A spinal interneuron (relay neuron) forms part of a reflex arc
|NeuroLex ID||Intrinsic neuron role|
An interneuron (also called relay neuron, association neuron, connector neuron or local circuit neuron) is one of the three classifications of neurons found in the human body. Interneurons create neural circuits, enabling communication between sensory or motor neurons and the central nervous system (CNS). They have been found to function in reflexes, neuronal oscillations, and neurogenesis in the adult mammalian brain. Interneurons can be further broken down into two groups: local interneurons, or relay interneurons. Local interneurons have short axons and form circuits with nearby neurons and analyze small pieces of information. Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions. The interaction between interneurons allow the brain to perform involved functions such as learning, and decisive behaviors.
Interneurons in the Central Nervous System
When contrasted with the peripheral nervous system (PNS), the neurons of the CNS, including the brain, are all interneurons. Interneurons in the CNS are primarily inhibitory, and use the neurotransmitter GABA or glycine. However, excitatory interneurons using glutamate in the CNS also exist, as do interneurons releasing neuromodulators like acetylcholine.
In 2008, a nomenclature for the features of GABAergic cortical interneurons was proposed, called Petilla terminology.
Interneurons of the spinal cord
- 1a Inhibitory Neuron: Found in Lamina VII. Responsible for inhibiting antagonist motor neuron. 1a spindle afferents activate 1a inhibitory neuron.
- 1b Inhibitory Neuron: Found in Lamina V, VI, VII. afferent or Golgi tendon organ activates it.
Interneurons of the cortex
- Parvalbumin-expressing interneurons
- CCK-expressing interneurons
- VIP-expressing interneurons
- SOM-expressing interneurons
Interneurons of the cerebellum
- Molecular layer interneurons (basket cells, stellate cells)
- Golgi cells
- Granule cells
- Lugaro cells
- Unipolar brush cells
Interneurons of the striatum
- Parvalbumin-expressing interneurons
- Cholinergic interneurons
- Tyrosine hydroxylase-expressing interneurons
- Calretinin-expressing interneurons
- Nitric oxide synthase-expressing interneurons
Interneurons and Reflexes
In a normal neural pathway, cerebral neurons found in the frontal lobe and primary motor cortex send a signal to motor neurons in the periphery (another term for the PNS) to elicit motor movements. The movement then sends a response signal back to the brain allowing the cerebrum to interpret the action. Of course this entire communication circuit occurs in a few milliseconds. In a reflex, however, signals from the periphery do not need to travel to the brain for interpretation, rather the signal reaches the spinal cord, who interprets the action.
The Withdrawal Reflex
This reflex deals with the removal of your hand from a painful stimulus such as a hot stovetop. In this situation, the sensory neurons include thermoreceptors and nociceptors to detect the presence of a hot stimulus or a painful stimulus. When the dendrites of a sensory neuron are stimulated by (for instance) a hot sensation, the neuron propagates an action potential down the neuron's axon to excite local interneurons. These interneurons communicate with other interneurons in the spinal cord to allow for a quick response. Once interpreted, the interneurons from the spinal cord send a response signal to the motor neurons in your hand. The effector motor neurons interact with the myofibrils in your hand's myocytes at the neuromuscular junction to elicit a physiological reaction. When the neuromodulator, acetylcholine, is released, it depolarizes your myofibrils, causing a muscle contraction. The muscle contraction causes you to withdrawal your hand from the hot stovetop.
The knee jerk reaction is a special type of reflex called a myotatic spinal reflex. In this situation, the sensory neurons include thermoreceptors and nociceptors to detect the presence of a hot stimulus or a painful stimulus. Once the stimulus is detected, a message is sent down the afferent axon to that neurons terminal buttons, which are located in the spinal cord. The terminal buttons of the sensory neuron release an excitatory neurotransmitter like glutamate which stimulates the interneuron, causing the interneuron to fire. Once interpreted, the interneurons from the spinal cord send a response signal to the motor neurons in your hand. The effector motor neurons interact with the myofibrils in your hand's myocytes at the neuromuscular junction to elicit a physiological reaction. When the neuromodulator, acetylcholine, is released, it depolarizes your myofibrils, causing a muscle contraction. The muscle contraction causes you to withdrawal a limb from the painful stimulus.
- Whittington MA, Traub RD, Kopell N, Ermentrout B, Buhl EH (2000). "Inhibition-based rhythms: experimental and mathematical observations on network dynamics". Int J Psychophysiol 38: 315–336. doi:10.1016/S0167-8760(00)00173-2. PMID 11102670.
- Carlson, Neil R. "Physiology of Behavior". Pearson Higher Education, 2013, p. 28.
- Kandel, E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science (4th ed.). New York City, New York: McGraw Hill Companies, Inc. p. 25
- Ascoli GA; Alonso-Nanclares L; Anderson SA et al. (July 2008). "Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex". Nat. Rev. Neurosci. 9 (7): 557–568. doi:10.1038/nrn2402. PMC 2868386. PMID 18568015.
- Koós T, Tepper JM. Nat Neurosci. 1999 May;2(5):467-72. PMID 10321252
- Zhou FM, Wilson CJ, Dani JA. J Neurobiol. 2002 Dec;53(4):590-605. Review. PMID 12436423
- English DF, Ibanez-Sandoval O, Stark E, Tecuapetla F, Buzsáki G, Deisseroth K, Tepper JM, Koos T. Nat Neurosci. 2011 Dec 11;15(1):123-30. doi: 10.1038/nn.2984. PMID 22158514
- Ibáñez-Sandoval O, Tecuapetla F, Unal B, Shah F, Koós T, Tepper JM. J Neurosci. 2010 May 19;30(20):6999-7016. doi: 10.1523/JNEUROSCI.5996-09.2010. PMID 20484642
- Tepper JM, Tecuapetla F, Koós T, Ibáñez-Sandoval O. Front Neuroanat. 2010 Dec 29;4:150. doi: 10.3389/fnana.2010.00150. PMID 21228905
- Carlson, Neil R. "Physiology of Behavior". Pearson Higher Education, 2013, p. 41.
- Purves, D., Augustine, G.J., Fizpatrick, D., Hall, W.C., LaMantia, A.S., McNamara, J.O., White, L.E. (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Assocaites, Inc. p. 12
- Carlson, N.R. (2013). Physiology of Behavior (11th ed.). New York City, New York: Pearson Education, Inc. p. 41