Thegrey columnsare three regions of the somewhat ridge-shaped mass ofgrey matterin thespinal cord.[1]These regions present as three columns: theanterior grey column,theposterior grey column,and thelateral grey column,all of which are visible in cross-section of the spinal cord.
| Grey columns | |
|---|---|
 Cross section of the spinal cord. The three grey columns make up the butterfly-shaped shaded region | |
| Details | |
| Identifiers | |
| Latin | columnae griseae |
| TA98 | A14.1.02.101 |
| TA2 | 6063 |
| FMA | 77867 |
| Anatomical terminology | |
The anterior grey column is made up ofAlpha motor neurons,gamma motor neurons,and small neurons thought to beinterneurons.[2]It affects theskeletal muscles.
The posterior grey column receives several types of sensory information regarding touch and sensation from receptors in the skin, bones, and joints, includingfine touch,proprioception,andvibration.[citation needed]It contains the cell bodies ofsecond-order sensory neuronsand their synapses with thepseudounipolarfirst-order sensory neurons(whose cell bodies are located within thesensory ganglia (a.k.a. dorsal root ganglia)).
The lateral grey column is only present in the thoracic region and upper lumbar segments (T1-L2). It containspreganglionic cell bodiesof theautonomic nervous systemand sensory relay neurons.
Structure
editAnterior grey column
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Theanterior grey column,(also known as the anterior horn of spinal cord and anterior cornu) is broad and of a rounded or quadrangular shape. Its posterior part is termed the base, and its anterior part the head, but these are not differentiated from each other by any well-defined constriction. It is separated from the surface of the spinal cord by a layer of white substance which is traversed by the bundles of the anterior nerve roots. In the thoracic region, the posterolateral part of the anterior column projects laterally as a triangular field, which is named thelateral grey column.It comprises three different types of neurons, two types oflower motor neuron– largeAlpha motor neurons,and mediumgamma motor neurons,and small neurons thought to beinterneurons.[2]These neurons differ in both theirmorphologyand in their patterns of connectivity.[3]They are organized in the same manner as the muscles they innervate.[4]
Alpha motor neurons
editAlpha motor neurons arelower motor neuronsthat innervateextrafusal muscle fibersto generate force atneuromuscular junctionsat the start of amuscle contraction.They have large cell bodies and receiveproprioceptiveinput.[3]They have been shown to reduce in population, but not in size with age.[2]Damage to these cell bodies can lead to severe muscle weakness and loss of reflexes, and is also associated withALS.[5][6]
Gamma motor neurons
editGamma motor neurons innervateintrafusal muscle fibersthat control the sensitivity ofmuscle spindlesto stretch. They have smaller cell bodies than Alpha motor neurons and do not receive proprioceptive input.[3]They have been shown to reduce in numbers but not size with age.[2]
Small neurons
editThe physiology of the small neurons in the anterior column is not well understood. Their effects can be bothexcitatoryandinhibitory.They are suspected to be interneurons and have been shown to reduce in size but not numbers with age.[2]
Clinical significance
It is these cells that are affected in the following diseases,[citation needed]–amyotrophic lateral sclerosis,spinal and bulbar muscular atrophy,Charcot–Marie–Tooth disease,progressive muscular atrophy,allspinal muscular atrophies,poliomyelitis,andWest Nile virus.
Pharmacological interaction
The anterior grey column is the target for somespasmolyticmedications.Norepinephrinerelease here, (as induced bycyclobenzaprine) reduces spasms by innervation (reducing nerve activity) ofAlpha motor neuronsvia interaction withgamma fibers.[7]
Posterior grey column
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Theposterior grey column,also known as the posterior (or dorsal) horn of spinal cord, is subdivided into six layers known asRexed laminae,based on the type of sensory information sent to each section.[8]
- Marginal nucleus of spinal cord(lamina I)
- Substantia gelatinosa of Rolando(lamina II)
- Nucleus proprius(laminae III, IV)
- Spinal lamina V,the neck of the posterior horn[9]
- Spinal lamina VI,the base of the posterior horn.
The other four laminae are located in the other two grey columns in the spinal cord.
The function of the spinal dorsal horn is to process and integrate sensory information from theperipheral nervous system.It receives inputs fromprimary afferent fibersand modulatory systems, and it projects to higher brain centers andmotor neurons.The dorsal horn circuitry is involved in various aspects of sensory processing, including discrimination, integration, and modulation ofnociceptiveand non-nociceptive signals. Dysfunction of the dorsal horn circuitry has been implicated in chronic pain conditions and other neurological disorders.[10]
Laminae I and II receive information fromafferent neuronsthat sense nociception, temperature, and itching, laminae III and IV are sent information from neurons that sense mechanical pressure, and laminae V and VI are sent information from proprioceptors.[11]It is known to be the primary relay point forhapticandnociceptivemessages.[12]The posterior horn is also known as a partially layered structure because only laminae I and II are well defined.
The column can also be separated by nociceptive and non-nociceptive senses. Laminae I and II are important in nociception, laminae III and IV are not involved nociception, and lamina V is involved in both nociception and non-nociception.[13]
The function of the spinal dorsal horn is to process and integrate sensory information from theperipheral nervous system.It receives inputs fromprimary afferent fibersand modulatory systems, and it projects to higher brain centers andmotor neurons.The dorsal horn circuitry is involved in various aspects of sensory processing, including discrimination, integration, and modulation ofnociceptiveand non-nociceptive signals. Dysfunction of the dorsal horn circuitry has been implicated in chronic pain conditions and other neurological disorders.
Lamina I
editLamina I is also known as themarginal nucleus of spinal cord.The majority of posterior column projection neurons are located in lamina I, however most neurons in this layer are interneurons.[14]The main areas these neurons innervate are thecaudal ventrolateral medulla(CVLM), thenucleus of the solitary tract(NTS), thelateral parabrachial area(LPb), theperiaqueductal grey matter(PAG), and certain regions in thethalamus.[12]The CVLM receives nociceptive andcardiovascularresponses.[15]The NTS receives cardio-respiratory inputs and affectsreflex tachycardiafrom noxious stimulation.[16]The LPb projects to theamygdalaandhypothalamusand is involved in the emotional response to pain.[17]The PAG develops ways to deal with pain and is a main target ofanalgesics.It projects to other parts of the brainstem.[18]The nuclei of the thalamus affect sensory and motivational aspects of pain.[19]The neurons of this lamina can be distinguished by their morphology aspyramidal,spindle,ormultipolar.[20]
Lamina II
editThis layer is also known as thesubstantia gelatinosa of Rolandoand has the highest density of neurons.[21]These neurons mediate the activity of nociceptive and temperature afferent fibers.[4]It is almost entirely made up of interneurons which can be further divided by their morphology. The four main morphological classes, based on the shape of their dendritic structure, are islet, central, vertical, and radial cells. The interneurons can also be divided by their function: excitatory or inhibitory. The excitatory interneurons releaseglutamateas their mainneurotransmitterand the inhibitory interneurons useGABAand/orglycineas their main neurotransmitter. The neurons of this layer are onlyC fibersand contain almost nomyelin.[22]
Laminae III and IV
editThese laminae are also known as thenucleus propriusand contain a much smaller density of neurons than lamina II.[21]There are projection neurons scattered throughout these layers.[14]MechanosensitiveA beta fibersterminate in these layers.[13]The layers receive input from lamina II and also control pain, temperature, and crude touch.[4]C fibers that control nociception and temperature and sensory information from mechanoreceptors are relayed here.[23]
Lamina V
editThis lamina is also known as the neck of the posterior column and receives information from mechanoreceptors and danger information from nociceptors.[23]It has different neurons in different regions. In the medial region it contains medium-sized triangular neurons and the lateral region contains medium-sized multipolar neurons.[21]
Lamina VI
editThis lamina is only found in thecervicalandlumbarregions of the spinal cord. It receives afferent input from muscle fibers and joints.[4]
Lateral grey column
editThelateral grey column,or the lateral horn of spinal cord, is part of thesympathetic nervous systemand receives input frombrain stem,organs, andhypothalamus.The lateral column is only present in the thoracic region and upper lumbar segments. The lateral grey column contains preganglionic cell bodies of the autonomic nervous system and sensory relay neurons.
Clinical significance
editNeurons in the anterior column have been shown to be affected byamyotrophic lateral sclerosis(ALS). The number of large Alpha motor neurons and medium gamma motor neurons was greatly reduced and the number of small neurons was either slightly or greatly reduced depending on the type of ALS.[24]
Muscular atrophyhas also been shown to have an effect on neurons of the anterior column. A large loss of large Alpha motor neurons, medium gamma motor neurons, and small neurons was recorded in cases of muscular atrophy.[25]
Damage to thelateral columncan result inHorner's syndrome.
Multiple system atrophy(MSA), has also been linked to the lateral grey column. MSA has been shown to reduce the cell count in the lateral column by over 50%.
Theposterior columnhas a prominent role in thepain system,it is the first central relay in the nociceptive pathway. Thefirst-order afferent neuroncarries sensory information to the second order neuron in the dorsal horn. The axon of the second order neuron, if it is a projection neuron and not an interneuron, then goes to the third order neuron in thethalamus.The thalamus is known as the "gateway to the cortex". The third order neuron then goes to thecerebral cortex.The afferent neurons are either A fibers or C fibers. A fibers are myelinated allowing for faster signal conduction. Among these there are A beta fibers which are faster and carry information about non-painful touch andA delta fiberswhich are slower and thinner than the A beta fibers. The C fibers are not myelinated and therefore slower.[14]C fibers that carry nociceptive signals can be divided into two types: fibers that containneuropeptides,likesubstance P,and fibers that do not contain neuropeptides.[26]The two types terminate in very different areas. Non-peptidergic C fibers are linked to the skin, where they innervate theepidermiswhile peptidergic C fibers innervate other tissues and deeper parts of the skin.[14]
There are two main types of nociceptive signals: sensory and affective.
Sensory
editSensory nociceptive signals provide information about what kind of stimulus (heat, mechanical, etc.) is affecting the body and also indicates where on the body the stimulus is. Sensory nociceptive neurons have a smallreceptive fieldto help pinpoint the exact location of a stimulus.[27]
Affective
editAffective nociceptive signals affect emotions. These signals go to thelimbic systemand tell the body to react to the danger stimulus (i.e. removing a hand from a hot stove). These neurons have larger receptive fields because the emotional reaction to most pain stimuli is similar.[27]
See also
edit- Beta motor neuron
- Gamma motor neuron
- Posterior column-medial lemniscus pathway
- Posterior horn of lateral ventricles
- Type Ia sensory fiber
References
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