The ventrobasal complex (VB) is a relay nucleus of the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

for

nociceptive Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an org ...

stimuli received from nociceptive

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

s. The VB consists of the

ventral posteromedial nucleus The ventral posteromedial nucleus (VPM) is a nucleus of the thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve f ...

(VPM) and the

ventral posterolateral nucleus The ventral posterolateral nucleus (VPL) is a nucleus of the thalamus. Together with the ventral posteromedial nucleus (VPM), ventral posterior inferior nucleus (VPI) and ventromedial posterior nucleus (VMpo), it constitutes the ventral posterior ...

(VPL). In some species the ventral posterolateral nucleus, pars caudalis is also a part of the VB.Natsu Koyama, Yasuo Nishikawa, Toshikatsu Yokota. Distribution of nociceptive neurons in the ventrobasal complex of macaque thalamus. Neuroscience Research 31 (1998) 39-51. The VB gets inputs from the

spinothalamic tract The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...

medial lemniscus In neuroanatomy, the medial lemniscus, also known as Reil's band or Reil's ribbon (for German anatomist Johann Christian Reil), is a large ascending bundle of heavily myelinated axons that decussate (cross) in the brainstem, specifically in the ...

, and corticothalamic tract.W.D. Willis Jr. et al. Projections from the marginal zone and deep dorsal horn to the ventrobasal nuclei of the primate thalamus. Pain 92 (2001) 267-276.Yasuyuki Nagumo, Yuichi Takeuchi, Keiji Imoto, Mariko Miyata. Synapse- and subtype-specific modulation of synaptic transmission by nicotinic acetylcholine receptors in the ventrobasal thalamus. Neuroscience Research 69 (2011) 203-213. The main output of the VB is the

primary somatosensory cortex In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the Human brain, brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield ...

. The VB serves as the main relay for nociceptive stimuli and the modulation of that stimuli to the primary somatosensory cortex. The modulation occurs through different types of receptors present in the VB.

VB Inputs

Spinothalamic tract (STT) cells that project from laminae I and V in the

lumbosacral The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates ...

area of 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 spi ...

project to the VPL in the VB. STT cells located in the cervical area of the spinal cord are the densest and project from the neck of the dorsal horn to the VPL of the VB. Most projections to the VB are contralateral while only a few projections to the VB are ipsilateral. Excitatory inputs to the VB are medial lemniscal (ML) and corticothalamic (CT) glutamatergic synapses. The ML is a sensory afferent input and the CT is from layer VI of the primary sensory cortex. The VB also gets inputs from areas in the brain stem which release

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. Part ...

(ACh) that can modulate activity in the VB.

VB Outputs

The VB has outputs to the primary somatosensory cortex.

VB Neurons

There are two types of nociceptive neurons that provide input to the VB: nociceptive specific (NS) neurons and

wide dynamic range neuron The wide dynamic range (WDR) neuron was first discovered by Mendell in 1966. Early studies of this neuron established what is known as the gate control theory of pain. The basic concept is that non-painful stimuli block the pathways for painful sti ...

(WDR). NS neurons respond specifically to a noxious mechanical stimulus, whereas WDR neurons respond to a graded mechanical stimulus. NS and WDR neurons within the VB are somatotopically organized. NS neurons are located more caudally in the VB, while WDR neurons are located more rostrally. All inputs into the VB are contralateral and have two different receptive fields within the VB. The VPM receptive field receives input from the contralateral trigeminal nerve and the VPL receptive field receives input from the contralateral spinal nerve. Each have NS and WDR neurons but terminate either caudally or rostrally respectively.

VB Modulation

Nicotinic ACh Receptors

Nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ne ...

(nAChRs) are present in the VB. Each nAChR can be made up of different subunits which can cause the receptor to respond to different stimuli. In the VB, nAChRs can contain the subunits α4, α5, α7, and β2. nAChRs that are made up of (α4β2)₂α5 are of interest because they decrease neurotransmitter release for corticothalamic (CT) synapses. When nAChRs are activated there is a decrease in synaptic transmission of glutamate from CT neurons. When CT synaptic transmission is decreased by activation of the nAChRs then the activated nAChRs in the VB can selectively enhance information to the somatosensory cortex through the medial lemniscal tract.

Mu-opioid Receptors

μ-opioid receptor (MORs) are inhibitory receptors that can cause a decrease in pain if activated and are expressed in the VB especially in the VPL.Daniel Humberto Pozza et al. Nociceptive behaviour upon modulation of mu-opioid receptors in the ventrobasal complex of the thalamus of rats. Pain 148 (2010) 492-502. When MORs are activated, by an agonist like

DAMGO DAMGO ( D-Ala2,_''N''-MePhe4,_Gly-ol.html" ;"title="small>D-Ala2, ''N''-MePhe4, Gly-ol">small>D-Ala2, ''N''-MePhe4, Gly-olenkephalin) is a synthetic opioid peptide with high μ-opioid receptor specificity. It was synthesized as a biologically stab ...

for example, pain-related behaviors are decreased for a certain amount of time. After 45 minutes rats that were given DAMGO show signs of increased pain behaviors suggesting that opiates activate a pronociceptive system which can lead to increased pain sensitivity after only having one dose of opiates administered. Decrease in pain-related behaviors can be attributed to the activation of MORs in the VB which activates an inhibitory circuit for pain by decreasing the amount or quality of information relayed to the somatosensory cortex. However, there is a complex mechanism between MORs and other receptors in the VB that can lead to a decrease in pain-related behaviors and thus further research is needed to understand exactly how this mechanism works.

GABA_B Receptors

GABA_B receptors are located in the VB. If a receptor is located presynaptically when activated it causes the suppression of neurotransmitter release. If the receptor is located postsynaptically then when activated it causes

inhibitory postsynaptic potential An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.Purves et al. Neuroscience. 4th ed. Sunderland (MA): Sinauer Associates, Incorporated; 2008. ...

.Catarina Soares Potes, Fani Lourenca Neto, Jose Manuel Castro-Lopes. Inhibition of pain behavior by GABAB receptors in the thalamic ventrobasal complex: Effect on normal rats subjected to the formalin test of nociception. Brain Research 1115 (2006) 37-47. When GABA_B receptors are activated or blocked by

baclofen Baclofen, sold under the brand name Lioresal among others, is a medication used to treat muscle spasticity such as from a spinal cord injury or multiple sclerosis. It may also be used for hiccups and muscle spasms near the end of life. It is ta ...

(agonist) or CGP35348 (antagonist) respectively there is a decrease in pain-related behaviors in a dose-dependent manner. That is, there is less pain-related behavior if a higher dose is given. It is not known if this mechanism is occurring presynaptically or postsynaptically. Further research is needed to distinguish between where the inhibition is being mediated.

References

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