS that provides myelination to axons in the PNS. Each individual Schwann cell sheath has only myelin sections between nodes of Ranvier and has phagocytic activity, which means it can engulf and clear cellular debris 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 tissue 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.
Neuroglia can be located in either the CNS or the PNS. Astrocytes and satellite cells provide support. Oligodendrocytes and Schwann cells provide insulation, microglia provide immune surveillance, and ependymal cells help create cerebrospinal fluid.