An overview of the peripheral nervous system (PNS) lectures as part of the physiology module.
Covers the structure of the PNS, its various receptors and the ways in which signals are sent across the body in pre- and post-ganglionic fibres.
The parasympathetic NS has fibres arising from the brainstem and
also from sacral spinal nerves. These fibres have long pre-ganglion
fibres and short post-ganglionic fibres. In terms of functions, the
parasympathetic is for ‘rest and digest’. In an emergency when
your eyes are dilated, your parasympathetic NS will work to
constrict them back to a relaxed state. Note that this NS isn’t just
about resting, it’s also about digesting, so it does increase the
activity in the GI tract.
In the sympathetic NS, there are short pre-ganglionic fibres and
long post-ganglionic fibres. Note that the adrenal medulla acts like
a ganglion in the sympathetic nervous system. This NS organises
the fight or flight responses such as increasing your heart rate.
Autonomic division: the sympathetic and
The whole system is generally organised so that higher fibres deal
the parasympathetic often have the same
with the head and neck and lower fibres deal with the GI tract and
effector tissues as they have opposing
heart (exception = vagus). Remember that both parasympathetic
effects on the body.
and sympathetic fibres typically innervate the same effector
Cranial nerves- there are 12 pairs of
organs but with opposing effects.
cranial nerves and they are named in the
order in which they leave the brain. The The post-ganglionic fibre, ends in pearl-like structures. As a whole,
olfactory and optic are however these structures are called telodendria, and within these pearl-like
considered part of the brain rather than structures are little swellings of varicosities. These varicosities
the PNS. Some have just sensory e.g the contain vesicles containing neurotransmitters.
optic and olfactory whilst the oculomotor
only have motor neurones. There are two main types of receptors in the
Spinal nerves- there are 31 pairs parasympathetic and sympathetic divisions. GPCRs
numbered preceded by a letter denoting
their region.
Both the parasympathetic and
PNS and ligand-gated ion channels.
sympathetic NS use a two-neurone
system linked by ganglia. There are two
types of ganglia : sensory (dorsal root
ganglion, cranial nerves) and autonomic
(sympathetic chain ganglia and terminal
ganglia).
The 1st neurone in the 2-neurone system
is before the ganglion – pre-ganglionic
and is myelinated (From Schwann cells
not oligodendrocytes). It synapses with
the cell body (in a ganglion) of a second
neuron (non-myelinated, post-
ganglionic), which innervates its effector Neurotransmitters: Noradrenaline has 2 main types of
adrenergic receptors which are further subdivided:
Neurotransmitters: Acetylcholine has 2 main
cholinergic receptors:
Nicotinic – found in skeletal muscles (in the somatic
division where a single fibre from CNS releases Ach in
the muscle), as well as parasympathetic and
sympathetic ganglions and the CNS. Excitatory
response that uses ligand-gated ion channels (Na+) They are
ionotropic. Has 2 binding domains for Ach. all GPCRs. The beta receptors all activate adenylyl
Muscarinic – found in smooth muscle, cardiac muscle cyclase, increasing cAMP. NAD binds to a receptor,
and glands. 5 different sub-types (M1-5) of which the leading to GDP being exchanged for GTP causing a
odd are excitatory and the even inhibitory. Uses GPCRs conformational change in the G protein. The α,β and γ
(metabotropic) so its domains are extracellular, subunits dissociate. The α subunit activates adenylyl
transmembrane and intracellular. cyclase which converts ATP to cAMP (also increasing
PKA). (Continues onto next page)
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