overview of pns Flashcards
how many sets of dendrites do sensory neurons have?
2 sets
where is the cell body of a sensory neuron?
on a t-junction off of the axon in the dorsal root ganglia
where do sensory neurons enter the spinal cord?
in the dorsal root
how do motor neurons exit the spinal cord?
via the ventral root
where are motor cell bodies found?
in the grey matter in the ventral root
how are spinal nerves formed?
from the fusion of dorsal and ventral roots
what happens to sensory axons when dorsal roots are severed between dorsal root ganglion and the spinal cord?
sensory axons cannot regenerate in the spinal cord
at what points can spinal nerves be damaged?
where they leave the spinal cord and pass through the IV foramina
at what spinal level is CSF sampling done?
L3/L4
what is myelin?
sheath of fatty insulation wrapped around axons
what cells produce myelin?
Schwann cells
how do Schwann cells produce myelin?
Schwann cells wrap themselves around the axon multiple times
Multiple layers act as an electrical insulator around the nerve
what is meant by myelin having a ‘low electrical capacitance’?
charge cant be stored so current is forced to the nodes
explain how myelin can lead to a demyelinating disease?
myelin has antigenic proteins on its surface
can trigger an autoimmune response
leads to demyelinating disease of peripheral nerves
what happens in a demyelinating disease?
- Schwann cell dies so there’s a larger node of Ranvier.
- AP can’t jump the gap causes intermittent conduction as node gets wider
- As the autoimmune disease dies down, the myelin regenerates
- Nerve isn’t damaged
what is a node of ranvier?
small gap in the myelin where the 2 sheaths meet
what is the endoneurium?
thin protective membrane which surrounds individual sensory/motor nerve fibres
what is the perineurium?
surrounds fascicles – groups of functionally related nerve fibres
what is the epineurium?
thick connective tissues protecting several fascicles bundles together with blood vessels
do small axons have their own Schwann cells or do they share?
share
do grown axons each have their own Schwann cell or do they share?
have their own
what are sensory receptors?
how the peripheral branches of the sensory nerve fibres end in the skin/muscle
what are the two types of sensory receptors?
free nerve endings
capsules
what are free nerve endings?
sensory nerve branches end up lying in the extracellular space between tissue cells
form a fine nerve plexus in dermis and other tissues
what stimuli do free nerve endings respond to?
chemical
what are capsules?
specialised connective tissue surrounds nerve endings
what does the capsule determine?
the kind of stimulus the nerve ending will be sensitive to e.g. slow pressure, vibration, stretch
what is the function of an A-alpha alpha-motoneuron?
motor to skeletal muscle
what is the function of A-alpha 1a/muscle spindle afferent?
sensory from muscle spindle
what is the function of A-alpha 1b/golgi tendon afferent?
sensory from Golgi tendon organ
what is the function of A-beta general sensory afferents?
sensory from skin, viscera etc.
sensory from secondary endings in muscle spindles
what is the function of A-gamma gamma-motoneurons?
motor to muscle spindles
what is the function of A delta nociceptor/thermoreceptor?
fast pain from skin, muscle, joints, thermoreceptors
what is the function of C nociceptor/thermoreceptor?
slow pain from skin, muscle, viscera, thermoreceptors
how does the amount of myelin change as you go from A-alpha to A-delta?
amount of myelin decreases
what are examples of encapsulated endings?
Meissner’s corpuscles
Pacinian corpuscles
Ruffini’s
what does Meissner’s corpuscles detect?
sensitive to touch
what do Pacinian corpuscles detect?
vibration and pressure
what does Ruffini’s detect?
pressure and proprioception
what is the main type of receptor in hairy skin?
hair follicle receptors - hybrid form of free and encapsulated endings
what do hair follicle receptors respond to?
hair displacement
what receptors does glabrous/non-hairy skin have?
Meissner’s corpuscles, Ruffini corpuscles and Pacinian corpuscles.
what are capsules made up of?
connective tissue
explain the process of a capsule forming
nerve fibre grows into tissue bare and unencapsulated
cytokines released from bare end of the nerve fibre
causes local connective tissue cells to form a capsule around it
what frequency do Pacinian corpuscles make nerve endings sensitive to?
high frequency > 50Hz
how are encapsulated sensory axon endings activated?
by physical distortion of their terminal membrane
explain the process of the initiation of sensory nerve fibres
when axon is bent, Na+ ions enter through mechanically sensitive sodium channels in their membrane –> depolarisation (receptor potential)
receptor potential triggers action potential
stronger receptor potential = high AP frequency
what is the maximum frequency of action potential firing limited by?
refractory period of the axon
what are rapidly adapting capsules?
only respond at the beginning of a stimulus – they fatigue after a second or to a sustained steady stimulus
name rapidly adapting capsules
Pacinian corpuscles
Meissner’s corpuscles
what are slow adapting capsules?
continue firing to a sustained stimulus but at a gradually reducing rate
name slow adapting capsules
Ruffini’s endings, Merkel’s disks
what is the receptive field?
the area of skin innervated by a single nerve fibre
where do we have small receptive fields?
on skin we use for tactile discrimination – e.g. our skin. Allows us to localise stimuli
where do we have large receptive fields?
areas not used for tactile discrimination - limbs
how does the size of the discriminative field change the more distal you go?
gets smaller
how does the brain localise a stimulus?
by processing info from many fibres simultaneously
what does a progressive loss of nerve fibres lead to?
leads to a progressive worsening in ability to localise stimuli as there is less and less overlap
name a condition where there is progressive loss of nerve fibres
diabetic neuropathy
what happens when a peripheral nerve is cut?
the distal part disconnected from its cell body and degenerates
distal Schwann cells unwrap themselves from dead fragments
divide to form a continuous line of cells lining the distal endoneurial sheaths
Proximal cut ends form growth cones
grow back down inside the sheaths guided by chemical factors (cell adhesion molecules) on the surface of Schwann cells
explain how an axon repairs itself after a peripheral nerve injury
microtubules transport growth-related materials down to the growth cone
actin filaments at the edge of the cone extend out in filopodia
tips of filopodia attach to the tissues
actin contracts - pulls cone towards denervated tissue
schwann cells proliferate behind the growth cone
start to wrap myelin around nerve fibre
at what rate do nerve fibres regenerate?
1.5mm/day
how does the likelihood of successful regeneration change as a nerve injury gets more distal?
increases
