Week 3 - PNS Flashcards
What does the CNS consist of?
brain and spinal cord
What is the function of the CNS ?
Integration and analysis of information, production of responses
What does the PNS consist of?
outside the CNS comprising nerves and ganglia
What is the function of the PNS ?
reception of information and transmission of response (including homeostatic regulation)
What is the PNS made up of?
sensory and motor components
What are the sensory components of the PNS?
- somatic and visceral sensory: sensory ganglia anf nerves, receptors
- special sensory eg. retina -eye
What are the motor components of the PNS?
- somatic motor: motor nerves
- visceral motor (autonomic): sympathetic/parasympathetic/enteric divisions, autonomic ganglia and nerves
Where do some motor components such as motor neuron cell bodies originate from?
brainstem and spinal cord
Where are the axons of the motor neurons (efferent) going to skeletal muscle found?
PNS
How do motor components enter and leave the spinal cord?
- motor (efferent) axons exit in ventral root
- sensory (afferent) axons enter in dorsal root
Where do motor (efferent) neurons have their cell bodies?
in the grey matter in the ventral spinal cord
Where are sensory neuron cell bodies located?
in the dorsal root ganglion - peripheral
- sent info into spinal cord via ganglion
What is the spinal cord?
the major origin of the PNS
How is the spinal cord protected?
cushioned by a layer of fat inside the vertebral canal, contained in meninges
What is the rout of nerve roots int he spine?
emerge from cord leaving the spine through intervertebral foramina
Ventral root contains
motory output
Dorsal root contains
sensory input
What are spinal nerves?
nerver roots fused together
Is the intervertebral foramen resistant to damage?
no, damaged easily - may lead to motor loss, sensory alterations
How many pairs of spinal nerves are there?
31 pairs
What do spinal nerves do?
- arise from spinal cord roots
- supply all parts of the body except the head
How are spinal nerves named?
according to their point of issue between vertebrae
Name all spinal nerves:
- 8 cervical (C1-C8)
- 12 thoracid (T1-T12)
- 5 lumbar (L1-L5)
- 5 sacral (S1-S5)
- 1 coccygeal (C0)
What arises from spinal nerves?
peripheral nerves - mixed motor and sensory
What peripheral nerve is in the trunk ?
single major intercostal nerve - from a single spinal level
What peripheral nerve is in the neck and limbs?
multiple spinal levels contribute to 4 plexi and multiple nerves
name all 4 flexi from top to bottom
- celvical plexus
- brachial plexus
- lumbar plexus
- sacral plexus
What is the difference between spinal and peripheral nerves?
Intercostal nerve comes from ventral ramus of spinal nerve
What does the spinal nerve divide into?
dorsal ramus and ventral ramus
What area is supplied by the dorsal ramus?
skin and muscles of the back
What area is supplied by the ventral ramus?
skin and muscles of chest abdomen and limbs
How do ventral rami supply the limbs?
via an interwoven network of nerve fibres - a nerve plexus
What are ramus communicans ?
further branches of the spinal nerves at spinal levels T1-L3
Where do ramus communicans connect to?
sympathetic chain - series of linked sympathetic ganglia, lying on either side of the vertebral column
What is a dermatome?
a strip of skin innervated by sensory axons of a single dorsal nerve root (or spinal nerve)
Do peripheral nerves match the territory of dermatomes?
no it doesn’t, as they have contributions form more than one dorsal root
What are nerve coverings made up of?
epineurium
perineurium
endoneurium
What is the epineurium?
- continuation of dura mater of the CNS
- provides mechanical strength for a nerve
What are fascicles?
axons in a nerve grouped into bundles
What is the perineurium?
continuation of arachnoid mater that surrounds fascicles
- cells have tight junctions
- form barrier between nerve and external environment
What is the endoneurium?
- continuous with Pia meta
- comprises vascular connective tissue surrounding the axons and their Schwann cells
What are Schwann cells?
- supportive glial cells in nerves
- vital for Acton function: maintain the environment ie. pH, glucose at correct levels
Do all nerve fibres need Schwann cells?
Yes
What do single myelinated fibres have?
a personal set of Schwann cells that wrap themselves around the axon supporting and forming myelin
How many unmyelinated cells can a single Schwann cell support?
up to 20
What difference is there in peripheral nerve fibres (axons)?
different sizes and different functions
What is the advantage of larger diameter fibres?
- have more myelin
- conduct action potentials more rapidly
What fibres have the slowest conducting?
unmyelinated C-fibres
What will later axons be?
motor neurones - to be quick, allowing for rapid movement
Name peripheral nerve fibres from large to smallest
lower motor neurone - alpha
Touch receptors
delta fibres - sharp pain, cold, preganglionic autonomic
C fibres - unmyelinated, slowest. slow pain, heat, itch, postganglionic autonomic
Somatosensory - muscle stretch reflex
contraction of extension muscle - kick
opposing muscle (flexor) must be inhibited - inhibitory interneuron
- receptor sensing stretch at terminals of sensory neuron in sensory root of ganglion
- in via dorsal root into spinal cord
What is signal transduction?
process of converting the energy of a stimulus into an electrical signal
How is single transduction achieved?
- in somatosensory afferent endings
- through the opening of cation channels
- producing depolarising receptor or generator potential
- sufficient magnitude: reaches threshold necessary for action potential firing by afferent
- rate of firing roughly proportional to magnitude of depolarisation
Do sensory nerve fibres have different endings?
Yes
What are most mechanoreceptive endings (AAlpha, Abeta) like?
- encapsulated by receptor cells and structures that help tune the afferent to particular features of stimulus
What are free nerve endings?
- lack specialised cells
- eg. Agamma, C fibres
How do free nerve endings transduce the signal?
- through receptor linked ion channels within their plasma membranes
- detect chemical changes in environment eg. pH or physical stimuli eg. heat, movement
What are afferents that detect chemical changes in environment eg. pH or physical stimuli eg. heat, movement?
- nociceptors - detecting injury of painful stimuli
- thermoreceptors
- some might be mechanoreceptors
Name all encapsulated endings of mechanoreceptors in skin
- Meissner’s afferents
- Merkel cell afferents
- Fuddini afferents
- Pacinian afferents
What are Meissner’s afferents?
detect skin motion, grip control
What do Merkel cell afferents do?
- detect eges, points, curvature
- form and texture perception
What do Ruffini afferents do?
- detect skin stretch
- tangential force, hand shape and motion direction
What do Pacinian afferents do?
- detect vibration
- perception of stint events through transmitted vibrations
- tool use
What is a receptive field?
the area of skin innervated by a single sensory afferent
Are there more densely innervated or less innervated areas in the body?
more less enervated - eg. forearm back, instead go densely innervated such as fingers and face
Does loss of one afferent produce an anaesthetic area?
no, because there is overlap
What is the 2 point discrimination threshold test ?
- measures min inter-stimulus distance required to perceive 2 simultaneous stimuli as distinct
- reflects somatic spatial acuity and receptor density
- areas where the most accurate sensory discrimination is needed have the lowest threshold eg, fingers, hand , face, lips
How is visceral sensory information conveyed to the CNS?
- dorsal root ganglia send visceral afferents through sympathetic nerves
- include mechanoreceptor specialisations or free endings sensitive to pressure or stretch, mechanical and harmful endings
What are sensory ganglia’s associated with?
- parasympathetic ns
- eg. ganglia of C11,10
- convey general visceral sensory input to CNS to coordinate visceral motor activity
What are proprioceptors?
- muscles, tendon, joined mechanoreceptors
What do proprioceptors and skin mechanoreceptors provide?
detailed, continuous information about the position of our limbs and other parts in space
What do proprioceptors do?
- encode information relating to stretch and tension of muscles
- sensation of position, movement and force
- essential for accurate performance of complex tasks
What are neuromuscular junctions?
synaptic connection between motor neuron and skeletal muscle fibre
MOTOR EFFERENT
