Nervous system
Homeostasis
maintains relatively stable internal environment
eg: temperature, oxygen levels, pH, levels of nutrients, waste products and electrolytes
cells which make-up body systems (organs, respiratory, renal etc..) maintain homeostasis
Deviation of homeostasis
sensory: sensory nerves/ cells
info sent to integrator (brain)
effector: processes activated by efferent nerves and hormone systems
Compensatory response re-establishing homeostasis
Divisions of the nervous system
Central Nervous system: brain and spinal cord
Peripheral nervous system (spinal nerves and cranial nerves): afferent and efferent fibres which relay signals between CNS and periphery (other parts of body
Autonomic nervous system
motor (efferent = away from CNS)
Sensory (afferent = towards CNS)
Somatic nervous system = voluntary and conscious
motor (efferent = away from CNS)
sensory (afferent - towards CNS)
involuntary/ sensory signals are not perceived consciously
heart
smooth muscle of blood vessels
visceral organs
lungs
intestine
bladder
glands
genitalia
sympathetic (fight or flight)
parasympathetic (rest or digest)
Two neutron chain
Pre-ganglionic neuron
post-ganglionic neuron
cell body located in the CNS
axon synapses with cell body of second neutrons, which lies in ganglion (cluster of neuronal cell bodies outside CNS)
axon innervates the effector organ
nerve fibres leave thoracic and lumbar regions of spinal cord
sympathetic ganglia lie close to spinal cord and form a body known as sympathetic trunk
short preganglionic fibres
long post-ganglionic fibres
ganglion closer to CNS
nerve fibres leave from brain and sacral regions of spinal cord
long preganglionic fibres
short postganglionic fibres
ganglion closer to effector organs
Advantages
preganglionic neuron can synapse onto many postganglionic neurons
can synapse onto both excitatory and inhibitory postganglionic neurons
rapid, reflex effects on output
skeletal muscles
Effect of increased sympathetic NS
increased HR and CO (increased contractility of ventricle in presence of NA
Constrict arteriole and venous smooth muscle = Increase BP
Pupils dilate to let more light in
respiratory airways open (relaxation of airway smooth muscle)
breakdown of glycogen = more energy for muscles and brain
increased sweating
redistribute blood to skeletal muscles (during exercise)
digestive and urinary activities shut down
activate adrenal gland (adrenal medulla)
release adrenaline and noradrenaline (catecholamines)
adrenal gland activation
preganglionic neuron (lumbar)
chrmaffin cell (modified postganglionic neuron) = no axon cause it secretes Adr (80%)/ NA (20%) to bloodstream
Dual innervation (exceptions)
Not all organs receive dual innervation; most innervated arterioles/ veins receive only sympathetic nerve fibres
sympathetic and parasympathetic stimulation are not always antagonistic in their effects
salivary glands have both sympathetic and parasympathetic innervation but both are stimulatory for saliva secretion
Autonomic conflict
opposite effects of sympathetic and parasympathetic on the heart are activated separately or sequentially
both systems activated simultaneously in certain situations leading to pathophysiological consequences
Eg: activate peripheral chemoreceptors, mild prolonged hypercapnia (raised blood CO2), the startle response and cold water immersion
Cold water immersion activates two powerful reflexes
Dive reflex
Cold shock response: immersion stimulates cutaneous cold receptors (whole body), which activates sympathetic system resulting in tachycardia
Face cold and wet (breath hold) which stimulates mechanism- and thermoreceptors and consequently activates the parasympathetic system resulting in profound bradycardia
two opposing influences acting on heart simultaneously resulting in arrhythmias
Pharmacology of the autonomic NS
Autonomic neurotransmitters
Activate target cells
released from varicosities (irregular expansions on the axon)
produce diffuse, non-directional release
Acetylcholine: major euro transmitter for all preganglionic-postganglionic synapses (sympathetic and parasympathetic) including activation of chromaffin cells (modified post-ganglionic neurons) of the adrenal medulla
Postganglionic-effector cell synapse
Parasympathetic: acetylcholine
sympathetic: norepinephrine/ noradrenaline
Breakdown of neurotransmitter/ hormones
NA is rapidly re-sequestered by sympathetic nerves (and then broken down by monoamine oxidase)
Act broken down by extracellular acetylcholinerases
circulating catecholamines: Adr/ NA inactivated by catechol-O-methyltransferase in the liver
Membrane receptors: binding of neurotransmitter induces a tissue-specific response by means of a second messenger system within cell
adrenergic receptors: bind NA/ Adr (released by adrenal medulla)
a1: vein/ arteriole smooth muscle
Contraction and stimulatory effect and higher affinity for NA
a2: endocrine pancreas
inhibit insulin release (inhibitory effect) and higher affinity for NA
b1: heart
increase rate and force and a stimulatory effect, also has similar affinity for Air/ NA
b2: airway smooth muscle
relaxation and inhibitory effect and higher affinity for Adr
Cholinergic receptors
Muscarinic acetylcholine
Nicotinic acetylcholine
named after agonist muscarine
sound on effector cells innervated by parasympathetic branch
blocked by atropine
named after the agonist nicotine
receptors found on: postganglionic cell bodies and post synaptic membranes of skeletal muscle cells