(1)
Department of Anaesthesia, Royal Free Hospital, London, UK
The pathway can be divided into peripheral and central.
Peripheral pathways are the first point of contact for the painful stimulus and respond in a coordinated fashion to form a response so as to prevent further damage caused by the noxious stimulus.
Primary sensory neurons are seen in the dorsal ganglia and are classified according to their size, axon diameter, velocity of conduction or extent of myelination.
1.1 Dorsal Horn
Neurons which are primarily involved with the sensory system lie in this part of the spinal cord grey matter. It comprises of C- and Aδ-fibre afferents (Lissauer’s tract). It is further organised in layers known as laminae (I to VI):
Lamina I, posterior marginal nucleus: responds to cutaneous and visceral stimulation.
Laminae II/III, substantia gelatinosa: subdivided into outer and inner part. It gets stimulated by thermal or mechanical stimulation.
Laminae III/IV/V, nucleus proprius: responds to low-threshold Aβ input.
Lamina VI, nucleus dorsalis: neurons respond to nonnoxious stimulation of joints.
Aδ and C fibres innervate laminae I and II. The information is transmitted to laminae IV, V and VI. Axons from here cross the midline and reach the central processing system through pathways in the anterior and lateral parts of the spinal cord.
Visceral afferents synapse in laminae I and V along with somatic afferents. This is known as convergence. Increased activity of afferents from visceral injury is interpreted as input from somatic afferents and is responsible for the referred pain.
The various neurons of dorsal horn are:
Projection neurons: terminate in supraspinal centres. They are involved with sensory information.
Propriospinal neurons: axons cover several spinal segments and participate in heterosegmental reflexes.
Interneurons: small cells which participate in the integration of information. They may be excitatory or inhibitory.
The neurons are functionally classified further according to their activity:
Low-threshold cells: they respond to non-painful stimuli and are located in lamina IV. They respond to touch, pressure and vibration.
Wide dynamic range cells: they have both low-threshold and high-threshold afferents. They respond to both nociceptive and non-nociceptive stimuli and change their firing frequency depending upon the stimulus of the afferent intensity. The neurons are seen mostly around lamina V. The neurons have large receptive fields. Wind-up: repetitive stimulation (interval of 1 s) of C fibres produces a state of continuous discharge. This is due to temporal summation of the action potentials and contributes to transient increase in response to nociceptive stimuli.Related posts:
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