An interneuron, also known as a switching neuron or intermediate neuron, is a nerve cell within the central nervous system (CNS). The function of the interneuron is to switch two neurons together. In a narrow sense, it is a sensory (afferent) and a motor (efferent) neuron.
What is an interneuron?
This medical field is covered by neurology and neuroscience. The term is derived from the Greek words “inter” = between and “neuron” = nerve. Interneurons are nerve cells with their terminal knobs (processes) located in a defined area of the central nervous system, where they are connected between two or more nerve cells. They do not have long axons and are therefore not able to transmit signals over long distances. Intermediate neurons exhibit an extraordinarily differentiated, functional and morphological diversity. Intermediate neurons have to cope with a complex variety of tasks, including controlling the input and output of the principal neurons (principal cells) and modifying the signaling currents between the individual cells. The complexity of these tasks can only be understood through the neuroarchitecture of the central nervous system, which has so far only been partially understood medically. Various attempts exist to classify interneurons, but so far with only moderate success, as no unified taxonomy is yet available.
Anatomy and structure
There are different types of neurons. Medicine distinguishes between unipolar, bipolar, pseudounipolar, and multipolar neurons. These are divided into sensory neurons, interneurons, and motor neurons. They act in close interaction in the human body. Sensory neurons are nerves and nerve fibers that transmit information to the spinal cord and brain by means of sensory organ receptors. Motoneurons (motor nerve cells) transmit impulses from the brain and spinal cord to the muscles and glands. They are responsible for an orderly movement of the human body. Interneurons are located between the sensory and motor nerve cells and perform an intermediary function. They then transmit incoming signals between the individual areas of the body and process this information in local circuits. Medicine distinguishes between local and intersegmental interneurons. Intersegmental neurons usually have much smaller cells with many interconnections than motoneurons or sensory neurons. These three types of neurons are arranged in the form of chains and networks. The simplest expression is the reflex arc. This forms the shortest connection between effector and receptor across the neurons of a defined neuronal excitatory circuit. The connection from the afferent (afferent direction of power) to the efferent neuron (efferent direction of conduction) occurs at the spinal level via a synapse in the anterior horn of the spinal cord. This form of reflex is called a monosynaptic reflex arc. Effectors are cells that trigger a specific effect. Most often, they are muscle cells that contract (contract) or relax in response to the incoming signal. The function of the reflex arc is to focus the incoming action potentials and ensure an orderly sequence of movements. In this case, it ensures that the muscles do not react to the incoming signal by contracting. Example of the effective cooperation of the various neurons: If the blood glucose level falls below a critical setpoint, the receptors present in the veins report this alarming state via the interneurons and the afferent neurons in the central nervous system. Via the efferent neurons, the central nervous system sends a command to the beta cells of the pancreas to secrete the hormone insulin. This substance then travels via the bloodstream to the liver, which converts glucose into water-soluble gycolene and stores it. This mechanism lowers the glucose concentration in the blood.
Function and tasks
Interconnection neurons receive input from other neurons and, after processing, transmit these excitatory or inhibitory signals to downstream neurons. They do not perform motor or sensory tasks. Interconnection neurons are located between motor neurons and sensory neurons in the form of functional chains or functional circuits.In the spinal cord, for example, poly- and oligosynaptic reflexes and Renshaw inhibition proceed via circuit neurons. This is a reverse inhibition in which motor neurons pass axon collaterals to the inhibitory interneurons, which inhibit the motor neuron from which the excitatory signal originated. This results in a limitation of the duration of excitation. The brain has interneurons in the form of neurons with relatively short axons (type II Golgi cells). These are arranged opposite projection neurons with long axons. The enteric nervous system (ENS) has sensory interneurons as derivatives of the neural crest. These, together with inhibitory and excitatory motor and sensory neurons, form a complicated system. For this reason, interneurons are often referred to as circuit neurons because they perform a mediating function between the neurons involved, relaying incoming signals between the different areas of the body and processing data much like a computer using local circuits. These signals are previously received by receptors (sensory cells) and converted into electrical signals so that the interneurons can process them. They compute information from different sources and pass the result to the downstream cell. They form the largest set of neurons in the human body. For example, the human retina has several layers of interneurons. These add up and evaluate the signals from the photoreceptors (rods and cones) that come in through the retina. During this process, each interneuron is linked to a large number of photoreceptors, which in turn are linked to many interneurons.
Diseases
A well-functioning nervous system is essential for the maintenance of various bodily functions. The ongoing exchange of communication between the brain, sensory organs, muscles, and neurons enables us to respond in a timely manner to the demands of our environment. These mechanics begin with the control of body temperature, respiration, blood circulation and movement. In addition, there are energy supply, metabolism and sensory functions. The special function of the nerve cells is the processing and transmission of incoming impulses, whereby the body’s reaction takes place independently without the involvement of the brain. Instead, the reflex arc in the spinal cord is responsible for information processing. In order to bring about a quick reaction to incoming information, an impulse is sent directly from the spinal cord and executed by the muscles involved. It seems that this mechanism is consciously controlled, which is due to the fact that the brain subsequently takes control of this muscle region. The nerve cells are equally important in learning new things. If the central nervous system no longer functions properly, or even only to a limited extent, this condition can result in a variety of complaints, since nerve cells are found throughout the body. These complaints can be both neurological and physiological, such as mental illnesses and disorders, back pain, restricted movement, muscular and intestinal disorders, or metabolic disorders.
