Posterior column-medial lemniscus pathway
Encyclopedia
The posterior column-medial lemniscus pathway (dorsal column-medial lemniscus pathway, dorsal white column-medial lemniscus system) is the sensory
pathway responsible for transmitting fine touch, vibration
and conscious proprioceptive
information from the body to the cerebral cortex
; as well as tactile pressure, barognosis
, graphesthesia
, stereognosis
, recognition of texture, kinesthesia and two-point discrimination
.
The name comes from the two structures that the sensation travels up: the posterior (or dorsal) columns of the spinal cord
, and the medial lemniscus
in the brainstem. Because the posterior columns are also called dorsal columns, the pathway is often called the dorsal column-medial lemniscus system, or DCML for short. (Also called posterior column-medial lemniscus or PCML pathway).
The PCML is pathway is composed of rapidly conducting, large, myelinated fibers
s, and allows us to feel fine textures and determine what an unknown object in our hands is without looking at it (stereognosis
).
s that lie in the dermis
of the skin
close to the epidermis. When these structures are stimulated by slight pressure, an action potential
is started. Alternatively, proprioceptive muscle spindles and other skin surface touch receptors such as merkel cells, Ruffini endings, Pacinian corpuscles, and hair follicle receptors (Peritrichial endings) may involve the first neuron in this pathway.
The action potential travels up an axon
(the cell body of the neuron
will be in a dorsal root ganglion
). (The neurons are classified as pseudo-unipolar, so they are regarded as having just one long process, which includes both a peripheral branch dendrite and central branch axon.) So the sensation travels from the skin, along the axon, past the neuronal cell body, and into the dorsal column of the spinal cord.
The axons continue inside the spinal cord, running up the posterior (dorsal) column. Axons from the lower body are most medial (closer to the midline), and run in the gracile tract of the spinal column. Sensory axons from the upper body enter the spinal cord later, specifically from T6 on up, so are more lateral and travel up the cuneate tract.
At the level of the closed medulla oblongata
, these axons synapse
with neurons in the gracile nucleus
and cuneate nucleus
.
. This crossing over is commonly referred to as the sensory decussation
.
At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in the posterior columns. For example, in the columns, lower limb
is medial, upper limb
is more lateral. At the medial leminiscus, axons from the leg are more ventral, arm fibres more dorsal. Fibres from the trigeminal nerve
(supplying the head) come in dorsal to the arm fibres, and travel up the lemniscus too.
The medial lemniscus rotates 90 degrees at the pons
. The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards.
The axons travel up the rest of the brainstem, and synapse at the thalamus
(at the ventral posterolateral nucleus
for sensation from the neck, trunk, and extremities, and at the ventral posteromedial nucleus
for sensation from the head).
, and again head and leg swap relative positions. The axons synapse in the primary sensory cortex, with lower body sensation most medial (e.g., the paracentral lobule) and upper body more lateral.
. Lesions to the posterior column-medial lemniscus pathway below the decussation of its fibers produce loss of sensation on the same side of the body as the lesion. Above the decussation produces loss of sensation on the opposite side of the body than the lesion.
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...
pathway responsible for transmitting fine touch, vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
and conscious proprioceptive
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
information from the body to the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
; as well as tactile pressure, barognosis
Barognosis
Barognosis, or baresthesia, is the ability of evaluating the weight of objects, or to differentiate objects of different weights by looking at them. It is the opposite of baragnosis, the inability of evaluating the weight of objects...
, graphesthesia
Graphesthesia
Graphesthesia is the ability to recognize writing on the skin purely by the sensation of touch. Its name derives from Greek graphē and aisthēsis...
, stereognosis
Stereognosis
Stereognosis is the ability to perceive and recognize the form of an object using cues from texture, size, spatial properties, and temperature...
, recognition of texture, kinesthesia and two-point discrimination
Two-point discrimination
Two-point discrimination is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination and reflects how finely innervated an area of skin is....
.
The name comes from the two structures that the sensation travels up: the posterior (or dorsal) columns of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, and the medial lemniscus
Medial lemniscus
The medial lemniscus, also known as Reil's band or Reil's ribbon, is a pathway in the brainstem that carries sensory information from the gracile and cuneate nuclei to the thalamus.-Path:...
in the brainstem. Because the posterior columns are also called dorsal columns, the pathway is often called the dorsal column-medial lemniscus system, or DCML for short. (Also called posterior column-medial lemniscus or PCML pathway).
The PCML is pathway is composed of rapidly conducting, large, myelinated fibers
Journey
Discriminative sensation is well developed in the fingers of humanHuman
Humans are the only living species in the Homo genus...
s, and allows us to feel fine textures and determine what an unknown object in our hands is without looking at it (stereognosis
Stereognosis
Stereognosis is the ability to perceive and recognize the form of an object using cues from texture, size, spatial properties, and temperature...
).
First neuron
This fine sensation is detected by Meissner's corpuscleMeissner's corpuscle
Meissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
s that lie in the dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
of the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
close to the epidermis. When these structures are stimulated by slight pressure, an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
is started. Alternatively, proprioceptive muscle spindles and other skin surface touch receptors such as merkel cells, Ruffini endings, Pacinian corpuscles, and hair follicle receptors (Peritrichial endings) may involve the first neuron in this pathway.
The action potential travels up an axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
(the cell body of the neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
will be in a dorsal root ganglion
Dorsal root ganglion
In anatomy and neuroscience, a dorsal root ganglion is a nodule on a dorsal root that contains cell bodies of neurons in afferent spinal nerves.-Unique unipolar structure:...
). (The neurons are classified as pseudo-unipolar, so they are regarded as having just one long process, which includes both a peripheral branch dendrite and central branch axon.) So the sensation travels from the skin, along the axon, past the neuronal cell body, and into the dorsal column of the spinal cord.
The axons continue inside the spinal cord, running up the posterior (dorsal) column. Axons from the lower body are most medial (closer to the midline), and run in the gracile tract of the spinal column. Sensory axons from the upper body enter the spinal cord later, specifically from T6 on up, so are more lateral and travel up the cuneate tract.
At the level of the closed medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
, these axons synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
with neurons in the gracile nucleus
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...
and cuneate nucleus
Cuneate nucleus
One of the dorsal column nuclei, the cuneate nucleus is a wedge-shaped nucleus in the closed part of the medulla oblongata. It contains cells that give rise to the cuneate tubercle, visible on the posterior aspect of the medulla...
.
Second neuron
The secondary neurons (that start in the nuclei) cross over to the other side of the medulla (as internal arcuate fibres) to form the medial lemniscusMedial lemniscus
The medial lemniscus, also known as Reil's band or Reil's ribbon, is a pathway in the brainstem that carries sensory information from the gracile and cuneate nuclei to the thalamus.-Path:...
. This crossing over is commonly referred to as the sensory decussation
Sensory decussation
The sensory decussation or decussation of the lemniscus is a decussation or cross over of axons from the gracile nucleus and cuneate nucleus. 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...
.
At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in the posterior columns. For example, in the columns, lower limb
Lower limb
The lower limb is a limb of the body.According to Terminologia Anatomica, it includes the pelvic girdle, buttocks, hip, and thigh, as well as the components distal to the knee.-References:...
is medial, upper limb
Upper limb
The upper limb or upper extremity is the region in an animal extending from the deltoid region to the hand, including the arm, axilla and shoulder.-Definition:...
is more lateral. At the medial leminiscus, axons from the leg are more ventral, arm fibres more dorsal. Fibres from the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...
(supplying the head) come in dorsal to the arm fibres, and travel up the lemniscus too.
The medial lemniscus rotates 90 degrees at the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
. The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards.
The axons travel up the rest of the brainstem, and synapse at the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
(at the ventral posterolateral nucleus
Ventral posterolateral nucleus
The ventral posterolateral nucleus is a nucleus of the thalamus. Together with the ventral posteromedial nucleus , VPI and VMpo, it constitutes the ventral posterior nucleus.-Input and output:...
for sensation from the neck, trunk, and extremities, and at the ventral posteromedial nucleus
Ventral posteromedial nucleus
-Inputs and outputs:It conveys information of the trigeminothalamic tract, from the solitary tract and the trigeminal nerve and projects to the postcentral gyrus....
for sensation from the head).
Third neuron
Neurons starting in the thalamus travel up the posterior limb of the internal capsuleInternal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
, and again head and leg swap relative positions. The axons synapse in the primary sensory cortex, with lower body sensation most medial (e.g., the paracentral lobule) and upper body more lateral.
Test
The pathway is tested with the Romberg's testRomberg's test
Romberg's test or the Romberg maneuver is a test used by doctors in a neurological examination, and also as a test for drunken driving. The exam is based on the premise that a person requires at least two of the three following senses to maintain balanced while standing:Proprioception ; Vestibular...
. Lesions to the posterior column-medial lemniscus pathway below the decussation of its fibers produce loss of sensation on the same side of the body as the lesion. Above the decussation produces loss of sensation on the opposite side of the body than the lesion.
External links
- http://www.ualberta.ca/~neuro/OnlineIntro/SysSensorySomatic.htm#Paths