The Peripheral Nervous System

Question 1

Describe general structure of peripheral nervous system

Answer 1

Composed of cranial nerves (12) and spinal nerves (31) which coordinate information between CNS and body. Has a sensory (afferent) and motor (efferent) division. Sensory is composed of sensory neurons which transducts signals from receptors to CNS. Efferent division has two further divisions of autonomic and somatic. Somatic controls voluntary movement while autonomic has 2 divisions: sympathetic and parasympathetic. Sympathetic mobilises body systems while parasympathetic conserves energy.

Question 2

What are areas controlled by a single somatic efferent spinal nerve called?

Answer 2

Dermatome: An area of skin that is supplied by a single spinal nerve
Myotome: Group of muscles innervated by a single spinal nerve.

Question 3

What areas do afferent and efferent nerves of the visceral/autonomic system innervate?

Answer 3

AFFERENT: Carries information from the viscera (thoracic, abdominal and pelvic organs)
EFFERENT can be divided into sympathetic and parasympathetic.
Sympathetic efferent: innervate viscera and periphery (vasculature and sweat glands)
Parasympathetic efferent: viscera only

Question 4

Define ganglion, nucleus and plexus

Answer 4

Ganglion: Collection of cell bodies outside the CNS
Nucleus: Collection of cell bodies inside the CNS
Plexus: network of interconnecting nerves

Question 5

Where are cell bodies found?

Answer 5

All afferent fibres have cell body in spinal ganglia. Visceral efferent fibres have cell bodies in peripheral ganglia. Somatic efferent fibres have cell bodies in cranial nerve cell bodies.

Question 6

What are fasciculi?

Answer 6

These are bundles of peripheral nerves, made of three layers of connective tissue which bind these bundles together. Individual axons are surrounded by endoneurium and each individual fascicle is covered in perineurium. The external vascular layer is called the epineurium which surrounds the entire nerve.

Question 7

How are peripheral nerves classified?

Answer 7

1. Conduction velocity – ABC, A is fastest

2. Axonal diameter (sensory only) – Roman numerals, I is largest

Question 8

How are peripheral nerves structurally adapted for purpose?

Answer 8

Myelinated neurones with largest diameter are the fastest and used for motor to skeletal muscle function, followed by sensory from muscle spindle. Unmyelinated neurones have endings for pain and temperature detection in peripheral nervous system.

Question 9

How can sensory receptors classified?

Answer 9

Can detect external/internal information so classified based on source of stimulus or mode of detection. Source of stimulus can be external (exteroreceptors) or internal (interoreceptors - proprioreceptors/enteroreceptors).

Question 10

Name 5 types of receptors and their functions

Answer 10

Chemoreceptors: Detect molecules which bind to receptor
Photoreceptor: detects light in retina
Thermoreceptor: Detect temperature in skin
Mechanoreceptor: Mechanical opening of ion channels e.g. touch receptors in skin
Nociceptor: Detect tissue damage, usually interpreted as pain

Question 11

What are proprioceptors and what types are there?

Answer 11

Proprioceptors: Provide information about joint, muscle and tendon positions
•Muscle spindles: Detect changes in muscle length and form basis of reflex action
•Golgi tendon organ: Detect changes in tension in tendons
•Joint receptors: Found in joint capsule and detect when movement starts and ends

Question 12

What is a motor unit?

Answer 12

Single motor neuron together with all the muscle fibres that it innervates, forming the smallest functional unit with which force is produced – each motor neuron supplies about 600 muscle fibres so stimulation of one motor unit causes contraction of all the muscle fibres in that unit.

Question 13

What is a reflex action

Answer 13

An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli e.g. knee jerk reaction where end of quadricep muscle is stimulated to contract by hitting the patella ligament

Question 14

How does a monosynaptic reflex work?

Answer 14

Proprioceptor sends action potentials via the afferent nerve back towards the spinal cord, cell body of this sensory neuron in dorsal root ganglion. Signal then travels towards ventral/anterior horn onto a lower motor neurone which is activated and synapses onto original muscle causing the contraction.

Question 15

What is antagonist inhibition?

Answer 15

Multiple branches of sensory neurones which cause activation of interneurons that inhibit antagonist muscle. This helps reflex be carried out.

Question 16

Describe spinal visceral efferent fibre outflow

Answer 16

S system has outflow in thoracolumbar region of spinal cord – between T1 to L2. PS system’s preganglionic neurones emerge from brainstem, and sacral portion of spinal cord.

Question 17

What is the role of visceral sensory fibres?

Answer 17

Relay sensory information from core. Detects pain, fullness and blood pressure. Enter the spinal cord between T1-L2. Associated cranial nerves are glossopharyngeal and vagus nerve.

Question 18

What is the role of visceral motor fibres?

Answer 18

Provides outflow to core and body. Controls pupils, sweat glands, salivary glands, heart muscle and airways/ Outflow is from thoracolumbar region (T1-L2) and craniosacral (CN 3,7,9,10).

Question 19

Where are the cell bodies of sympathetic fibres found?

Answer 19

Thoracolumbar cord has an extra part of grey matter emerging known as the lateral horn which contains cell bodies of sympathetic outflow. As they are efferent fibres emerge from anterior aspect of cord and go off into some structures called sympathetic ganglia – these run outside the vertebral column and lie in the back of the chest.

Question 20

Describe route of innervation going out to periphery

Answer 20

If innervation going out to supply periphery (sweat glands, blood vessels etc.), it either synapses onto a secondary/post-ganglionic neuron which then travels up and out with mixed spinal nerve to distribute to skin. Or, can travel down to a sympathetic ganglia where it will synapse at that post-ganglionic neurone which will distribute signal to skin.

Question 21

What are ramus communicans?

Answer 21

There are small connecting nerves between sympathetic ganglia and the main nerves – known as ramus communicans which are branches. White ramus called so as it appears whiter in colour because the preganglionic neurone has a myelin sheath – this can then synapse and PGN may be an unmyelinated neurone which when it goes back to the mixed spinal nerve to be distributed, goes in a branch called grey ramus communicans.

Question 22

Describe sympathetic outflow to heart

Answer 22

At T1-4, preganglionic neurone emerges and heads out to sympathetic trunk out into white ramus where it synapses and sends a postganglionic nerve into the heart. This PGN is unmyelinated and collection of nerves surrounding heart known as cardiac plexus.

Question 23

Describe sympathetic outflow to viscera

Answer 23

Thoracic spinal nerves have pre-ganglionic neurones that go into the sympathetic chains but no synapse in the SC. The preganglionic nerve travels straight towards gut in greater, lesser and least splanchnic nerves which are just collections of neurones. Once close to organ, synapse in the ganglia which sits around the aorta – these are called pre-aortic ganglia where cell bodies of the post ganglionic neurones can be found. These send their fibres out to the organs themselves.

Question 24

What is the essential difference between sympathetic outflow to heart vs viscera?

Answer 24

The synapse that takes place between pre-ganglionic neurone and post-ganglionic neurone occurs in one of the paravertebral chain for the heart while for viscera, preganglionic neurones go through paravertebral chains and synapse in pre-aortic ganglia.

Question 25

What is a general rule of sympathetic outflow?

Answer 25

If sympathetic outflow going to periphery/heart, outflow goes into chains either side of vertebral column and synapse in the column (can be at diff levels) BUT cell bodies of postganglionic neurones found in these chains. If sympathetic outflow going to gut, it goes through sympathetic chains but doesn’t synapse there so cell bodies found in ganglia situated anterior to aorta (pre-aortic ganglia)

Question 26

What is the craniosacral outflow?

Answer 26

Craniosacral outflow occurs through cranial nerves 3,7,9 and 10 with specialised ganglia where the PreSN and PSN synapse. Vagus nerve travels out of cranial cavity and supplies heart with parasympathetic outflow + some gut structures. Sacral parasympathetic outflow causes pelvic functions

Question 27

What is the effect of symapethetic innervation on the eye, salivary glands, heart and lungs?

Answer 27

Eye = Dilation
Salivary glands = Inhibition of saliva
Heart = Increase heart rate
Lungs = Bronchodilation

Question 28

What is the effect of parasymapethetic innervation on the eye, salivary glands, heart and lungs?

Answer 28

Eye = Constriction
Salivary Glands = Stimulation of saliva
Heart = Lower heart rate
Lungs = Bronchoconstriction