2.10 - Peripheral Nervous System

Question 1

What can the PNS be divided into?

Answer 1

• somatic and autonomic nervous systems
• autonomic –> sympathetic and parasympathetic
• sensory (afferent) nerves carry information towards CNS
• motor (efferent) nerves carry information away from CNS

Question 2

How many pairs of cranial and spinal nerves are there?

Answer 2

• 12 pairs of cranial nerves
• 31 pairs of spinal nerves

Question 3

What do somatic afferent and efferent nerves convey information from and to?

Answer 3

• somatic afferent nerves convey information from skin, skeletal muscle and joints
• somatic efferent nerves convey information to skeletal muscles

Question 4

What are dermatomes and myotomes?

Answer 4

• dermatome - an area of skin that is supplied by a single spinal nerve
• myotome - group of muscles innervated by a single spinal nerve

Question 5

What do visceral (autonomic) nerves convey information from and to?

Answer 5

• visceral afferent nerves carry information from the viscera (thoracic, abdominal and pelvic organs) to the CNS
• visceral efferent nerves can be divided into sympathetic and parasympathetic
• sympathetic efferent nerves innervate the viscera (organs) and periphery (vasculature and sweat glands)
• parasympathetic efferent nerves innervate the viscera (organs) only

Question 6

What is a ganglion?

Answer 6

• a collection of cell bodies outside the CNS
• all afferent (somatic and visceral) fibres have their cell bodies in spinal ganglia
• visceral efferent nerves synapse in a peripheral ganglion

Question 7

What is a nucleus?

Answer 7

Collection of cell bodies inside the CNS

Question 8

What is a plexus?

Answer 8

Network of interconnecting nerves

Question 9

How are peripheral nerves arranged at spinal vertebrae?

Answer 9

• arranged in fasciculi - a bundle with layers of connective tissue surrounding it
• three layers of connective tissue:
  1. individual axons (covered in Schwann cells) have the endoneurium around them
  2. axons bundle together to form a fascicle which is surrounded by the perineurium
  3. fascicles bundle together to form the spinal nerve, with the epineurium around it

Question 10

How can peripheral nerves be classified?

Answer 10

• two classification systems
• one based on conduction velocity - uses letters A, B & C (A is the fastest)
• one based on axonal diameter (sensory only) - uses Roman numerals I-IV (I is the largest diameter)
• myelinated neurones have bigger axonal diameters and faster conduction velocities (e.g. motor to skeletal muscle; sensory from muscle spindle), thin unmyelinated neurones are the opposite (e.g. postganglionic autonomic fibres; sensory from free nerve endings for pain and temperature)

Question 11

What are sensory receptors and how can they be classified by source of stimulus?

Answer 11

• they detect external or internal information
• can be classified by source of stimulus:
• exteroceptors respond to external stimuli e.g. pain, temperature, touch, pressure
• internal receptors can be proprioceptors or enteroceptors and respond to internal stimuli
• proprioceptors respond to movement and joint position
• enteroceptors respond to movement through gut and blood pH

Question 12

How can sensory receptors be classified by mode of detection?

Answer 12

• transduction - changing a signal to electrical impulses
• chemoreceptors - detector molecules which bind to receptor e.g. in olfactory bulb
• photoreceptors - light in retina
• thermoreceptors - temperature in skin
• mechanoreceptors - mechanical opening of ion channels e.g. touch receptors in skin
• nociceptors - tissue damage, interpreted as pain

Question 13

Where are proprioceptors located?

Answer 13

• muscle spindles - small sensory organs in skeletal muscles, detect changes in muscle length = whether they have been stretched, and they are the basis for simple reflex actions
• Golgi tendon organs - detect changes in tension in tendons (the more a muscle contracts against a bone, the more tension)
• joint receptors - found in joint capsules surrounding joints, detect start and end of movement

Question 14

What is a neuromuscular junction?

Answer 14

Specialised synapse between a motor neurone and a muscle fibre

Question 15

What are motor units?

Answer 15

• a single motor neurone together with all the muscle fibres that it innervates
• smallest functional unit with which to produce force
• stimulation of one motor unit causes contraction of all the muscle fibres in that unit
• approximately 420,000 motor neurones and 250 million skeletal muscle fibres; on average each motor neurone supplies about 600 muscle fibres
• some neurological conditions when motor nerves are lost cause reinnervation of muscle fibres that have been denervated

Question 16

What is a reflex action?

Answer 16

An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli

Question 17

Describe the reflex action when a tendon hammer hits the patella ligament

Answer 17

1. stretching stimulates sensory receptor (muscle spindle)
2. sensory neurone activated
3. sensory neurone activates motor neurone within the spinal cord (integrating centre)
4. motor neurone is activated
5. effector (quadricep muscle of thigh) contracts

Question 18

What is the autonomic nervous system?

Answer 18

• has visceral afferent and efferent parts
• efferent part has sympathetic and parasympathetic
• have broadly opposing actions (fight or flight, rest and digest)
• have pre- and post-ganglionic neurones
• release acetylcholine at pre-ganglionic synapses
• release noradrenaline (S) or acetylcholine (PS) at post-ganglionic neurones
• S nerves in spinal cord T1-L2
• PS nerves in brain and in base of spinal cord

Question 19

What are visceral sensory neurones?

Answer 19

• relay sensory information from the core (afferent)
• dull pain, fullness, blood pressure
• run to T1-L2, S2-S4 and cranial nerves IX & X

Question 20

What are visceral motor neurones?

Answer 20

• outflow to core and body wall (efferent)
• controls pupils, sweat glands, salivary glands, heart muscle, airways
• sympathetic outflow = thoracolumbar outflow (pre-ganglionic sympathetic neurones only emerge from T1-L2)
• parasympathetic outflow = craniosacral outflow (cranial nerves III, VII, IX, X) - pre-ganglionic neurones emerge from brainstem and sacral portion of spinal cord

Question 21

What is the anatomy of sympathetic outflow to the periphery?

Answer 21

• the thoracolumbar cord in the middle (T1-L2) has an extra part of grey matter in the middle (lateral/anterolateral horn) - contains cell bodies of preganglionic neurones
• the outflow (efferent fibres) emerge from anterior aspect of the cord and go into sympathetic ganglia and sympathetic chains
• these chains (aka paravertebral trunks) are outside the protection of the vertebral column and lie inside the chest at the back

Question 22

Neurones going to the periphery that are coming out of thoracolumbar cord and entering sympathetic chains can do one of two things - what are they?

Answer 22

• can synapse onto secondary (postganglionic) neurone that can travel up mixed spinal nerve of cervical cord region to be distributed to the skin it needs to innervate
• can travel down to another sympathetic ganglia where it will synapse onto a secondary (postganglionic) neurone that will be distributed to skin via mixed spinal nerve

Question 23

What is a ramus communicans?

Answer 23

• in both cases the preganglionic neurone first travels a little way down the mixed spinal nerve at the thoracolumbar region and then turns into a small interconnective nerve into the sympathetic ganglia - called a ramus communicans
• there are two - white and grey
• white - where the preganglionic neurone goes into which is myelinated so looks white
• grey - where the postganglionic neurone crosses into the mixed spinal nerve and is unmyelinated so looks grey

Question 24

What is the anatomy of sympathetic outflow to the heart?

Answer 24

• between T1-T4, sympathetic preganglion neurone goes out towards sympathetic chain via white ramus
• T1 can send out a preganglionic neurone into the sympathetic ganglia where it goes up to another ganglia in the cervical region where it synapses onto a postganglionic neurone that it sends out to heart
• collection of nerves surrounding heart is called the cardiac plexus

Question 25

What is the anatomy of parasympathetic outflow to viscera?

Answer 25

• also called the craniosacral outflow, via some cranial nerves in head (III, VII, IX, X)
• specialised ganglia where preganglionic neurones synapse with postganglionic neurones to go off and do things like lacrimation (tear production), constrict pupils, cause parotid/submandibular/sublingual salivary glands to produce saliva
• vagus nerve travels out of cranial cavity and supplies heart with PS outflow and some gut structures
• sacral preganglionic nerves (S2-S4) go to some pelvic viscera to carry out excretion / activation of pelvic viscera like bladder and rectum