Local Anaesthesia: Neurophysiology Pain : Pain : –An unpleasant sensory and emotional experience associated with actual or potential tissue damage. –Two parts, pain perception and pain reaction Pain perception: Pain perception: –Pain is perceived by free nerve endings present in the skin & mucous membrane –These are called nociceptors.

Local Anaesthesia: Neurophysiology Nociceptors: Nociceptors: –Receptors that are sensitive to a noxious or potentially noxious stimuli. –Two main types: Mechanoreceptors Mechanoreceptors – Present only in the skin and mucous membrane. – Activated by high-intensity mechanical and thermal stimuli. – connected to type A delta (A-δ) fibres conducting at 5 – 10 m/s. Polymodal nociceptors: Polymodal nociceptors: – Free nerve ending, present widely in skin and deeper tissues. – Activated by mechanical, chemical and thermal stimuli. – connected to type C fibres, conduct at 0.5 – 2 m/s.

Local Anaesthesia: Neurophysiology Types of stimulus: Types of stimulus: –Physical –Mechanical –Thermal –Chemical Threshold stimulus: Threshold stimulus: –The minimum environmental change that is sufficient to excite the nociceptors to transmits an impulse. –The impulse is self-propagating and of equal intensity (All or none rule)

Local Anaesthesia: Neurophysiology Pain Reaction: Pain Reaction: –Is the integration and appreciation of pain by the CNS. –The parts involved are thalamus, hypothalamus and sensory cerebral cortex. –Reaction varies from one person to another and from time to time in the same person.

Local Anaesthesia: Neurophysiology –Types of reaction: Vocal Vocal Flexion reflexes. Flexion reflexes. Postural adjustment. Postural adjustment. Changes in vital signs. Changes in vital signs. Recall and past experience Recall and past experience –Pain (reaction) Threshold: High pain threshold – minimal response to painful stimuli. High pain threshold – minimal response to painful stimuli. Low pain threshold – Maximum response to painful stimuli. Low pain threshold – Maximum response to painful stimuli.

Local Anaesthesia: Neurophysiology –Factors controlling pain threshold: Emotional states Emotional states Fatigue, lower the threshold. Fatigue, lower the threshold. Fear and apprehension, patient and operator. Fear and apprehension, patient and operator. Age, older patients tolerate pain better. Age, older patients tolerate pain better. Sex, men versus women. Sex, men versus women. Racial, North European versus African. Racial, North European versus African.

Local Anaesthesia: Neurophysiology Nerve pathways Nerve pathways –Types of nerves: A-Fibres: A-Fibres: –Myelinated fibres. –Size 3 – 20 µm in diameter. –Conduction velocity up to 120m/s. –Subdivided into:  A-β Carry non-pain sensation.  A- δ Small thinly myelinated, 3 – 5 µm in diameter and conduct “First pain” sensation at 5 – 10 m/s.

Local Anaesthesia: Neurophysiology B-Fibres: B-Fibres: –Myelinated fibres –Size µm in diameter. –Conduct at 3 – 5 m/s. –Associated with Autonomic Nervous system. C-fibres: C-fibres: –Unmyelinated fibres. –Size 0.25 – 2.5 µm –Conduct at 0.5 – 2 m/s. –Convey “second pain” sensation.

Local Anaesthesia: Neurophysiology First & second pain: First & second pain: – Two sets of primary afferent neurones convey nociceptive information to the dorsal horn of the spinal cord. The high threshold mechanoreceptors are connected to type A-δ. The high threshold mechanoreceptors are connected to type A-δ. –Pain is first, fast, sharp, short duration, well localized and accompanied with flexion The polymodal nociceptors are connected to the unmyelinated C-fibres. The polymodal nociceptors are connected to the unmyelinated C-fibres. –Pain is second, slow, aching, long duration, diffuse and accompanied by tonic reflexes.

Local Anaesthesia: Neurophysiology Features First pain Second pain Stimulation Heat & pinprick Tissue damage Nerve fibres A- δ small myelinated C-Fibres unmyelinated C-Fibres unmyelinated Conduction 5 – 15 m/s 0.5 – 2 m/s Distribution Body surfaces, mucosa All tissues, except brain Effect of Morphine Very little effect Suppress pain and abolish spasm

Local Anaesthesia: Neurophysiology Theories of Pain Theories of Pain –Specificity theory: Specific pain system, relaying pain impulses from nociceptors to the sensory cerebral cortex. Specific pain system, relaying pain impulses from nociceptors to the sensory cerebral cortex. Specific free nerve endings are activated by tissue damage, PPS released, and an impulse is generated. Specific free nerve endings are activated by tissue damage, PPS released, and an impulse is generated. Pass along a specific pain pathway to reach the dorsal root of the spinal cord. Pass along a specific pain pathway to reach the dorsal root of the spinal cord. Enter the spinal cord and relay in the substantia gelatinosa in the grey matter. Enter the spinal cord and relay in the substantia gelatinosa in the grey matter.

Local Anaesthesia: Neurophysiology Ascend 2 – 3 vertebrae in the spinal cord and then cross the midline to the opposite spino- thalamic tract to the thalamus. Ascend 2 – 3 vertebrae in the spinal cord and then cross the midline to the opposite spino- thalamic tract to the thalamus. A third order neurone leave thalamus to the sensory cortex at the post-central gyrus. A third order neurone leave thalamus to the sensory cortex at the post-central gyrus. –Fails to explain clinical characteristics of pain e.g. phantom limb and post-herpetic and trigeminal neuralgia. –Surgical intervention does not relieve these pains. –Does not explain referred pain.

Local Anaesthesia: Neurophysiology The Gate-control theory: The Gate-control theory: –A “gate” exist in the substantia gelatinosa in the dorsal horn root of the spinal cord –It is represented by a “T” cell-Transmitting cell and for Trigeminal nerve, it is in the caudal division of the spinal nucleus. –A balance exist between the A- δ and C-fibres and the large A- β fibres.

Local Anaesthesia: Neurophysiology Activities in the large A-β exercise an inhibitory effect on the T-cell “close the gate” to pain impulses. Activities in the large A-β exercise an inhibitory effect on the T-cell “close the gate” to pain impulses. Activities in the small A- δ and C-fibres “open the gate” and facilitate transmission of painful impulses via the T-cell. Activities in the small A- δ and C-fibres “open the gate” and facilitate transmission of painful impulses via the T-cell. –Therefore, the level of activity in the gate is controlled, in part, by the activity in the A-β fibres.

Local Anaesthesia: Neurophysiology Activities in the higher centres of the brain, Limbic, emotion fear and anxiety, and reticular activating systems, thought and memory, pass from to the cortex to the T-cell, blocking the gate for pain. Activities in the higher centres of the brain, Limbic, emotion fear and anxiety, and reticular activating systems, thought and memory, pass from to the cortex to the T-cell, blocking the gate for pain. Nerve impulses pass from the reticular activating system, via the thalamus, to the cerebral cortex have both a facilitating and inhibitory effect. Nerve impulses pass from the reticular activating system, via the thalamus, to the cerebral cortex have both a facilitating and inhibitory effect.

Local Anaesthesia: Neurophysiology