Physiological Control Systems - StudentVIP
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PHYL2001
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
PHYL2001
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
PHYL2001
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
PHYL2001
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
PHYL2001
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
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