The autonomic nervous system functions below consciousness to control visceral functions like the heart and digestive system. It transmits messages via neurohumoral transmission where nerves release chemical messengers called neurotransmitters at synapses. This involves nerve impulses triggering neurotransmitter release from synaptic vesicles, binding to and activating receptors, and termination of the neurotransmitter through reuptake or degradation. Neurotransmitters can have excitatory or inhibitory effects depending if they cause depolarization or hyperpolarization of the post-synaptic membrane.

1. Autonomic Nervous System

2. ORGANIZATION AND FUNCTION
• The autonomic nervous system (ANS)
functions largely below the level of
consciousness and controls visceral functions.
• The ANS consists of afferents, centre and
efferents nerves.

6. NEUROHOMORAL TRANSMISSION
• Neurohumonal transmission implies that
nerves transmit their message across synapses
and neuroeffector junctions by the release of
humoral (chemical) messengers.

7. Steps in neurohumoral transmission
• Impulse conduction:
• The resting transmembrane potential (70 mV negative
inside) is established by high K+ permeability of axonal
membrane and high axoplasmic concentration of this
ion coupled with low Na* permeability and its active
extrusion from the neurone.
• Stimulation or arrival of an electrical impulse causes a
sudden increase in Na+ conductance thus
depolarization and overshoot (inside becoming 20 mV
positive).
• Then K* ions move out in the direction of their
concentration gradient and repolorization occurs.

8. • Tetrodotoxin (from puffer fish) and saxitoxin
(from certain shell-fish) selectively abolish
release in Na+ conductance in nerve fibres and
thus block impulse conduction.

9. • Transmitter release
– The transmitter(excitatory or inhibitory) is stored
in prejunctional nerve endings within synaptic
vesicles.
– Nerve impulse promotes fusion of vesicular and
axonal membranes through Ca+2 entry which
fluidizes membranes.
– All contents of the vesicle transmitter, enzymes
and other proteins) are extruded (exocytosis) in
the junctional cleft.

10. • The release process can be modulated by the
transmitter itself and by other agents through
activation of specific receptors located on the pre-
junctional membrane.
• e.g. noradrenaline (NA) release is inhibited by NA (alfa-
2 receptor),
• Dopamine, adenosine, prostaglandins and enkephalins
-,While isoprenaline (B2 receptor) and angiotensin (AT1
receptor) increase NA release.
• Similarly, alfa-2 and muscarinic agonists inhibit
acetylcholine release at autonomic neuroeffector sites
(but not in ganglia and skeletal muscles)

11. • Transmitter action on postiunctional membrane
– The released transmitter combines with specific
receptors on the postjunctional membrane and
depending on its nature induces an excitatory
postsynaptic potential (EPSP) or an inhibitory
postsynaptic potential (IPSP).
• EPSP Increase in permeability of all cations so there is Na+ or
Ca+2 influx (through fast or slow channels) causes
depolarization followed by K+ efflux.
• These ionic movements are passive as the flow is down the
concentration gradients.

12. • IPSP Increase in permeability to smaller ions,
i.e. K+ and Cl- only, so that K+ moves out and
Cl- moves in (in the direction of their
concentration gradients) resulting in
hyperpolarization.

13. • Postjunctional activity
– A suprathreshold EPSP generates a propagated
postjunctional AP which results in nerve impulse
(in neurone), contraction (in muscle) or secretion
(in gland).
– An IPSP stabilizes the postjunctional membrane
and resists depolarizing stimuli.

14. • Termination of transmitter action
• Following its combination with the receptor, the
transmitter is either locally degraded (e.g. ACh) or is
taken back into the prejunctional neurone by active
uptake or diffuses away (e.g. NA,GABA).
• Specific carrier proteins like norepinephrine
transporter (NET), dopamine transporter (DAT),
serotonin transporter (SERT) are expressed on the
axonal membrane for this purpose.
• The rate of termination of transmitter action govems
the rate at which responses can be transmitted across
a junction (1 to 1000/sec)