The central process o f the axon passes to the spinal cord via the dorsal root. A single dorsal root is responsible for information from a particular area o f the body. This area is known as the dermatome. The spinal cord is split up into 31 spinal segments. A segment contains the dorsal root (containing afferents) as well as the ventral root (containing efferents). The segm ents are the cervical, thoracic, lumbar, sacral and coccygeal.
It is generally thought that there is functional separation between the dorsal and ventral roots, in that the dorsal roots contain afferent sensory fibres and the ventral roots contain efferent motor fibres. However, a variety o f studies have shown that this separation is not absolute. In a study in cats a small proportion o f the afferent fibres, predominantly small in diameter, enter not by the dorsal roots but via the ventral root and have been shown to terminate in the marginal zone and substantia gelatinosa (Light and Metz 1978). In this same study some larger diameter fibres were also shown to enter the spinal cord via this route but terminate in the nucleus proprius (Light and Metz 1978). It has been suggested by Kim et al. (1987) that these afferents entering the ventral root may be a possible third branch o f these neurones or could be afferents arising from the pia matter. Stimulation o f these ventral root afferents have been shown to give rise to changes attributed to the
response to painful sensations in awake animals (Longhurst et al. 1980).
The large fibres enter via the dorsal columns and the smaller fibres via a bundle known as Lissauer’s tract (Molander et al. 1984). Lissauer’s tract also contains axons o f cells originating in the dorsal horn, from marginal as w ell as SG neurones. Both large and small fibre types split into two and may project both rostral and caudal to the dorsal root before entering the spinal cord.
1.2.8: The Spinal Cord.
The spinal cord is divided into white and grey matter. The white matter contains axons which tend to be myelinated, whereas the grey matter contains cell bodies and their processes. The grey matter is further divided into ten laminae, first described in the cat by Rexed (1952) and subsequently by others in the rat (Molander et al. 1984; Wall 1967b). When viewed in section these laminae can be seen as layers o f functionally distinct cells. They also form columns o f functionally related cells that extend the length o f the cord. The different laminae contain different intrinsic cells, receive different inputs and may also have outputs to the ventral horn and to the brain.
These laminae are not separated by hard distinct borders. Often the cell types making up each proposed lamina may be integrated to some extent. Lamina I forms a rim around the most dorsal part o f the grey matter and white matter. This lamina contains large horizontal neurones know) as the marginal cells o f Waldeyer and historically has been called the W aldeyer’s layer or marginal zone. These cells are large but few. They have long dendrites which mainly pass over the surface o f the dorsal horn, occasionally entering the SG and form a vague boundary between the white and grey matter. The dendrites, as w ell as covering the outer surface o f the dorsal horn also join Lissauer’s tract for up to 5-6 segments where they join the grey matter again (Cervero and Iggo 1980). There are also more smaller neurones located here which may make up the more dorsally located SG cells. These cells are more prolific in number than the marginal cells. As well as these intrinsic, propriospinal cells the various laminae also receive afferent inputs. The marginal plexus contains the processes o f afferent neurones, superficial neurones as w ell as the processes o f deeper cells. The Aô-fibres terminate in lamina I entering via this marginal plexus (Light and Perl 1979). Some o f the visceral inputs terminate here as well as laminae Ho, V and X. Visceral inputs usually synapse onto the same cells that concurrently receive inputs from the skin and muscle. This is known as convergence, since a variety o f inputs project to the same second order neurone. This is one o f the reasons that visceral pain is often diffuse and hard to localize as well as the fact that visceral
afferents only represent 1 0% of the afferent input into the spinal cord and they also
tend to have a greater rostro-caudal distribution than afferents from the skin.
Lam ina II of the spinal cord is also known as the substantia gelatinosa and historically has been called Rolandi substance (see Cervero and Iggo 1980; Pearson 1968; Rexed 1952). The substantia gelatinosa comprises of lamina Ilouter (which is more dorsal and contains densely packed cells) and lamina Ilinner (less compact and more ventrally located). The literature is somewhat confusing as lamina Hi has also been called lamina III; here it will be referred to as lamina Hi. The intrinsic cells here are predominantly stalk and islet cells but there are also arboreal cells, border cells and spiny cells.
The stalk cells have classically been called limiting cells, since their spines are short. Their cell bodies are in Ho near the border with lamina I and they send their dendrites towards the ventral horn. The stalk cells act to relay transmission from the superficial lamina to deeper laminae, and are presum ed to be mainly excitatory, although the presence of enkephalin has been shown in some of these cells (Bennett et al. 1980).
The islet cells have their cell bodies in Ho and inner and their dendrites extend in a rostro-caudal plane. The islet cells are inhibitory cells and play an important role in the control of the presynaptic terminals of afferent inputs, via axo axonic connections as well as a post-synaptic inhibitory control via axo-dendritic connections. It is presumably activation of these cells by the collaterals of the large diameter fibres that control the inputs of predominantly the Aô-fibre terminals and to a lesser extent the C-fibre terminals and underlies the “Gate Theory” of control first put forward by Melzack and Wall (1965) (Alvarez et al. 1992; Bemardi et al. 1995; Melzack and Wall 1965).
The other intrinsic cells are the arboreal cells found in lamina Ho and have extensive dendritic branches in Ho as well as Hi and I. The border cells are not very common and their dendrites arborize in laminae Ho and Hi. The spiny cells send dendrites to laminae I-IH.
As well as these cells intrinsic to the spinal cord, afferents also terminate in these laminae. Hair follicle afferents are the only large diam eter afferents to terminate in lamina Hi although the medial part of lamina II is crossed by large nerve fibres entering from the dorsal white column (Molander et al. 1984). The innocuous C-fibres terminate in lamina Hi and the noxious C-fibre terminals are in lamina Ho. However, it should be noted that although (C-fibres do not terminate in deeper lamina,
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cells located in laminae V and V l^ a n receive C-fibre inputs since they extend their dendrites into more dorsal laminae (Fitzgerald and Wall 1980). Afferent terminals from Aô-fibres are few in this lamina, though some have been shown to terminate in
lamina Ho (Molander et al. 1984).
The border between lamina Ili and III is irregular and often hard to visualize since it contains densely packed cells. Lamina III-VI make up the deep dorsal horn. Cells located here send their dendrites to deeper laminae or into superficial lamina. Inputs arising from cutaneous mechano and proprioception terminate in lamina III and IV (SAI mechano, FAI, FAII and hair follicles). Lamina IV forms the base o f the head o f the dorsal horn. The cells found here are diffusely arranged, are large and extend their dendrites into more superficial lamina. This means that they can receive inputs from afferents terminating more dorsally as well as
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receiving inputs directly into this lamina. There are/|fe fine afferents terminating in this lamina. Muscle stretch receptors, joint receptors and SAQ terminate in lamina IV to V n and IX. Lamina V forms the neck o f the dorsal horn and Lamina VI forms the base o f the dorsal horn, both receives inputs from thick myelinated fibres. More ventral laminae (VII-IX) have mainly efferents o f visceral and somatic motor neurones. Lamina X is located close to the central canal.
Cells within the spinal cord can send projections to other areas o f the spinal cord (propriospinal) or out o f the spinal cord to areas of the brain, projection neurones. The large fibres, as well as sending collaterals into the dorsal horn at their point o f entry, ascend ipsilaterally in the dorsal columns to terminate on second order neurones in the medullary dorsal column nuclei. They then cross over to the contralateral side to the thalamus and cortex. The small fibres may project both rostral and caudal along the dorsal root before entering the spinal cord where they terminate in the dorsal horn o f the spinal cord in superficial lamina. These afferents synapse with intemeurones which relay their information to deeper projection neurones, to dendrites o f more ventrally located projection neurones or back to spinothalamic neurones in laminae I. Projection neurones from laminae I, III and IV transmit noxious and thermal sensations to the brain in the anterolateral white matter. This incudes the spinoreticular, spino-mesencephaUc and the spinothalamic tracts. These project to the reticular formation o f the brain stem (spinoreticular) or the parabrachial area in the midbrain (spinomescenphalic) then on to the thalamus and limbic systems.
As well as relaying information from the periphery to supraspinal sites the spinal cord is also under the control o f supraspinal structures. Descending pathways from the brain stem provides tonic inhibition of cells responding to noxious stimuli within the spinal cord (Wall 1967b). Ascending tracts can activate these descending controls which arise from activation of the periaqueductal gray, rostral ventromedial medulla and dorsolateral pontomesencephalic tegmentum. These descending controls release serotonin, noradrenaline and op ioid s to inhibit
nociresponsive neurones.