Gracile nucleus

Located in the medulla oblongata, the gracile nucleus is one of the dorsal column nuclei that participate in the sensation of fine touch and proprioception of the lower body (legs and trunk). It contains second-order neurons of the posterior column-medial lemniscus pathway, which receive inputs from sensory neurons of the dorsal root ganglia and send axons that synapse in the thalamus.

The neurons contained within the nucleus form a visible bump called the gracile tubercle on the posterior side of the closed medulla at the floor of the fourth ventricle.

The gracile nucleus and fasciculus carry epicritic, kinesthetic, and conscious proprioceptive information from the lower part of the body (below the level of T6 in the spinal cord). The counterpart to the gracile nucleus and fasciculus is the cuneate nucleus and fasciculus, which carries the same type of information, but from the upper body (above T6, excepting the face and ear - the information from the face and ear is carried by the principal sensory nucleus of trigeminal nerve).

Gracile nucleus
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Section of the medulla oblongata at the level of the decussation of the pyramids. Gracile nucleus is #8.
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Dissection of brain-stem. Dorsal view. ("nucleus gracilis" is labeled on left, second from the bottom.)
Details
Identifiers
Latinnucleus gracilis
NeuroNames766
NeuroLex IDbirnlex_2643
TAA14.1.04.202
FMA72602
Anatomical terms of neuroanatomy

Additional images

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Decussation of pyramids.

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Scheme showing the course of the fibers of the lemniscus; medial lemniscus in blue, lateral in red.

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Transverse section passing through the sensory decussation.

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Deep dissection of cortex and brain-stem.

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The sensory tract.

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Fourth ventricle. Posterior view. Deep dissection.

External links

Anterior external arcuate fibers

The anterior external arcuate fibers (ventral external arcuate fibers) vary as to their prominence: in some cases they form an almost continuous layer covering the medullary pyramids and olivary body, while in other cases they are barely visible on the surface.

Most of them reach the surface by way of the anterior median fissure, and arch backward over the pyramid.

Reinforced by others which emerge between the pyramid and olive, they pass backward over the olive and lateral district of the medulla oblongata, and enter the inferior peduncle.

As the fibers arch across the pyramid, they enclose a small nucleus which lies in front of and medial to the pyramid.

This is named the arcuate nucleus, and is serially continuous above with the pontine nuclei in the pons; it contains small fusiform (spindle-shaped) cells, around which some of the arcuate fibers end, and from which others arise.

Cervicoaxillary canal

The Cervicoaxillary canal is the passageway that extends between the neck and the upper extremities through which the long thoracic nerve and other structures pass.Its structure is defined by being posteriorly bordered by the scapula, anteriorly by the clavicle, and medially by the first rib. The long thoracic nerve traverses this passageway in addition to axillary blood vessels and the brachial plexus. This nerve arises in the neck from the fifth, sixth, and seventh cervical roots, C5, C6, and C7. It then enters the canal in the axilla.

Clava

Clava can refer to:

Mu Boötis, a triple star system in the constellation Boötes

Clava cairn, a type of Bronze Age circular chamber tomb cairn

Gracile nucleus, an area of the brain that participate in the sensation of fine touch and proprioception of the lower body

The club-like segments at the end of some insect antennae

Clava (hydrozoa), a genus of marine hydroid

Corticobulbar tract

The corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata (also called "bulbar") region, and are primarily involved in carrying the motor function of the non-oculomotor cranial nerves. The corticobulbar tract is one of the pyramidal tracts, the other being the corticospinal tract.

Cuneate fasciculus

The cuneate fasciculus, fasciculus cuneatus, cuneate tract, (tract of Burdach, named for Karl Friedrich Burdach) is a tract of nerves in the dorsal column of the spinal cord that primarily transmits information from the upper part of the body (the neck, trunk, and arms). It is part of the dorsal column-medial lemniscus pathway.

Cuneate nucleus

One of the dorsal column nuclei, the cuneate nucleus is a wedge-shaped nucleus in the closed part of the medulla oblongata, in the brainstem. It contains cells that give rise to the cuneate tubercle, visible on the posterior aspect of the medulla. It lies laterally to the gracile nucleus and medial to the spinal trigeminal nucleus in the medulla.

Dorsal column nuclei

In neuroanatomy, the dorsal column nuclei are a pair of nuclei in the dorsal columns in the brainstem. The name refers collectively to the cuneate nucleus and gracile nucleus, which are present at the junction between the spinal cord and the medulla oblongata. Both nuclei contain second-order neurons of the dorsal column-medial lemniscus pathway, which carries fine touch and proprioceptive information from the body to the brain.

The gracile nucleus is medial to the cuneate nucleus; its neurons receive afferent input from dorsal root ganglion sensory neurons subserving the lower trunk and limbs, while neurons of the cuneate nucleus receive connections from dorsal root neurons innervating the upper body. Neurons of the dorsal column nuclei send axons that form the internal arcuate fibers, crossing over at the sensory decussation to form the medial lemniscus, ultimately synapsing with third-order neurons of the thalamus.

Because each nucleus contains a large population of neurons, the dorsal column nuclei give rise to characteristic bumps or tubercles when viewing the posterior side of the intact brainstem. The cuneate nucleus gives rise to the cuneate tubercle, while the gracile nucleus gives rise to the gracile tubercle.

Dorsal column–medial lemniscus pathway

The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway (PCML)) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord, to the medulla where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

There are three groupings of neurons that are involved in the pathway: first-order neurons, second-order neurons, and third-order neurons. The first-order neurons are sensory neurons located in the dorsal root ganglia, that send their afferent fibers through the two dorsal columns – the gracile fasciculus, or gracile tract, and the cuneate fasciculus or cuneate tract. The first-order axons make contact with second-order neurons at the gracile nucleus and the cuneate nucleus in the lower medulla. The second-order neurons send their axons to the thalamus. The third-order neurons are in the ventral nuclear group in the thalamus and fibres from these ascend to the postcentral gyrus.

Sensory information from the upper half of the body is received at the cervical level of the spinal cord and carried in the cuneate tract, and information from the lower body is received at the lumbar level and carried in the gracile tract. The gracile tract is medial to the more lateral cuneate tract.

The axons of second-order neurons of the gracile and cuneate nuclei are known as the internal arcuate fibers and when they cross over the midline, at the sensory decussation in the medulla, they form the medial lemniscus. All of the axons in the DCML pathway are rapidly conducting, large, myelinated, alpha delta fibers.

Gracile fasciculus

The gracile fasciculus (fasciculus gracilis, tract of Goll or gracile tract) is a nerve tract a bundle of nerve fibers) in the dorsal column-medial lemniscus pathway of the spinal cord and carries information from the lower parts of the body. The gracile fasiculus is one of many ascending tracts which carry received sensory information to the brain via the spinal cord. It is also one of the dorsal columns, the other being the cuneate fasciculus.

Gracility

Gracility is slenderness, the condition of being gracile, which means slender.

It derives from the Latin adjective gracilis (masculine or feminine), or gracile (neuter) which in either form means slender, and when transferred for example to discourse, takes the sense of "without ornament", "simple", or various similar connotations.In his famous "Glossary of Botanic Terms", B. D. Jackson speaks dismissively of an entry in earlier dictionary of A. A. Crozier as follows: Gracilis (Lat.), slender. Crozier has the needless word "gracile". However, his objection would be hard to sustain in current usage; apart from the fact that "gracile" is a natural and convenient term, it is hardly a neologism; the Shorter Oxford English Dictionary gives the source date for that usage as 1623.

In the same entry for Gracile, the Shorter Oxford English Dictionary remarks: Recently misused (through association with grace) for Gracefully slender. This misuse is unfortunate at least, because the terms gracile and grace are completely unrelated: the etymological root of grace is the Latin word gratia from gratus, meaning pleasing and nothing to do with slenderness or thinness.

Internal arcuate fibers

The internal arcuate fibers are the axons of second-order sensory neurons that compose the gracile and cuneate nuclei of the medulla oblongata. These second-order neurons begin in the posterior grey column in the spinal cord. They receive input from first-order sensory neurons, which provide sensation to many areas of the body and have cell bodies in the dorsal root ganglia of the dorsal root of the spinal nerves. Upon decussation (crossing over) from one side of the medulla to the other, also known as the sensory decussation, they are then called the medial lemniscus.

The internal arcuate fibers are part of the second-order neurons of the posterior column-medial lemniscus system, and are important for relaying the sensation of fine touch and proprioception to the thalamus and ultimately to the cerebral cortex.

List of regions in the human brain

The human brain anatomical regions are ordered following standard neuroanatomy hierarchies. Functional, connective, and developmental regions are listed in parentheses where appropriate.

Medulla oblongata

The medulla oblongata (or medulla) is a long stem-like structure located in the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers and therefore deals with the autonomic functions of breathing, heart rate and blood pressure.

During embryonic development the medulla oblongata develops from the myelencephalon. The myelencephalon is a secondary vesicle which forms during the maturation of the rhombencephalon, also referred to as the hindbrain.

The bulb is an archaic term for the medulla oblongata and in modern clinical usage the word bulbar (as in bulbar palsy) is retained for terms that relate to the medulla oblongata, particularly in reference to medical conditions. The word bulbar can refer to the nerves and tracts connected to the medulla, and also by association to those muscles innervated, such as those of the tongue, pharynx and larynx.

Posterior external arcuate fibers

The posterior external arcuate fibers (dorsal external arcuate fibers) take origin in the accessory cuneate nucleus; they pass to the inferior peduncle of the same side.

It carries proprioceptive information from the upper limbs and neck. It is an analogue to the dorsal spinocerebellar tract for the upper limbs. In this context, the "cuneo-" derives from the accessory cuneate nucleus, not the cuneate nucleus. (The two nuclei are related in space, but not in function.)

The term "cuneocerebellar tract" is sometimes used to collectively refer to the posterior external arcuate fibers.The term "cuneocerebellar tract" is also used to describe an exteroceptive and proprioceptive components that take origin in the gracile and cuneate nuclei; they pass to the inferior peduncle of the same side.It is uncertain whether fibers are continued directly from the gracile and cuneate fasciculi into the inferior peduncle.

Posterior spinal artery

The posterior spinal artery (dorsal spinal arteries) arises from the vertebral artery in 25% of humans or the posterior inferior cerebellar artery in 75% of humans, adjacent to the medulla oblongata. It supplies the grey and white posterior columns of the spinal cord.

Pronator quadratus muscle

Pronator quadratus is a square shaped muscle on the distal forearm that acts to pronate (turn so the palm faces downwards) the hand.

As it is on the anterior side of the arm, it is innervated by a branch of the median nerve, the anterior interosseous nerve (roots C8 and T1 with T1 being primary). Arterial blood comes via the interosseous artery.

Sensory decussation

The sensory decussation or decussation of the lemniscus is a decussation or crossover of axons from the gracile nucleus and cuneate nucleus, which are responsible for fine touch, proprioception and two-point discrimination of the body. The fibres of this decussation are called the internal arcuate fibres and are found at the superior aspect of the closed medulla superior to the motor decussation. It is part of the second neuron in the posterior column–medial lemniscus pathway.

Taenia of fourth ventricle

In the brain, the taenia of the fourth ventricle (lingula, tenia of fourth ventricle) are two narrow bands of white matter, one on either side, which complete the lower part of the roof of the fourth ventricle.

Each consists of a vertical and a horizontal part.

The vertical part is continuous below the obex with the gracile nucleus, to which it is adherent by its lateral border.

The horizontal portion extends transversely across the inferior peduncle, below the striæ medullares, and roofs in the lower and posterior part of the lateral recess; it is attached by its lower margin to the inferior peduncle, and partly encloses the choroid plexus, which, however, projects beyond it like a cluster of grapes; and hence this part of the tænia has been termed the cornucopia.

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