Peripheral Nervous System

Question 1

PNS consists of?

Answer 1

Everything but the brain and the spinal cord

Question 2

PNS function?

Answer 2

Communicating centre between the body and the CNS
Divided in to:
Sensory - sensory neurons, sens receptors to CNS
Motor - motor neurons, CNS to effectors

Motor divided into:
Autonomic - involuntary responses
Somatic - voluntary movement

Autonomic divided into:
Sympathetic - mobilses body system, fight/flight
Parasympathetic - conserves energy, rest+digest

Somatic Sensory and Somatic Motor are found in ALL spinal nerves

Autonomic are found in MOST spinal nerves

Question 3

How does the PNS develop?

Answer 3

Neural Ectoderm gives rise to the CNS
Non-Neural Ectoderm to the Skin

At Non-Neural/Neural Junction = neural fold = neural crest cells.
From Neural Crest Cells - sensory neurons arise from here.

Question 4

Sensory Neurons?

Answer 4

Pseudo-Unipolar (1 process but divided into 2)
Dendrites located in the sensory regions (skin) where receptors are.
Axon projects to CNS direction of conduction (central process)

A fibres: myelinated somatic
B fibres: myelinated visceral
C fibres: unmyelinated somatic and visceral pain afferents

The cell body of the somatic sensory axon is in the Dorsal Root Ganglion

Question 5

What is a ganglion?

Answer 5

A collection of nerve cell bodies

Question 6

Describe the Somatic Nervous system?

Answer 6

Consists of 2 neuron types:
Upper Motor neuron of the CNS exclusively
Lower Motor Neuron of the PNS exclusively - multipolar motor neuron
synpase = in brainstem/CNS

Route begins in the primary motor cortex, can be voluntary or reflexive, nerves terminate in the neuromuscular junction to produce motor response.

Question 7

Describe the PNS nerves?

Answer 7

43 pairs

12 Cranial

31 Spinal - come out of the intervetebral foramen (cervical, thoracic, lumbar, sacral, coccygeal)

Question 8

Describe the Autonomic Nervous system?

Answer 8

Efferent Nerves and Ganglia stimulating effectors outside our control.
Subdivided into Sympathetic and Parasympathetic
Has 3 Neuron types

1) neuron from hypothalamic nuclei to brain stem nuclei/spinal cord
2) preganglionic neuron from bs nuclei/spinal cord to autonomic ganglia (preSYNAPTIC neuron if from vagal nerve + sacral parasympathetic nerves) - MYELINATED
3) Postganglionic neuron from autonomic ganglia to visceral effectors (postsynaptic neuron if associated with vagus nerve or sacral parasympathetic nerves) - UNMYELINATED

The Preganglionic Autonomic Motor Neuron sends its axon through the VENTRAL ROOT and into the spinal nerve

Question 9

What is Myelin?

Answer 9

Originates from Schwann cell wraps around axon
Major Dense Line = Schwann cell cytoplasm is condensed
Minor Dense Line = plasma membrane of the SC
Junctions = Nodes of Ranvier (site of saltatory conduction)

A fibre = 100 layers
C fibre = Only cytoplasm of Schwann cell ( 1 layer) (call it unmyelinated)

Question 10

Speed of Impulse propagation?

Answer 10

Neurons in the PNS are grouped based on:
Diameter
Signal Conduction
Velocity
Myelination State (Aalpha,beta,delta,gamma,B + C)

Propagation speed of nerve not related to strength of Impulse:
Larger fibres = faster impulse due to size (less resistance)
Myelinated fibres = faster impulses (saltatory conduction)

Question 11

Types of fibres and their properties?

Answer 11

A fibres (5-20 um, 130m/sec)

• myelinated, somatic
• sensory + motor to skeletal muscle

B fibres - medium (2-3 um, 15m/sec)

• myelinated, visceral
• sensory + autonomic preganglionic

C fibres - smallest (0.5-1.5um, 2m/sec)

• unmyelinated
• sensory + autonomic motor neurons

Question 12

Describe Autonomic Ganglia?

Answer 12

An autonomic ganglion is a cluster of nerve cell bodies (a ganglion) in the autonomic nervous system. The two types are sympathetic ganglion and parasympathetic ganglion.

Question 13

Describe Autonomic Parasympathetic Ganglia?

Answer 13

4 Cranial Parasympathetic ganglia, synpasing pre+post parasympathethic neurons:

• Ciliary
• Submandibular
• Otic
• Pterygopalatine

Question 14

Describe Autonomic Sympathetic Ganglia?

Answer 14

Sympathetic Ganglion:
- Paravertebral ganglia, are located just ventral and lateral to the spinal cord.
- 22-23 pairs of these ganglia,
interaction of the nerve fibres in the sympathetic nervous system begins at the spinal cord
- Here four options are available to the fibres:
(1) they can run up the chain and synapse,
(2) they can synapse at the level of entry,
(3) they can pass straight through and synapse elsewhere - such as in the case of T5-12 (the splanchnic nerves), or
(4) they can enter the chain and descend to synapse.

exiting the sympathetic chain, the fibres enter a less-myelinated gray ramus communicans

Question 15

What is a Dermatome?

Answer 15

Areas of skin supplied by a SINGLE sensory spinal nerve root.
Looking at the Dorsal Root of the Spinal Nerve
Stacked along the thorax and abdomen
Longitudinally along the limbs
Can have overlap of dermatomes
Clinical significance: can determine the site of spinal damage by simple pin prick exam

Question 16

What about Visceral Afferents

Answer 16

conduct sensory impulses (usually pain or reflex sensations) from the internal organs, glands, and blood vessels to the central nervous system.

• not part of the ANS, but uses sympathethic nervous system to get to the spinal dorsal root.

This explains the phenomenon of referred pain. is pain perceived at a location other than the site of the painful stimulus.
- associated AT THE CORRESPONDING DERMATOME(S) OF THE SPINAL DORSAL ROOT(S)

Question 17

What is a myotome?

Answer 17

A myotome is the group of muscles that a single spinal nerve ROOT innervates.

• supplied by the VENTRAL ROOT.
• clinically significant to test for root level muscle weakness

Question 18

Peripheral Neuropathies?

Answer 18

• Associated with damage/disease to the PNS nerves
• may impair sensation, movement, gland, organ function
• sensory nerves (sensation): pain, tingling, numbness
• motor nerves (mvt): weakness to hands + feet
• autonomic nerves (involuntary function): changes to heart rate or blood pressure

Question 19

PNS nerve structure?

Answer 19

Cross section:

Axon
Endoneurium - around each individual axon + loose collagen fibrils
Perineurium - around fascicle (neuron bundle), gives main tensile strength to nerve
Epineurium - Around the entire nerve, Dense, collagenous Blood supply Some fatty tissue

Question 20

Causes of Peripheral Neuropathies?

Answer 20

Mononeuropathy: refers to a single nerve being affected,
Polyneuropathy: refers to several nerves being affected
Causes can be metabolic, toxic, inflammatory, traumatic, genetic
~30% of neuropathies are “idiopathic” - i.e. of unknown origin

Question 21

Peripheral Neuropatheis - Neuropraxia?

Answer 21

• reversible conduction block
• selective demyelination of the axon sheath
• endoneurium and axon still intact - e.g., nerve compression

Question 22

Peripheral Neuropathies - Axonotomesis?

Answer 22

• demyelination and axon loss
• epineurium and perineurium remain intact
• still some continuity within the nerve
• degeneration occurs below and slightly proximal to the site of injury

Question 23

Peripheral Neuropathies - Neurotmesis?

Answer 23

most severe form of nerve injury
associated with complete nerve division and disruption
commonly seen after toxic or ischemic injuries
Damage to the epineurium (around the entire nerve) - no nerve growth

Question 24

What is Wallerian Degeneration?

Answer 24

a process that results when a nerve fiber is cut or crushed and the part of the axon distal to the injury (i.e. farther from the neuron’s cell body) degenerates.
This is also known as anterograde or orthograde degeneration.