Neurons

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. 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.

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.