PHYSIOLOGY OF PAIN AUTONOMIC NERVOUS SYSTEM, HYPOTHALAMUS, Dr Joan Kumar Asst Prof –Physiology

PHYSIOLOGY OF PAIN Pain is an unpleasant sensory experience. Pain sensation is a protective mechanism which informs the organism about the damage occurring in the tissues. The interpretation of pain involves physical, physiological & psychological factors. It is specific sensation with specific receptors, Nerve fibres and Pathway. It shows very little adaptation and is associated with feelings It may be associated with ANS responses like Nausea, vomiting etc.

RECEPTORS Receptors are free nerve endings which are excited by noxious stimuli. Damaged tissue release chemical substances like histamine etc which cause excitation of the receptors and generation of action potential in the nerve fibres. NERVE FIBRES 2 types of nerve fibres carry pain impulses. Aδ mylelinated fibre which carry pain due to mechanical stimuli.(fast pain) C unmyelinated fibres which carry pain resulting from chemical and thermal stimuli. (slow pain)

PAIN PATHWAYS Free nerve endings carry pain stimuli enter via dorsal nerve root and terminate in the posterior horn,of spinal cord. From here nerves cross to opposite side and ascend the spinal cord as the LATERAL SPINO THALAMIC TRACT. These end in VPL nucleus of thalamus. From the face the trigeminal nerve carries the pain and ends to the VPM nucleus of thalamus. The thalamus is the subcortical center for pain perception. From the thalamus III order neurons go to the sensory areas in the cortex and cingulate gyrus for higher interpretation.

TYPES OF PAIN Visceral pain Distention/ inflammation/ ischemia of Viscera cause pain. Pain carried by autonomic fibres. Headache Irritation of meninges, dural sheath, rise in Intracranial pressure, etc cause headache. Migraine headache due to distention of cerebral blood vessels. Muscle pain Caused by ishchemia (Occlusion of blood supply.) Accumulation of ‘P’ factor thought to be K+. Lactic acid accumulation also causes pain. Restoration of blood flow washes away the metabolites and relieves the pain.

REFFERED PAIN Pain from viscera which is referred or felt in somatic superficial structures some distance away. Eg: Ischemic pain of Heart- referred to left arm and shoulder. Pain in inflamed Appendix referred to umbilicus Theories of Referred pain. 1.Dermatomal Rule Somatic and visceral structures arise from a common dermatome and are innervated by same segment of spinal nerve.During embryonic development the viscera may move away carrying its nerve supply. Hence visceral pain is usually referred to a structure developed from the same embryonic segment 2. Convergence Theory Both somatic and visceral impulses converge onto the same cell at Posterior Horn. But brain used to receiving only somatic impulses. Hence visceral pain produced peripheral pain also.

AUTONOMIC NERVOUS SYSTEM The autonomic nervous system innervates the internal organs and modulates and maintains the internal environment or Homeostasis. It is involuntary in nature. The autonomic outflow has 2 divisions- the Sympathetic and Parasympathetic. Structurally and functionally these 2 pathways are different Each division has 2 efferent neurons in the pathways. Preganglionic neuron in the CNS-brain or spinal cord- which has connected by preganglionic nerve fibre to Post ganglionic neuron in periphery. This is turn give rise to the postganglionic nerve fibre which supplies the viscera.

SYMPATHETIC NERVOUS SYSTEM Preganglionic neurons : Present in lateral cell column of T1 to L3 spinal cord segments. Also called Thoracolumbar outflow. The preganglionic nerves fibres emerge via the anterior nerve root of spinal cord and reach the Lateral (Paravertebral) Sympathetic Ganglia chains or prevertebral ganglia. There are 3 –Coeliac, superior and inferior mesentric. From here the Post ganglionic nerves then arise and supply the organs. Preganglionic neurons secrete Acetylcholine. Postganglionic neurons secretes norepinephrine 4 types of receptors  1,  2,  1,  2 present all over the body.

PARASYMPATHETIC SYSTEM Pre ganglionic neurons :- 1) cranial motor nuclei of III, VII, IX & X nerves & 2) S2 – S4 spinal segments. Hence also called Craniosacral outflow. Preganglionic fibres arise from here and reach parasympathetic ganglia situated near the organs. These ganglia contain Post ganglionic neurons which give rise to the short postganglionic nerve fibres that supply the organs. Preganglionic &Postganglionic neurons secrete Acetylcholine. 2 types of receptors – Nicotinic & Muscarinic.

Functions Sympathetic nervous system is stimulated during stress and its activity helps to cope with stress. It prepares the organism for fight or flight. Parasympathetic action is individualized and discrete and lasts for a shorter time. Mainly concerned with conservation and restorative processes. Most viscera have dual innervation by both sympathetic and parasympathetic. If one division causes excitation, the other causes inhibition. Thus a balance is maintained for homeostasis.

APPLIED PHYSIOLOGY Hyperalgesia – Stimuli normally causing minor pain produces an exaggerated response Allodynia - Normal stimuli like touch causes pain. Analgesia – Absence of pain sensation Hydrocephalus – Obstruction to CSF flow causing increased pressure. Usually congenital which causes enlargement of the head in small babies. Cerebrovascular accident –can be either cerebral infarction or intracranial hemorrhage.This can cause cerebral hypoxia.

Hypothalamus The hypothalamus lies on either side of the third ventricle, immediately above the pituitary (hypophysis) and below the thalamus The hypothalamus controls visceral, autonomic, endocrine and emotional responses. It integrates visceral and somatic functions and thus regulates the homeostasis. It is connected to the anterior pituitary by blood vessels and to the posterior pituitary by nerve fibres.

NUCLEI OF HYPOTHALAMUS These can be divided into 4 groups. Anterior Group Preoptic nucleus Suprachiasmatic nucleus Supra optic nucleus Para ventricular nucleus Middle Group Middle group merging in the middle The median eminence Posterior Group

Connections of Hypothalamus Hypothalamus Limbic cortex Hippo- campus Amyg- dala Frontal cortex Globus pallidus Thalamus Reticular formation retina Ant nuc of thalamus Mid brain Reticular formation Dorsomedi al nuc of thalamus Frontal cortex Post pituitary Afferents Efferents

Functions of hypothalamus 1. Control of Autonomic Nervous system – Head ganglion of ANS Stimulation of anterior hypothalamus produces parasympathetic reactions like Salivation,  HR,  BP,  GIT motility and secretion. Pupillary constriction etc. Hence this area is designated as Parasympathetic Center. Stimulation of posterior hypothalamus and lateral hypothalamus cause sympathetic reactions like- Pupillary dilatation,  HR,  BP, vasoconstriction, piloerection etc. - Hence this area is designated as Sympathetic center

2. Control of body temperature Thermostat of the body.  Anterior hypothalamus – heat loss center(preoptic nucleus):  Posterior hypothalamus – heat conservation center. “ Normal body temperature due to balance between the two”. 3. Control of food intake & Regulation of body weight  Lateral hypothalamus – Appetite center.  Ventro medial nucleus – Satiety center. Balance between the two centers keeps the body weight under control. 4. Control of water intake and regulation of water balance 5. Control of both anterior and posterior pituitary 6. Control of emotional behavior 7. Control of Sleep

8. Control of sexual behavior 9. Control of Circadian Rhythm  Circadian rhythm – diurinal fluctuations in magnitude of most body functions.  Egs. Secretion of ACTH, sleep wake cycle, etc  Suprachiasmatic nucleus controls Circadian rhythm- influenced by visual impulses reaching the Hypothalamus via optic chiasma.