A spinal interneuron, found in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

, relays signals between (afferent)

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...

s, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of

grey matter Grey matter is a major component of the central nervous system, consisting of neuronal Soma (biology), cell bodies, neuropil (dendrites and unmyelinated axons), Glia, glial cells (astrocytes and oligodendrocytes), synapses, and Capillary, capil ...

in the spinal cord.

Structure

The grey column of the

appears to have groups of small

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

s, often referred to as spinal interneurons, that are neither primary sensory cells nor

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...

s. The versatile properties of these spinal interneurons cover a wide range of activities. Their functions include the processing of

sensory Sensory may refer to: Biology * Sensory ecology, how organisms obtain information about their environment * Sensory neuron, nerve cell responsible for transmitting information about external stimuli * Sensory perception, the process of acquiri ...

input, the modulation of motor neuron activity, the coordination of activity at different spinal levels, and the relay of sensory or

proprioceptive Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neuron ...

data to the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

. There has been extensive research on the identification and characterization of the spinal cord interneurons based on factors such as location, size, structure, connectivity, and function. Generally, it is difficult to characterize every aspect of the neuronal anatomy of a vertebrate's spinal cord. This difficulty is due not only to its structural complexity but also to the morphology and the connectivity of neurons. For instance, in the spinal cord of a 19-day-old rat embryo, at least 17 different subclasses of interneurons with

ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

projections were found. In addition, 18 types of commissural interneurons have been identified on the basis of morphology and location.

Location

In particular, the cell bodies of the spinal interneurons are found in the grey matter of the spinal cord, which also contains the motor neurons. In 1952, the grey matter of the cat's spinal cord was investigated, and it was shown to have ten distinct zones referred to as Rexed laminae. Eventually, the lamination pattern was also observed in several species including humans. Rexed laminae VII and

VIII 8 (eight) is the natural number following 7 and preceding 9. In mathematics 8 is: * a composite number, its proper divisors being , , and . It is twice 4 or four times 2. * a power of two, being 2 (two cubed), and is the first number of t ...

are locations where most of the interneurons are found. Medulla spinalis - Substantia grisea - English

Development

In the mouse's

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

alar plate Daminozide—also known as aminozide, Alar, Kylar, SADH, B-995, B-nine, and DMASA,—is a plant growth regulator, a chemical sprayed on fruit to regulate growth, make harvest easier, and keep apples from falling off the trees before they ripen ...

, six

progenitor In genealogy, the progenitor (rarer: primogenitor; german: Stammvater or ''Ahnherr'') is the – sometimes legendary progenitor, legendary – founder of a family, Kinship, line of descent, clan or tribe, Nobility, noble house, or ethnic group. ...

domains give rise to dI1-dI6 neurons and two classes of dorsal interneurons. In addition, in the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

half of the neural tube, four classes of (CPG) interneurons known as V0, V1, V2, and V3 neurons are generated. V0 neurons are commissural neurons that extend their axons rostrally for 2-4 spinal cord regions in the embryonic spinal cord. V3 neurons are excitatory commissural interneurons that extend

caudally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

projecting primary axons. The V1 neurons are inhibitory interneurons with axons that project

ipsilaterally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and rostrally. V2 neurons, which include a population of

glutamatergic Glutamatergic means "related to glutamate". A glutamatergic agent (or drug) is a chemical that directly modulates the excitatory amino acid (glutamate/aspartate) system in the body or brain. Examples include excitatory amino acid agonist, excitato ...

V2a neurons V, or v, is the twenty-second and fifth-to-last Letter (alphabet), letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in Englis ...

and inhibitory

V2b neurons V, or v, is the twenty-second and fifth-to-last letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''vee'' (pronounced ), plura ...

, project ipsilaterally and caudally across multiple spinal cord regions. The class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons.

Function

The integration of the sensory feedback signals and central motor commands at several levels of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

plays a critical role in controlling movement. Research on cat's spinal cord has shown that at the spinal cord level sensory afferents and descending motor pathways converge onto common spinal interneurons. Human studies since the 1970s have documented how this integration of motor commands and sensory feedback signals is used to control muscle activity during movement. During locomotion, the sum of convergent inputs from the

central pattern generator Central pattern generators (CPGs) are self-organizing biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. They are the source of the tightly-coupled patterns of neural activity that drive rhythmic and stereo ...

(CPG), sensory feedback, descending commands and other intrinsic properties turned on by different neuromodulators give rise to the activity of the interneurons. Further, this interneuronal activity was either recorded directly or inferred from the modulation of response in their postsynaptic targets, most often motoneurons. The most efficient way to gate sensory signals in reflex pathways is to control the firing level of interneurons. For example, during locomotion, the interneuronal activity is modulated via excitation or inhibition depending on the reflex pathways. Thus, different patterns of interneuronal activity will determine which pathways are open, blocked, or modulated.

Neurotransmitter

The sensory information that is transmitted to the spinal cord is modulated by a complex network of

excitatory In neuroscience, an excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of postsynaptic membrane potential, caused by the ...

and inhibitory interneurons. Different neurotransmitters are released from different interneurons, but the two most common neurotransmitters are GABA, the primary inhibitory neurotransmitter and

glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...

, the primary

neurotransmitter.

Acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

is a neurotransmitter that often activates interneurons by binding to a receptor on the membrane.

Cell types

Renshaw cells

Renshaw cells Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron. * They receive an excitatory collateral from the alpha neuron's axon as they emerge from the motor ...

are among the first identified interneurons. This type of interneuron projects onto α-motoneurons, where it establishes inhibition by expressing its inhibitory neurotransmitter glycine. However, some reports have indicated that Renshaw cells synthesize calcium-binding proteins calbindin-D28k and parvalbumin. Further, during spinal reflex, Renshaw cells control the activity of the spinal motoneurons. They are excited by the axon collaterals of the motor neurons. In addition, Renshaw cells make inhibitory connections to several groups of motor neurons, Ia inhibitory interneurons as well as the same motor neuron that excited them previously. Furthermore, the connection to the motor neurons establishes a

negative feedback Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...

system that may regulate the

firing rate Rate of fire is the frequency at which a specific weapon can fire or launch its projectiles. This can be influenced by several factors, including operator training level, mechanical limitations, ammunition availability, and weapon condition. In m ...

of the motor neurons. Moreover, the connections to the Ia inhibitory interneurons may modulate the strength of the

reciprocal inhibition Reciprocal inhibition describes the relaxation of muscles on one side of a joint to accommodate contraction on the other side. In some allied health disciplines, this is known as reflexive antagonism. The central nervous system sends a message to ...

to the antagonist motor neuron.

Ia inhibitory interneuron

Joints are controlled by two opposing sets of muscles called extensors and

flexors A flexor is a muscle that flexes a joint. In anatomy, flexion (from the Latin verb ''flectere'', to bend) is a joint movement that decreases the angle between the bones that converge at the joint. For example, one’s elbow joint flexes when one ...

that must work in synchrony to allow proper and desired movement. When a

muscle spindle Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

is stretched and the

stretch reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. The reflex functions to maintain the muscle at a constant length. The term deep tendon reflex is ...

is activated, the opposing muscle group must be inhibited to prevent from working against the agonist muscle. The spinal interneuron called Ia inhibitory interneuron is responsible for this inhibition of the antagonist muscle. The Ia afferent of the muscle spindle enters the spinal cord, and one branch

synapses In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

on to the alpha motor neuron that causes the agonist muscle to contract. Thus, it results in creating the behavioral reflex. At the same time, the other branch of the Ia afferent

on to the Ia inhibitory interneuron, which in turn synapses the

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their ...

of the antagonist muscle. Since Ia interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing. Thus, it prevents the antagonist muscle from contracting. Without having this system of

, both groups of muscles may contract at the same time and work against each other. This results in spending a greater amount of energy as well. In addition, the reciprocal inhibition is important for mechanism underlying voluntary movement. When the antagonist muscle relaxes during movement, this increases efficiency and speed. This prevents moving muscles from working against the contraction force of antagonist muscles. Thus, during voluntary movement, the Ia inhibitory interneurons are used to coordinate muscle contraction. Further, the Ia inhibitory interneurons allow the higher centers to coordinate commands sent to the two muscles working opposite of each other at a single joint via a single command. The interneuron receives the input command from the corticospinal descending axons in such a way that the descending signal, which activates the contraction of one muscle, causes relaxation of the other muscles.

Ib inhibitory interneuron

The

autogenic inhibition reflex The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-indu ...

is a spinal reflex phenomenon that involves the

Golgi tendon organ The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a mu ...

. When tension is applied to a muscle, group Ib fibers that innervate the Golgi tendon organ are activated. These afferent fibers project onto the spinal cord and synapse with the spinal interneurons called Ib inhibitory interneurons. This spinal interneuron makes an inhibitory synapse onto the alpha motor neuron that innervates the same muscle that caused the Ib afferent to fire. As a result of this reflex, activation of the Ib afferent causes the alpha motor neuron to become inhibited. Thus, the contraction of the muscle stops. This is an example of a disynaptic reflex, in which the circuitry contains a spinal interneuron between the sensory afferent and the motor neuron. The activities of the extensor and flexor muscles must be coordinated in the autogenic inhibition reflex. The Ib afferent branches in the spinal cord. One branch synapses the Ib inhibitory interneuron. The other branch synapses onto an excitatory interneuron. This excitatory interneuron innervates the

that controls the antagonist muscle. When the agonist muscle is inhibited from contracting, the antagonist muscle contracts.

Excitatory interneurons

An important reflex initiated by cutaneous receptors and

pain receptor A nociceptor ("pain receptor" from Latin ''nocere'' 'to harm or hurt') is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sens ...

s is the

flexor reflex The withdrawal reflex (nociceptive flexion reflex or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of ...

. This reflex mechanism allows for quick withdrawal of the body parts, in this case a limb, from the harmful stimulus. The signal travels to the spinal cord and a response is initiated even before it travels up to the brain centers for a conscious decision to be made. The reflex circuit involves the activation of the Group III afferents of pain receptors due to a stimulus affecting a limb, e.g. a foot. These afferents enter the spinal cord and travel up to the

lumbar region In tetrapod anatomy, lumbar is an adjective that means ''of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum.'' The lumbar region is sometimes referred to as the lower spine, or as an area of the back in ...

, where they synapse an excitatory interneuron. This interneuron excites the alpha motor neuron that causes contraction of the thigh flexor muscle. Also, Group III afferent travels up to L2 vertebra, where they branch onto another excitatory interneuron. This interneuron excites the alpha motor neurons, which then excite the hip flexor muscle. This synchronized communication allows for the removal of the whole leg from the painful stimulus. This is an example of the spinal cord circuitry coordinating movement at several joints simultaneously. In addition, during flexor reflex, when the knee joints and hip joints are flexed, the antagonist extensor muscles must be inhibited. This inhibitory effect is achieved when Group III afferents synapse inhibitory interneurons that in turn synapse the alpha motor neurons innervating the antagonists muscle. The flexor reflex not only coordinates the activity of the leg being removed but also the activity of the other leg. When one leg is removed, the weight of the body needs to be distributed to the opposite leg to maintain the body’s balance. Thus, the flexor reflex incorporates a

crossed extension reflex The crossed extensor reflex or crossed extensor response or crossed extension reflex is a reflex in which the contralateral limb compensates for loss of support when the ipsilateral limb withdraws from painful stimulus in a withdrawal reflex. Duri ...

. A branch of the Group III afferent synapse an excitatory interneuron, which extends its axon across the midline into the

contralateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

spinal cord. At that location, the interneuron excites the alpha motor neurons that innervate the extensor muscles of the opposite leg. This allows for balance and body posture to be maintained.

References

{{reflist Neurons Spine Neuroanatomy