AUTONOMIC NERVOUS SYSTEM - PHYSIOLOGY

Describe how the ANS is influenced by the central nervous system

ANS regulated by several levels of CNS •cerebral cortex: emotions & sensory experiences •hypothalamus (major visceral motor control centre) - nuclei for hunger, thirst, thermoregulation, sex drive •midbrain, pons, and medulla oblongata - nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes •spinal cord reflexes - defecation and micturition reflexes integrated in cord and the brain can inhibit these responses consciously

Name the receptors for these neurotransmitters and explain their autonomic effects

Adrenergic receptor types and their effects: •Response to adrenergic stimulation varies from tissue to tissue - some are stimulated to contract, others to dilate •depends on the receptors activated •2 major classes: alpha and beta - several subtypes: a1, a2, b1, b2 •compounds have been developed that can selectively bind to each subtype & promote or block the action at each receptor (agonists & antagonists). Used to determine which receptors on each tissue with many important medical applications Beta - adrenergic receptors: •usually have inhibitory effects on target organ (e.g. NE relaxes and dilates the bronchioles when it binds to b-adrenergic receptors of the bronchial smooth muscle) •However has excitatory effect when NE binds to the b-adrenergic receptors of cardiac muscle •2 receptor subclasses of b-adrenergic receptors which mediate different effects from the same neurotransmitter: Relaxation of smooth muscle (b2) and, Increased force of contraction of cardiac muscle/increased heart rate (b1)

Name the receptors for these neurotransmitters and explain their autonomic effects

Alpha-adrenergic receptors: •Usually have excitatory effects on target organ •Stimulation of a-adrenergic receptors usually causes contraction of smooth muscle (e.g. vasoconstriction of deep coronary arteries, blood vessels of most viscera, blood vessels of skin and contraction of uterine smooth muscle •Different effects from same neurotransmitter and receptor relate to the different subclasses of a-adrenergic receptors (a1 or a2 ) •Note that generally a1 & b1 receptors are excitatory & a2 & b2 receptors are inhibitory Flight or fight example of receptor effects: •increased heart rate & contractility (b1) (excitation) •dilation of bronchioles of lung (b2) - smooth muscle relaxation (inhibition) •dilation of arterioles in skeletal muscle (b2) - Smooth muscle relaxation (inhibition) •reduced blood flow to viscera & skin (a1) - smooth muscle in blood vessels constrict (excitation) •See Table 15.5 of Saladin for specific effects of sympathetic & parasympathetic NS

Name the receptors for these neurotransmitters and explain their autonomic effects

Cholinergic receptor types and their effects: Cholinergic neurons (release ACh as a neurotransmitter) •All somatic motor neurons: excitatory •All preganglionic neurons: excitatory •Most postganglionic parasympathetic neurons: usually excitatory but may be inhibitory (e.g. slows HR) •(Some postganglionic sympathetic neurons) Nicotinic Receptors: •somatic NS: found at neuromuscular junction •ANS: found in autonomic ganglia - acted on by Ach from preganglionic neurons •blocked by curare •Work by opening ligand-gated ion channels •always excitatory in the target cell Muscarinic Receptors: •found on effector organs •acted on by Ach from postganglionic neurons •blocked by atropine •can be excitatory (e.g. on smooth muscle or glands), or inhibitory (e.g. on heart) - depends on receptor subtype (M1-5)

Explain how the ANS controls many target organs using dual innervation and give an example

Dual Innervation: •Most visceral organs receive both sympathetic & parasympathetic innervation •Both divisions do not normally innervate an organ equally Effects may be: •antagonistic •co-operative Antagonistic effects - oppose each other, either by: 1)exerted through dual innervation of same effector with differences in receptor type •heart rate decreases (parasympathetic) •heart rate increases (sympathetic) 2)exerted because each division innervates different cells of same organ also with different receptor types •pupillary dilator muscle (sympathetic) dilates pupil •constrictor pupillae (parasympathetic) constricts pupil Examples: Dual innervation of the iris: •Sympathetic stimulation of the radial muscles to dilate pupils •Parasympathetic stimulation of circular muscles to constrict pupils Cooperative effects - 2 divisions act on slightly different effectors to produce a unified effect: 1)Parasympathetic innervation increase salivary serous cell secretion (watery component of saliva) 2)Sympathetic innervation increase salivary mucous cell secretion (mucus component of saliva)

State 5 actions of the parasympathetic nervous system

Functions of parasympathetic nervous system: Parasympathetic NS: "rest & digest" Broadly opposite to the sympathetic nervous system: •decrease in heart rate •decrease in pulmonary flow (bronchoconstriction) •redirect blood to digestive system & away from skeletal muscles etc •penile erection •increased glandular secretion (nasal, lacrimal, salivary) •increased peristalsis and digestive enzyme activity •relaxation of gut sphincters Localized responses, not mass activation

State 5 actions of the sympathetic nervous system

Functions of sympathetic NS: Sympathetic NS: "fight or flight" Prepare the body for intense physical activity such as in emergencies Wide range of actions throughout body: •increase heart rate, •increase pulmonary airflow (bronchodilation) •redirect blood to vital areas e.g. muscles & lungs (& away from intestines, skin etc) •inhibit digestive system (decreased peristalsis) •increase blood glucose concentration •increased metabolism Generally activated as a whole (mass activation)

Name the neurotransmitters used at the different ANS synapses

Neurotransmitters of the ANS: Effects of ANS are determined by types of neurotransmitters released and types of receptors on target cells Sympathetic has longer lasting effects - neurotransmitters persist in synapse and some reach the bloodstream Preganglionic fibres •All preganglionic fibres release acetylcholine as neurotransmitter (both sympathetic and parasympathetic NS) Postganglionic fibres: •Sympathetic NS uses norepinephrine (noradrenaline) as its neurotransmitter for nearly all of its postganglionic fibres (adrenergic neurons) •A small amount of postganglionic fibres secrete acetylcholine (cholinergic neurons) e.g. sweat glands, piloerector muscles •Parasympathetic NS uses acetylcholine exclusively

Explain how the ANS control is exerted in the absence of dual innervation and give an example

Organs without dual innervation: Some effectors receive only sympathetic input & no parasympathetic input •adrenal medulla •arrector pili muscles •sweat glands •many blood vessels Regulation by frequency of action potentials (sympathetic tone): There is a baseline firing frequency and change from this determine the response •increase in firing frequency = vasoconstriction •decrease in firing frequency = vasodilation Sympathetic control of vasomotor tone allows the shift of blood flow from one organ to another as needed •sympathetic stimulation increases blood to skeletal and cardiac muscles & reduced blood to skin Example of organs without dual innervation: •Blood vessels to skin vasoconstrict to minimize bleeding if injury occurs during stress or exercise •Sympathetic division prioritizes blood flow to skeletal muscles and heart in times of emergency •Called sympathetic tone or vasomotor tone (partial constriction of blood vessel)

Name the receptors for these neurotransmitters and explain their autonomic effects

Receptor types and their effects: Receptors can be classified into two major categories: 1.Adrenergic (binds with epinephrine & norepinephrine) Two main types of adrenergic receptors are: •Alpha receptors •Beta receptors 2.Cholinergic (binds with acetylcholine) Two types of cholinergic receptors are: •nicotinic receptors •muscarinic receptors •Any nerve fibre that secretes Ach is called a cholinergic fibre and the receptor that binds to it is called a cholinergic receptor. •Any nerve fibre that secretes NE is called a adrenergic fibre and the receptor that binds to it is called a adrenergic receptor.