Overview of the PNS Flashcards
What does the PNS consist of?
sensory and motor nerves from endings in skin/ muscle to point where they enter/ leave the spinal cord
Where do motor nerve fibres arise from?
Cell bodies in spinal cord
How is the spinal cord protected?
Protected in spinal canal. lamina bone laterally and spinous process dorsally.
where do sensory neruones from peripheral nerves enter spinal cord?
dorsal root
where are sensory cell bodies located?
outside the spinal cord in their dorsal root ganglion
where do motor neurones exit the spinal cord?
ventral root
where are motor cell bodies located?
grey matter in ventral horn of spinal cord
how are spinal nerves formed?
fusion of dorsal and ventral roots
where are dorsal root ganglia located?
in the intervertebral foramina
how is oxygen supplied to peripheral nerves?
peripheral nerves have arteries within them which are branches of spinal cord arteries
one peripheral nerve arises from _____ spinal root(s)
at least 2
one root contributes to ______ peripheral nerve(s)
at least 2
define dermatome
area of skin innervated by spinal nerve or corresponding dorsal root
what is meant by lower motor neurone?
motor nerve cells which connect with skeletal muscle
where are the cell bodies of lower motor neurons, and where do their axons project?
cell bodies in ventral horn
axons project out ventral root and down peripheral nerve to synapse with skeletal muscle fibres
how many axons emerge from lower motor neuron cell body?
one
define endoneurium
thin, protective membrane that surround individual nerve fibres
define nerve fascicle
groups of functionally related nerve fibres collected together
what surround nerve fascicle?
perineurium
what does a whole peripheral nerve made up of?
several fascicles bundled together with blood vessels, all surrounded by epineurium
where are the majority of skin receptors found?
epidermal-dermal junction
describe the type of receptors found of glabrous skin
variety of receptors that respond to pressure, light touch, vibration
describe type of receptors found on hairy skin
hair follicle receptors that respond to hair displacement
how are non-voltage gated sodium channels opened on the end of sensory nerves?
mechanical bending
how is action potential initiated in sensory nerve fibres (4)
opening of non voltage gated sodium channels produces depolarisation called receptor potential.
this causes current to flow to first node
at first node, current raises potential so node becomes depolarised.
when there is significant depolarisation, action potential is initiated.
where does the receptor potential generally generate first action potential in sensory nerve fibres?
the first node of Ranvier where regenerative sodium channels start to occur normally
What is the effect of a higher intensity of stimulus?
higher frequency of action potentials
define conduction velocity
speed with which an action potential travels along a nerve fibre
what is equation for conduction velocity of myelinated fibres
CV= 6*diameter
what is the equation for conduction velocity of unmyelinated fibres
CV=diameter (approx)
Function of A-beta fibres?
form variety of specialised endings in skin
Define encapsulated mechanoreceptors?
Connective tissue capsule surrounding A-beta fibres which tunes them to detect different types of mechanical stimulation
What are Meissner’s corpuscles?
nerve endings in the skin that detect light touch- rapidly adapting receptors
Where are meissner’s corpuscles most concentrated?
thick hairless skin such as finger pads
What are Merkel receptors?
mechanoreceptors which give information on light pressure and position- slowly adapting receptors
What are Pacinian corpuscles?
rapidly adapting mechanoreceptors that detect high frequency vibration of the skin
What are Ruffini corpuscles
slowly adapting receptors that detect heavy pressure on the skin
Define receptive field (RF)
area of skin that can activate a single sensory nerve fibre
Why does damage to a single nerve fibre not render skin anaesthetic?
Because receptive fields on individual nerves overlap so each skin in part of at least 2 receptive fields
Effect of smaller receptive fields? How can this be tested?
Better localisation of stimuli on the skin. Two point discrimination test
What do you cal nerve endings without a capsule?
free nerve endings
What do A-gamma and C fibres detect?
Both movement and chemical changes in the environement. Signal temperature and pain
Describe what happens when a peripheral nerve is cut (4)
distal Shwann cells unwrap themselves and divide to form continuous line of cells
proximal cut ends of nerve fibres form growth cones and grow back down inside the sheaths guided by cell adhesion molecules
Shwann cells proliferate and wrap myelin around regenerating nerve fibres
with time, nerve fibres enlarge but may never reach original diameter
