Nervous Tissue Definition
Nervous tissue is the term for groups of organized cells in the nervous system, which is the organ system that controls the body’s movements, sends and carries signals to and from the different parts of the body, and has a role in controlling bodily functions such as digestion. Nervous tissue is grouped into two main categories: neurons and neuroglia. Neurons, or nerves, transmit electrical impulses, while neuroglia do not; neuroglia have many other functions including supporting and protecting neurons.
Function of Nervous Tissue
Nervous tissue makes up the nervous system. The nervous system is subdivided in several overlapping ways. The central nervous system (CNS) is composed of the brain and spinal cord, which coordinates information from all areas of the body and sends nerve impulses that control all bodily movements.
The peripheral nervous system (PNS) consists of peripheral nerves that branch all throughout the body. It connects the CNS to the rest of the body and is directly responsible for controlling movements of specific parts of the body; for example, just before arm movement, the CNS sends nerve impulses to the PNS nerves in the arm, which causes the arm to move.
Another subdivision of the nervous system is into the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). The SNS activates in order to stimulate a fight-or-flight response in an organism when that organism encounters a threat and must decide whether to fight or flee from it.
The nerves of the SNS have diverse effects on different parts of the body. Activation of the SNS causes the pupils of the eyes to dilate, inhibits digestion, increases sweat secretion, and increases the heart rate. Conversely, the PSNS is activated during moments of “rest and digest”, when an organism is not facing an immediate threat. Nerves of the PSNS work to stimulate activities that can occur at rest such as digestion, waste excretion, and sexual arousal, and they also decrease the heart rate.
The enteric nervous system (ENS) controls the gastrointestinal tract (digestive tract). This division of the nervous system, along with the SNS and PSNS, are collectively referred to as the autonomic nervous system (ANS). The ANS regulates activities that are performed unconsciously; we don’t have to think about digesting food for it to occur, for example. By contrast, the somatic nervous system (SoNS) controls voluntary body movements. It is made up of afferent and efferent nerves that send signals to and from the CNS, causing voluntary muscle contraction to occur.
Types of Nervous Tissue
Neurons are cells that can transmit signals called nerve impulses, or action potentials. An action potential is a quick rise and fall in the electrical membrane potential of the neuron, which transmits signals from one neuron to the next. These are the different types of neurons:
Sensory, or afferent neurons, relay information from the PNS to the CNS; different types of sensory neurons can detect temperature, pressure, and light.
Motor, or efferent neurons, send signals from the CNS to the PNS; these signals provide information to sensory neurons to “tell” them what to do (e.g., initiate muscle movement).
Interneurons connect sensory and motor neurons to the brain and spinal cord; they act as connectors to form neural circuits and are involved with reflex actions and higher brain functions like decision-making.
While neurons can be specialized and look very different from one another, they each have components in common. Each neuron has a soma, or cell body, that contains the nucleus. Dendrites, finger-like projections that receive nerve impulses, branch off from the soma.
The axon is a larger projection that branches off from the soma. Nerve impulses travel along the axon in the form of an action potential. The axon splits into axon terminals, which branch off to other neurons. Neurotransmitters are released from the ends of the axon terminals, and these travel across the synaptic cleft to reach receptors on the dendrites of other neurons. In this way, neurons communicate with each other and can send signals that reach many other neurons.
Neuroglia, or glial cells, are cells that support neurons, supply them with nutrients, and get rid of dead cells and pathogens such as bacteria. They also form insulation between neurons so that electrical signals do not get crossed, and can also aid the formation of synaptic connections between neurons. There are several types of neuroglia:
Astroglial cells, also called astrocytes, are star-shaped cells found in the brain and spinal cord. They provide nutrients to neurons, maintain ion balance, and remove unneeded excess neurotransmitters from the synaptic cleft.
Ependymal cells are also found in the CNS. There are two types of ependymal cells. Non-ciliated ependymal cells form cerebrospinal fluid, while ciliated ependymal cells help the cerebrospinal fluid circulate. Cerebrospinal fluid cushions the brain and spinal cord.
Oligodendrocytes are found in the CNS and provide physical support to neurons. They form a myelin sheath around some neurons in the CNS. The myelin sheath is a fatty substance wrapped around the axons of some neurons; it provides electrical insulation.
Schwann cells also form myelin sheaths around some neurons, but they are only found in the PNS. Neurons that are myelinated can conduct electrical impulses faster than non-myelinated neurons.
Microglial cells, or microglia, are small macrophage cells in the CNS that protect against disease by engulfing pathogens through phagocytosis (“cell eating”). They can also destroy infected neurons and promote the regrowth of neurons. All of the other types of neuroglia above are larger and collectively called macroglia.