| 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 | |
Thegrey columns are three regions of the somewhat ridge-shaped mass ofgrey matter in 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.
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 neurons and 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 bodies of theautonomic nervous system and sensory relay neurons.
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 theirmorphology and in their patterns of connectivity.[3] They are organized in the same manner as the muscles they innervate.[4]
Alpha motor neurons arelower motor neurons that innervateextrafusal muscle fibers to generate force atneuromuscular junctions at the start of amuscle contraction. They have large cell bodies and receiveproprioceptive input.[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 innervateintrafusal muscle fibers that control the sensitivity ofmuscle spindles to 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]
The physiology of the small neurons in the anterior column is not well understood. Their effects can be bothexcitatory andinhibitory. 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 somespasmolytic medications.Norepinephrine release here, (as induced bycyclobenzaprine) reduces spasms by innervation (reducing nerve activity) ofalpha motor neurons via interaction withgamma fibers.[7]
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]
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 fibers and 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 ofnociceptive and 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 neurons that 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 forhaptic andnociceptive messages.[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 fibers and 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 ofnociceptive and non-nociceptive signals. Dysfunction of the dorsal horn circuitry has been implicated in chronic pain conditions and other neurological disorders.
Lamina 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 andcardiovascular responses.[15] The NTS receives cardio-respiratory inputs and affectsreflex tachycardia from noxious stimulation.[16] The LPb projects to theamygdala andhypothalamus and 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]
This layer is also known as thesubstantia gelatinosa of Rolando and 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 releaseglutamate as their mainneurotransmitter and the inhibitory interneurons useGABA and/orglycine as their main neurotransmitter. The neurons of this layer are onlyC fibers and contain almost nomyelin.[22]
These laminae are also known as thenucleus proprius and contain a much smaller density of neurons than lamina II.[21] There are projection neurons scattered throughout these layers.[14] MechanosensitiveA beta fibers terminate 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]
This 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]
This lamina is only found in thecervical andlumbar regions of the spinal cord. It receives afferent input from muscle fibers and joints.[4]
Thelateral grey column, or the lateral horn of spinal cord, is part of thesympathetic nervous system and 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.
Neurons 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 atrophy has 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 column can 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 column has a prominent role in thepain system, it is the first central relay in the nociceptive pathway. Thefirst-order afferent neuron carries 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 fibers which 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 theepidermis while 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 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 field to help pinpoint the exact location of a stimulus.[27]
Affective nociceptive signals affect emotions. These signals go to thelimbic system and 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]
This article incorporates text in thepublic domain frompage 753 of the 20th edition ofGray's Anatomy(1918)
