Overview of the Peripheral Nervous System Flashcards
Is the distinction between the central and peripheral nervous systems arbitrary?
No, the nerve cells have different immune and antigenic properties
Where are nerves often damaged?
When they enter/leave the spinal (vertebral) canal, as this ordinarily works to protect the spinal cord
How might damage to vertebrae or IV discs lead to damage of sensory nerves?
Dorsal root ganglia may lie in the intervertebral foramen and therefore damage can crush these spinal nerves
How do peripheral nerves obtain a blood supply?
Have small arteries within them to supply oxygen to the nerves themselves, and these arteries are branches of the spinal cord arteries
What is a dermatome?
The area of skin innervated by a spinal nerve
Describe lower motor neurones
Connect directly with skeletal muscle and have cell body in ventral horn (axons project from ventral root and synapse on muscle) and have a single axon projecting from the cell body which has a number of dendrites
Describe upper motor neurones
Emerge from the brainstem or brain and synapse with lower motor neurones
Describe sensory neurones
Have two sets of dendrite-like processes (one in periphery, other in spinal cord), cell body is T junction off of the axon and lies in the DRG, they end in the skin or muscle as sensory receptors which have a specialised connective tissue capsule
Briefly describe myelination of a peripheral nerve
During growth of peripheral nerve the Schwann cell wraps itself many times around the axon to gradually squeeze out the cytoplasm until multiple layers of cell membranes are left which is then known as the myelin sheath
What is endoneurium?
A thin protective membrane surrounding individual sensory and motor fibres
What is perineurium?
Groups of functionally-related nerve fibres are collected in fascicles and surrounded by layer of CT
What is epineurium?
Deep fascia that surrounds a whole peripheral nerve which consists of several fascicles bundled together with blood vessels
Where are sensory receptors predominantly in the skin
Epidermal-dermal junction
How are action potentials initiated in sensory nerve fibres?
Receptors have non-voltage-gated sodium channels which are opened by mechanical bending, when this occurs –> sodium influx via mechanosensitive channels –> graded depolarisation of a few mV –> generator potential formed –> generates action potentials in the axon where sodium-channels start to occur (first node of Ranvier)
What is a generator potential?
The graded depolarisation formed in sensory afferents as a result of stimulation (mechanical bending)
What does a greater generator potential lead to?
The greater the generator potential, the greater the frequency of action potentials produced
What type of receptor are free sensory nerve endings?
Chemoreceptors
Describe free sensory nerve endings
Form very fine nerve plexus which will then respond to chemical stimuli such as changes in pH, or chemicals such as peptides in ECF
What is ‘noxious’ stimuli?
Actually, or potentially damaging tissue event
What is the nerve fibre for free nerve endings?
C or Aδ
Define conduction velocity
Speed at which an action potential travels along a nerve fibre
How do you calculate conduction velocity?
Myelinated fibres: 6 x diameter
Unmyelinated fibres = diameter
Describe and give examples of nerves with Aα nerve fibres
Largest diameter and velocity: alpha motorneurones (motor to skeletal muscle), 1a afferents (sensory from muscle spindle) and Ib afferents (sensory from Golgi tendon organ)
Describe and give examples of nerves with Aβ nerve fibres
2nd fastest velocity; general sensory afferents (sensory from skin, viscera and secondary endings)
Describe and give examples of nerves with Aγ nerve fibres
Medium velocity; gamma motoneurones (motor to muscle spindles)
Describe and give examples of nerves with Aδ nerve fibres
Small diameter myelinated fibres; nociceptor and thermoreceptors (carries ‘fast’ pain from skin, muscle, joints and thermoreceptors)
Describe and give examples of nerves with C nerve fibres
Unmyelinated fibres; nociceptor or thermoreceptor (carries ‘slow’ pain from skin, muscle, viscera and thermoreceptors)
Describe nerve regeneration
Nerve fibres of PNS regenerate at about 1.5mm/day, however some never regenerate’ the further distal the nerve injury is, the more likely there is to be successful regeneration
Explain how action potentials are generated in motor neurones
Sensory neurones synapse on the dendrites and cause a small depolarisation (EPSP) and summation (spatial or temporal) leads to the development of an action potential in the motor neurone
Describe temporal summation
when one input fires two EPSPs close together in time, this may trigger an AP
Describe spatial summation
when two different synaptic inputs are active at the same time so the two EPSPs add together and may trigger an AP
Explain the biochemistry of summation on motor neurones
Summed EPSPs are due to an inward ionic current (sodium influx) caused by sensory axons releasing glutamate anion into the synapse and causing the AMPA receptor to open, allowing sodium influx into the cell. Current flows down and depolarises the whole cell, and the action potential begins at the axon hillock.
