| Spinal interneuron | |
|---|---|
 Spinal interneuron integrates sensory-motor input | |
| Anatomical terminology |
A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. [1] Most interneurons are found in the grey column, a region of grey matter in the spinal cord.
The grey column of the spinal cord appears to have groups of small neurons, often referred to as spinal interneurons, that are neither primary sensory cells nor motor neurons. [2] The versatile properties of these spinal interneurons cover a wide range of activities. Their functions include the processing of sensory input, the modulation of motor neuron activity, the coordination of activity at different spinal levels, and the relay of sensory or proprioceptive data to the brain. 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. [2] 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 axon projections were found. In addition, 18 types of commissural interneurons have been identified on the basis of morphology and location. [3] [4]
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 are locations where most of the interneurons are found. [5]
In the mouse's dorsal alar plate, six progenitor domains give rise to dI1-dI6 neurons and two classes of dorsal interneurons. [6] In addition, in the ventral half of the neural tube, four classes of (CPG) interneurons known as V0, V1, V2, and V3 neurons are generated. [6] V0 neurons are commissural neurons that extend their axons rostrally for 2-4 spinal cord regions in the embryonic spinal cord. [6] V3 neurons are excitatory commissural interneurons that extend caudally projecting primary axons. [6] The V1 neurons are inhibitory interneurons with axons that project ipsilaterally and rostrally. [6] V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ipsilaterally and caudally across multiple spinal cord regions. [6] The class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons. [6]
| CPG interneurons | Type | Axon projection in embryonic cord |
|---|---|---|
| V0 | Commissural | Rostrally |
| V1 | Inhibitory (Renshaw cells and Ia interneurons) | Rostrally and ipsilaterally |
| V2 | Glutamatergic V2a and Inhibitory V2b | Ipsilaterally and caudally |
| V3 | Excitatory Commissural | Caudally |
The integration of the sensory feedback signals and central motor commands at several levels of the central nervous system plays a critical role in controlling movement. [7] 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. [7] 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. [7] During locomotion, the sum of convergent inputs from the central pattern generator (CPG), sensory feedback, descending commands and other intrinsic properties turned on by different neuromodulators give rise to the activity of the interneurons. [8] Further, this interneuronal activity was either recorded directly or inferred from the modulation of response in their postsynaptic targets, most often motoneurons. [8] 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. [8] Thus, different patterns of interneuronal activity will determine which pathways are open, blocked, or modulated. [8]
The sensory information that is transmitted to the spinal cord is modulated by a complex network of excitatory and inhibitory interneurons. Different neurotransmitters are released from different interneurons, but the two most common neurotransmitters are GABA, the primary inhibitory neurotransmitter and glutamate, the primary excitatory neurotransmitter. [9] [10] Acetylcholine is a neurotransmitter that often activates interneurons by binding to a receptor on the membrane. [11]
Renshaw cells are among the first identified interneurons. [12] This type of interneuron projects onto α-motoneurons, where it establishes inhibition by expressing its inhibitory neurotransmitter glycine. [12] [13] However, some reports have indicated that Renshaw cells synthesize calcium-binding proteins calbindin-D28k and parvalbumin.[ clarification needed ] 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. [13] Furthermore, the connection to the motor neurons establishes a negative feedback system that may regulate the firing rate of the motor neurons. [13] Moreover, the connections to the Ia inhibitory interneurons may modulate the strength of the reciprocal inhibition to the antagonist motor neuron. [13]
Joints are controlled by two opposing sets of muscles called extensors and flexors that must work in synchrony to allow proper and desired movement. [14] When a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent from working against the agonist muscle. [12] [14] The spinal interneuron called Ia inhibitory interneuron is responsible for this inhibition of the antagonist muscle. [14] The Ia afferent of the muscle spindle enters the spinal cord, and one branch synapses on to the alpha motor neuron that causes the agonist muscle to contract. [14] Thus, it results in creating the behavioral reflex.
At the same time, the other branch of the Ia afferent synapses on to the Ia inhibitory interneuron, which in turn synapses the alpha motor neuron of the antagonist muscle. [14] Since Ia interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing. Thus, it prevents the antagonist muscle from contracting. [14] Without having this system of reciprocal inhibition, 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. [14] 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. [14] 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. [14] 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. [12] [13] [14] [15]
The autogenic inhibition reflex is a spinal reflex phenomenon that involves the Golgi tendon organ. [14] 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. [14] 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. [14] This is an example of a disynaptic reflex, in which the circuitry contains a spinal interneuron between the sensory afferent and the motor neuron. [13] [14]
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 alpha motor neuron that controls the antagonist muscle. When the agonist muscle is inhibited from contracting, the antagonist muscle contracts. [14]
An important reflex initiated by cutaneous receptors and pain receptors is the flexor reflex. [14] 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. [14] 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, where they synapse an excitatory interneuron. [14] 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. [14] 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. [14] This inhibitory effect is achieved when Group III afferents synapse inhibitory interneurons that in turn synapse the alpha motor neurons innervating the antagonists muscle. [14]
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. A branch of the Group III afferent synapse an excitatory interneuron, which extends its axon across the midline into the contralateral 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. [14]
A group of commissural interneurons present in lamina VIII in mid-lumbar segments mediates excitation of contralateral motoneurons by reticulospinal neurons. [9] These neurons receive monosynaptic inputs from ipsilateral reticular formation and are not directly activated by group II afferents. [15]
Another class of lamina VIII commissural neurons includes a group that is activated by both reticulospinal and vestibular systems. These cells can also be activated indirectly by group I and group II afferents. [16] These cells have also been shown to be active during locomotion.
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers, or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.
A motor 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 control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.
The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.
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 processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.
The grey column refers to a somewhat ridge-shaped mass of grey matter in the spinal cord. This presents as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which are visible in cross-section of the spinal cord.
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.
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.
The 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 the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.
The caridoid escape reaction, also known as lobstering or tail-flipping, refers to an innate escape mechanism in marine and freshwater crustaceans such as lobsters, krill, shrimp and crayfish.
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 the agonist muscle to contract. The tension in the antagonist muscle is activated by impulses from motor neurons, causing it to relax.
A gamma motor neuron, also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of (Aγ) fibers going to the muscle. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles.
In neuroscience, Golgi cells are inhibitory interneurons found within the granular layer of the cerebellum. They were first identified as inhibitory in 1964. It was also the first example of an inhibitory feedback network in which the inhibitory interneuron was identified anatomically. These cells synapse onto the dendrite of granule cells and unipolar brush cells. They receive excitatory input from mossy fibres, also synapsing on granule cells, and parallel fibers, which are long granule cell axons. Thereby this circuitry allows for feed-forward and feed-back inhibition of granule cells.
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 contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
The Mauthner cells are a pair of big and easily identifiable neurons located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex. The cells are also notable for their unusual use of both chemical and electrical synapses.
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-induced. The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon. When the tension is extreme, the inhibition can be so great it overcomes the excitatory effects on the muscle's alpha motoneurons causing the muscle to suddenly relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.
Group A nerve fibers are one of the three classes of nerve fiber as generally classified by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated.
Cutaneous, superficial, or skin reflexes, are activated by skin receptors and play a valuable role in locomotion, providing quick responses to unexpected environmental challenges. They have been shown to be important in responses to obstacles or stumbling, in preparing for visually challenging terrain, and for assistance in making adjustments when instability is introduced. In addition to the role in normal locomotion, cutaneous reflexes are being studied for their potential in enhancing rehabilitation therapy (physiotherapy) for people with gait abnormalities.
Presynaptic inhibition is a phenomenon in which an inhibitory neuron provides synaptic input to the axon of another neuron to make it less likely to fire an action potential. Presynaptic inhibition occurs when an inhibitory neurotransmitter, like GABA, acts on GABA receptors on the axon terminal. Or when endocannabinoids act as retrograde messengers by binding to presynaptic CB1 receptors, thereby indirectly modulating GABA and the excitability of dopamine neurons by reducing it and other presynaptic released neurotransmitters. Presynaptic inhibition is ubiquitous among sensory neurons.
An axo-axonic synapse is a type of synapse, formed by one neuron projecting its axon terminals onto another neuron's axon.