The cellular laminae, also known as Rexed laminae, are the nine layers of gray matter that make up the spinal cord. These layers are organized in two main ways: by nuclear regions (we will discuss this briefly only as a way to identify laminar regions) and by the laminae themselves. In total, there are ten cellular laminae, each responsible for receiving and transmitting different information. Some of them are unique to certain vertebral levels whereas others extend over multiple. Here is a breakdown of the cellular laminae, the information they receive and transmit, and the spinal cord nuclei they correspond to.
Brief Anatomy of the Cellular Laminae
The cellular laminae were given the name “Rexed laminae” due to the discovery of Bror Rexed, who identified layers of grey matter in the spinal cord that were arranged by structure and function, instead of simple location. This discovery was made in the 1950s and is an alternative method of organizing regions of spinal cord neuronal activity to the six spinal cord nuclei (marginal zone, substantia gelatinosa, nucleus proprius, dorsal nucleus of Clarke, interomediolateral nucleus, lateral motor neurons, and medial motor neurons).
All of the primary sensory neurons that enter and inform the spinal cord originate in the ganglia that are located in the intervertebral foramina (openings of the vertebral column). (Remember that the term “ganglia” refers to collections of neuron cell bodies outside of the central nervous system, not completely inside like interneurons.)
The processes of these nerve cells as it relates to peripheral nervous system functions include the collection and distribution of sensory information from multiple receptors throughout the body; while central nervous system functions involve the bundles of nerve transmitting information up the spinal cord to the brain, thalamus, and cerebral cortex.
Fibers that convey specific types of sensation follow distinct pathways: Impulses involved with pain and noxious stimuli mostly terminate in Laminae I and II, while impulses that pick up tactile (touch) sensory information end in Laminae IV or on the processes of cells in Laminae IV. Signals of stretch receptors (i.e. muscle spindles, tendon organs) end in parts of laminae V, VI, and VII, and so on.
Almost all parts of the spinal cord’s gray matter contain interneurons responsible for forming neuronal connections between numerous groups of nerve cells. The axons of quite a few of these neurons are relatively short, which is not a huge problem, given that they are only distributed locally (in close anatomical proximity to the neuron cell body). Still, there are other interneuronal axons that extend over multiple segments of the spinal cord. Some interneurons are involved in the characterization of action potentials and stimuli responses, while others play key roles in the transmission of neuronal information and in reflexes.
Breakdown of the Functions of the Cellular Laminae
These laminae form the dorsal horns and receive sensory input of several different types. Do keep in mind that these layers are what make up the whole of the gray matter butterfly shape in the middle of the spinal cord, and are ordered dorsally to ventrally (back toward the spinous processes to the front toward the vertebral body).
Laminae I-VI comprise the entirety of the dorsal horn, where, alternatively, the spinal cord nuclei, marginal zone (MZ), substantia gelatinosa (SG), and nucleus proprius (NP) are located. Recall that sensory nerve fibers, transmitting information into the CNS and up to the brain, synapse onto interneurons in the dorsal gray horn.
Lamina I makes up the tip of the dorsal horn and is responsible for receiving noxious or thermal stimuli (noxious refers to stimuli that indicate an event that is either certainly or potentially damaging to tissues/organs. These stimuli can be mechanical, chemical, or thermal). Lamina I sends information to the brain via the lateral spinothalamic tract and corresponds to the spinal cord nucleus known as the marginal zone.
The lateral spinothalamic tract is one of four tracts comprising the entirety of the spinothalamic tract, also known as the ventrolateral or anterolateral system. The sensory tract as a whole transmits information from the skin to the thalamus.
Lamina II is involved in the reception of all sensory-related stimuli (both noxious and non-noxious), and specifically moderates the sensation of pain. This lamina maintains direct communication with Laminae III and IV and corresponds to the spinal cord nucleus known as the substantia gelatinosa.
Lamina III controls the sensations of proprioception (awareness of the positioning of body parts) and light touch (note that different pressures associated with touch involve different sensory receptors and nerve pathways). The cells of this lamina are connected with Laminae IV, V, and VI, and in part, corresponds to the spinal cord nucleus, nucleus proprius.
Lamina IV is involved in the reception and distribution of non-noxious (stimuli that do not pose or warn of a threat) sensory information and processing. Lamina IV maintains a direct connection to Lamina II, despite their non-continuous positions. This makes up the remainder of the correspondence of the nucleus proprius.
Lamina V relays sensory information – particularly nociceptive (meaning potentially painful) sensory signals – to the brain via the lateral spinothalamic tracts. This lamina also receives descending information from the brain via the corticospinal and rubrospinal tracts.
To give you an idea of descending neuronal information that is passed through Lamina V: the corticospinal tract carries motor commands from the brain to the periphery. Given the sensory information taken in by Lamina V, it can be deduced that motor commands may be those such as distancing yourself from a source of high heat (a noxious stimulus), for example.
The rubrospinal tract assists in motor functions, as it is believed to be involved in the activation of flexor muscles and the inhibition of extensor muscles.
Lamina VI is only present in the cervical and lumbosacral enlargements and contains many relatively small interneurons that are involved in the modulation of spinal reflexes and reception of sensory information from muscle spindles, specifically those that are involved with proprioception. It sends gathered information to the brain via the ipsilateral spinocerebellar pathways. This lamina does not correspond with any spinal nuclei.
Lamina VII, along with Lamina IX, form the middle region between the dorsal and ventral roots, medial to the position of the lateral gray horns.
This lamina is one example of a layer that changes configuration based on the level of the vertebral column observed. It receives information directly from Laminae II through VI, and from viscera (internal organs of the main cavities of the body, especially those in the abdomen such as the intestines/digestive tract). Lamina VII relays motor information back to the viscera and gives rise to cells that are involved in the function and regulation of the autonomic nervous system. This lamina partially corresponds to the dorsal nucleus of Clarke and the lateral and medial motor neurons.
Although Lamina IX corresponds to the middle region between the dorsal and ventral horns along with Lamina VII, it composes more of the ventral horn alongside Lamina VIII. In fact, when Laminae VIII and IX are observed in combination with Lamina VII, the three together comprise the entirety of the ventral gray horn. Somatic motor neurons originate in the ventral gray horn, and so give rise to the ability to voluntarily control body parts (for the most part).
Lamina VIII is another lamina that varies in configuration based on the level of the spinal cord in which it is occurring. It is most prominent in the cervical and lumbar vertebral levels. Lamina VIII is involved in the modulation of motor output to skeletal muscles and partially corresponds to the lateral and medial motor neurons.
Lamina IX is composed of clusters of large alpha motor neurons that innervate striated muscles, and small gamma motor neurons that innervate the elements of muscle spindles that are responsible for contraction. (The axons of both alpha and gamma motor neurons emerge via the ventral roots.)
This lamina also varies in configuration based on the vertebral level on which it occurs. It distributes motor information to skeletal muscles and partially corresponds to the lateral and medial motor neurons.
Laminae X makes up the center of the gray matter butterfly and surrounds the central canal.
Lamina X The cells that surround the central canal of the spinal cord form what is often referred to as Lamina X.
Lamina X surrounds the central canal of the spinal cord and is known as the grey commissure. This is where axons decussate from one side of the midline of the spinal cord to the other. Lamina X mostly corresponds with the spinal cord nucleus, the interomediolateral nucleus
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- Rexed laminae