PHYSIOLOGY OF PAINAUTONOMIC NERVOUS SYSTEM,

HYPOTHALAMUS,

Dr Joan KumarAsst 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.

RECEPTORSReceptors 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 FIBRES2 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 umbilicusTheories of Referred pain.1.Dermatomal RuleSomatic 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 SYSTEMThe 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 sensationHydrocephalus – 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-campu

s

Amyg-dala

Frontal

cortex

Globus pallidu

sThalamu

s

Reticular

formation

retina

Ant nuc of

thalamus

Midbrain

Reticular

formation

Dorsomedial nuc

of thalamus

Frontal

cortex

Post pituitar

y

Afferents

Efferents

Functions of hypothalamus1. 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 pituitary6. Control of emotional behavior7. 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.