Pain sensation Pain receptors are also called as nociceptors They are located at the ends of small 'C unmyelinated or myelinated A delta fibers a) Definition Pain sensation is unpleasant but protective sensation aroused by noxious stimuli that damage or can damage body tissues b) Physiology (properties & reaction) Purpose or importance: Protective Stimulus: noxious (chemicals like- Ach, bradykinin, serotonin, hydrogen ions, potassium ions, prostaglandins or mechanical or thermal) Receptors: free nerve endings (polymodal receptors) Adaptation : non or slow adapting receptors Nerve fibers: fast pain is carried by A-delta nerve fibers while slow pain by 'C' type. Neurotransmitters: glutamic acid (at spinal cord) for fast pain, substance P (at spinal cord) for slow pain and Lewis P factor for muscle pain, Pathway: lateral spinothalamic (specific neo spinothalamic for fast pain and diffuse and non specific paleo spinothalamic for slow pain) Reaction : pain is a
Physiological anatomy of Spinal cord
Spinal cord lies in the vertebral canal. Its length is 45-50 cms and has diameter of about 2 cm.
The whole length of spinal cord is divided into 31 segments. Each segment gives rise to a pair of spinal nerve. Depending on the region, the spinal nerves are named and numbered.
- Cervical 8 segments: C1-C8
- Thoracic 12 segments: T1-T12
- Lumbar 5 segments: L1-L5
- Sacral 5 segments: S1-$5
- Coccygeal 1 segment: Cx
- Total 31 pairs of spinal nerves,
Arrangement of gray and white matter in spinal cord:
In cross section, the spinal cord contains gray matter inside and white matter outside.
Gray matter:
The gray matter looks like English letter 'H'. The dorsal and ventral projections of grey matter look like horns and hence they are called dorsal horn and ventral horn
In addition in the thoracic and upper lumbar segments of spinal cord there is 'lateral horn'.The grey matter contains cell bodies of neurons. They can be classified into: Dorsal horn cells: Substantia gelatinosa of Rolando (SGR) cells and Clarke's column of cells.
Ventral horn cells: Alpha motor neurons, Beta motor neurons, Gamma motor neurons.
Lateral horn cells: Present only in the thoracic and upper lumbar segments. They give rise to pre ganglionic sympathetic fibers
White matter:
The white matter of spinal cord contains nerve fibers. These fibers are arranged in groups, having similar functions and are called tracts.
White matter present on the ventral aspect of spinal cord is called as ventral white column on the lateral side, lateral white column and on the dorsal side, dorsal white column,
The spinal cord is the primary centre of reflex action for the trunk and limbs consisting of main conducting paths to and from higher centers in the spinal cord and the brain.
In spinal cord dorsal roots are sensory and ventral (anterior) roots are motor, this is termed as Bell-Magendie law,
Functions of Spinal cord -
-Anterior and posterior grey horns of the spinal cord perform motor & sensory functions of respective part of body,
-Stretch reflex (basic reflex for regulation of tone, posture & equilibrium)
-Golgi tendon reflex (protective reflex for excessive tension in muscle)
-Flexor withdrawal reflex (protective reflex for painful stimuli)
Postural and locomotive reflexes - positive supporting reaction, stepping and walking including galloping reflex
Scratch reflex & Autonomic reflexes including mass reflex
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