It has been suggested that Anterior grey column be merged into this article. (Discuss) Proposed since July 2024. |
It has been suggested that Posterior grey column be merged into this article. (Discuss) Proposed since July 2024. |
Grey columns | |
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Details | |
Identifiers | |
Latin | columnae griseae |
TA98 | A14.1.02.101 |
TA2 | 6063 |
FMA | 77867 |
Anatomical terminology |
The grey columns are three regions of the somewhat ridge-shaped mass of grey matter in the spinal cord. [1] These regions present as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which are visible in cross-section of the spinal cord.
The anterior grey column is made up of alpha motor neurons, gamma motor neurons, and small neurons thought to be interneurons. [2] The posterior grey column is divided into several of the Rexed laminae. [3] The lateral grey column is only present in the thoracic region and upper lumbar segments (T1-L2). The lateral grey column contains preganglionic cell bodies of the autonomic nervous system and sensory relay neurons.
The anterior grey column, (also known as the anterior horn of spinal cord and anterior cornu) comprises three different types of neurons, two types of lower motor neuron – large alpha motor neurons, and medium gamma motor neurons, and small neurons thought to be interneurons. [2] These neurons differ in both their morphology and in their patterns of connectivity. [4] They are organized in the same manner as the muscles they innervate. [5]
Alpha motor neurons are lower motor neurons that innervate extrafusal muscle fibers to generate force at neuromuscular junctions at the start of a muscle contraction. They have large cell bodies and receive proprioceptive input. [4] 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 with ALS. [6] [7]
Gamma motor neurons innervate intrafusal muscle fibers that control the sensitivity of muscle spindles to stretch. They have smaller cell bodies than alpha motor neurons and do not receive proprioceptive input. [4] 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 both excitatory and inhibitory. They are suspected to be interneurons and have been shown to reduce in size but not numbers with age. [2]
The posterior grey column, also known as the posterior (or dorsal) horn of spinal cord, is divided into several laminae, based on the type of sensory information sent to each section. [3] Laminae I and II are sent information from afferent 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. [8] It is known to be the primary relay point for haptic and nociceptive messages. [9] 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. [10]
Lamina I is also known as the marginal nucleus of spinal cord. The majority of posterior column projection neurons are located in lamina I, however most neurons in this layer are interneurons. [11] The main areas these neurons innervate are the caudal ventrolateral medulla (CVLM), the nucleus of the solitary tract (NTS), the lateral parabrachial area (LPb), the periaqueductal grey matter (PAG), and certain regions in the thalamus. [9] The CVLM receives nociceptive and cardiovascular responses. [12] The NTS receives cardio-respiratory inputs and affects reflex tachycardia from noxious stimulation. [13] The LPb projects to the amygdala and hypothalamus and is involved in the emotional response to pain. [14] The PAG develops ways to deal with pain and is a main target of analgesics. It projects to other parts of the brainstem. [15] The nuclei of the thalamus affect sensory and motivational aspects of pain. [16] The neurons of this lamina can be distinguished by their morphology as pyramidal, spindle, or multipolar. [17]
This layer is also known as the substantia gelatinosa of Rolando and has the highest density of neurons. [18] These neurons mediate the activity of nociceptive and temperature afferent fibers. [5] 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 release glutamate as their main neurotransmitter and the inhibitory interneurons use GABA and/or glycine as their main neurotransmitter. The neurons of this layer are only C fibers and contain almost no myelin. [19]
These laminae are also known as the nucleus proprius and contain a much smaller density of neurons than lamina II. [18] There are projection neurons scattered throughout these layers. [11] Mechanosensitive A beta fibers terminate in these layers. [10] The layers receive input from lamina II and also control pain, temperature, and crude touch. [5] C fibers that control nociception and temperature and sensory information from mechanoreceptors are relayed here. [20]
This lamina is also known as the neck of the posterior column and receives information from mechanoreceptors and danger information from nociceptors. [20] 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. [18]
This lamina is only found in the cervical and lumbar regions of the spinal cord. It receives afferent input from muscle fibers and joints. [5]
The lateral grey column, or the lateral horn of spinal cord, is part of the sympathetic nervous system and receives input from brain stem, organs, and hypothalamus. 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 by amyotrophic 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. [21]
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. [22]
Damage to the lateral column can result in Horner'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%.
The posterior column has a prominent role in the pain system, it is the first central relay in the nociceptive pathway. The first-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 the thalamus. The thalamus is known as the "gateway to the cortex". The third order neuron then goes to the cerebral 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 and A delta fibers which are slower and thinner than the A beta fibers. The C fibers are not myelinated and therefore slower. [11] C fibers that carry nociceptive signals can be divided into two types: fibers that contain neuropeptides, like substance P, and fibers that do not contain neuropeptides. [23] The two types terminate in very different areas. Non-peptidergic C fibers are linked to the skin, where they innervate the epidermis while peptidergic C fibers innervate other tissues and deeper parts of the skin. [11]
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 small receptive field to help pinpoint the exact location of a stimulus. [24]
Affective nociceptive signals affect emotions. These signals go to the limbic 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. [24]
In physiology, nociception, also nocioception; from Latin nocere 'to harm/hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, convert it to a molecular signal, and recognize and characterize the signal to trigger an appropriate defensive response.
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of 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 as well as the sleep–wake cycle. "Medulla" is from Latin, ‘pith or marrow’. And "oblongata" is from Latin, ‘lengthened or longish or elongated'.
Afferent nerve fibers are axons of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections arrive at a particular brain region.
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.
A nociceptor is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.
The spinothalamic tract is a nerve tract in the anterolateral system in the spinal cord. This tract is an ascending sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.
The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.
A dorsal root ganglion is a cluster of neurons in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal root ganglia.
The posterior grey column is one of the three grey columns of the spinal cord. It is a pronounced, dorsolaterally-oriented ridge of gray matter in either lateral half of the spinal cord. When viewed in transverse section, it is termed the posterior horn or dorsal horn.
The spinocerebellar tract is a nerve tract originating in the spinal cord and terminating in the same side (ipsilateral) of the cerebellum.
The nucleus raphe magnus is one of the seven raphe nuclei. It is situated in the pons in the brainstem, just rostral to the nucleus raphe obscurus.
The apex of the posterior grey column, one of the three grey columns of the spinal cord, is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando, which contains both neuroglia cells, and small neurons. The gelatinous appearance is due to an abundance of neuropil with a very low concentration of myelinated fibers. It extends the entire length of the spinal cord and into the medulla oblongata where it becomes the spinal trigeminal nucleus.
The Rexed laminae comprise a system of ten layers of grey matter (I–X), identified in the early 1950s by Bror Rexed to label portions of the grey columns of the spinal cord.
Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
The spinoreticular tract is a partially decussating (crossed-over) four-neuron sensory pathway of the central nervous system. The tract transmits slow nociceptive/pain information from the spinal cord to reticular formation which in turn relays the information to the thalamus via reticulothalamic fibers as well as to other parts of the brain. Most (85%) second-order axons arising from sensory C first-order fibers ascend in the spinoreticular tract - it is consequently responsible for transmiting "slow", dull, poorly-localised pain. By projecting to the reticular activating system (RAS), the tract also mediates arousal/alertness in response to noxious stimuli. The tract is phylogenetically older than the spinothalamic ("neospinothalamic") tract.
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal cord is hollow and contains a structure called the central canal, which contains cerebrospinal fluid. The spinal cord is also covered by meninges and enclosed by the neural arches. Together, the brain and spinal cord make up the central nervous system.
Group A nerve fibers are one of the three classes of nerve fiber as generally classified by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated.
A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.
The raphespinal tract is an unmyelinated descending serotonergic tract involved in pain modulation. It is a descending pain-inhibiting pathway; it is a component of the reticulospinal tract.