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Physiological Control Systems

Autonomic Nervous System • ANS aims to maintain homeostasis • Most autonomic sensory signals are not perceived consciously • Autonomic pathways have two

neurons o Preganglionic neuron can

synapse onto many postganglionic neurons

o Single preganglionic neuron can synapse onto both excitatory and inhibitory postganglionic neurons

• Autonomic nervous system consists of three divisions: o Sympathetic

§ Nerve fibres leave from thoracic and lumbar regions of spinal cord

§ Most sympathetic ganglia lie close to the spinal cord in the sympathetic trunk

o Parasympathetic § Nerve fibres leave

from the brain and sacral regions of the spinal cord

§ Ganglia lie within or near the innervated organ

o Enteric (gut)

Sympathetic Nervous Activation • Sympathetic and parasympathetic systems generally exert opposite effects

on a particular organ • Dual innervation of an organ allows precise control of an organ’s activity • Sympathetic and parasympathetic stimulation are not always antagonistic in

their effects o Sympathetic and parasympathetic stimulation on salivary glands both

stimulate saliva secretion

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Parasympathetic Nervous Activation • Parasympathetic stimulation dominates in quiet, relaxed situations – rest and

digest • Concerned with “general housekeeping”

o E.g. Digestion, emptying of urinary bladder, secretion o Brings body back to normal after stress has passed – rapidly slows down

activities enhanced by sympathetic stimulation

Autonomic Conflict • There is evidence that both systems

can be activated simultaneously in certain situations

• Cold water immersion activates two powerful reflexes

o Dive Reflex – facial immersion stimulates mechano- and thermoreceptors which activate parasympathetic system resulting in bradycardia

o Cold Shock Response – immersion stimulates cutaneous cold receptors resulting in tachycardia

Pharmacology of the Autonomic Nervous System

• Autonomic neurotransmitters activate target cells and are released from varicosities

• Acetylcholine is the major neurotransmitter for all preganglionic-postganglionic synapses, including the activation of chromaffin cells of the adrenal medulla

• The postganglionic-effector cell synapse is mediated by acetylcholine and noradrenaline

• The effects of acetylcholine and noradrenaline are short-lived because

o Noradrenaline is rapidly resequestered by the sympathetic nerves

o Acetylcholine is rapidly broken down by extracellular acetylcholinesterases

o The circulating catecholamines, adrenaline and noradrenaline are inactivated by Catechol-O-methyltransferase in the liver

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Membrane Receptors • Specific effects of sympathetic and parasympathetic stimulation depend on

the type of specific receptors found on the target cell membrane • Binding of neurotransmitters induces a tissue specific response by means of a

second messenger system within the cell Noradrenaline Receptors

• Adrenergic receptors bind to noradrenaline and adrenaline • There are four major types:

Acetylcholine Receptors • Known as cholinergic receptors with two types: o Muscarinic acetylcholine receptors § Found on effector cells and

innervated by parasympathetic branch § Blocked by atropine

o Nicotinic acetylcholine receptors § Found on postganglionic bodies and post synaptic membranes

of skeletal muscle cells

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Endocrine Control Systems • Operates through chemical messengers known as hormones • Endocrine hormones are secreted directly into the circulation

o Exocrine hormones are different and are secreted into ducts

Hormones • Belong to a class of regulatory molecules that are synthesized in specialised

cells • May be found as single cells within an organ or collected into a distinct

endocrine gland Peptide Hormones

• Lipophobic and bind to surface membrane receptors to activate signalling pathways

• Stored in small secretory granules or vesicles Steroid Hormones

• Class of lipids derived from cholesterol o Includes cortisol, aldosterone, testosterone and progesterone

• Affect carbohydrate metabolism, water balance and reproduction

• Are lipid soluble and act by binding to specific intracellular receptors, increasing the rate of synthesis of specific proteins

Tyrosine Hormones • Derived from amino acid tyrosine • Adrenaline and noradrenaline • Thyroid hormones

The Hypothalamic-Hypophyseal System • Organizes appropriate hormonal responses to stimuli from higher centres of

the CNS • Sensory neurons send information to the CNS • CNS signals the hypothalamus which then sends information to the pituitary

gland o The infundibulum joins the hypothalamus and pituitary

• Hormones are manufactured in the hypothalamus o Travel down the axon to the neurohypophysis (posterior pituitary) o Released into capillaries in response to neural stimulation

• Oxytocin and vasopressin are the main hormones of the neurohypophysis • The adenohypophysis (anterior pituitary gland) does not have important

neural connections with the hypothalamus

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Cortisol • Secreted by the zona fasciculata of the

adrenal cortex at a rate of 10mg daily in humans

• Exhibits a circadian rhythm • Cortisol is important for normal homeostatic

control of metabolic processes and fluid balance

• Involved in getting out of bed in the morning in conjunction with the sympathetic system

• Also vital for resistance to stress o During stress, cortisol increases glucose and oxygen supply to skeletal

muscles, heart and brain o Lipolysis is activated o Immune, reproductive and digestive functions are suppressed

Overproduction of Cortisol – Cushing’s Syndrome

• Can be due to: o Over secretion of CRH by hypothalamic tumour o Over secretion of ACTH by pituitary gland tumour o Negative feedback problem: ARH/ACTH secretion insensitive to raised

cortisol levels • This can lead to diabetes mellitus and central obesity • The only treatment is to target the cause and remove the tumour • Addison’s Disease is the opposite and features the underproduction of cortisol

PHYL2001