3 - Overview of the Peripheral Nervous System

Question 1

What endings does the PNS include and to what point?

Answer 1

PNS includes the sensory and motor nerves from endings to point where they enter/leave spinal cord

Question 2

• What 2 nervous systems does the PNS include?
• Which foramen do the nerves from the PNS enter?
• How are the nerves named and why is the cervical region special?

Answer 2

• Autonomic and enteric nervous system
• Intervertebral foramina
• Nerves are given the name of the vertebra immediately above them (except for the cervical nerves C1-C7 that are named by the vertebra below them)

Question 3

Where do motor nerve fibres arise from?

Answer 3

Cell bodies in the spinal cord and is protected in the spinal canal, with lamina bone of vertebra laterally and spinous process dorsally.

Question 4

How many peripheral nerve arises from how many spinal roots?

Answer 4

1 peripheral nerve arises from at least 2 spinal roots

Question 5

What are myelinated axons surrounded by?
What does the perineurium surround?
What does the epineurium surround?

Answer 5

• Surrounded by endoneurium - thin connective tissue layer
• Nerve fibre bundled into fascicles
• All nerve fascicles together with blood vessels. Epineurium = thickest layer of connective tissue with lots of collagen

Question 6

Describe the Dura. Covers what? Continuous with?

Answer 6

Dura covers cord in central canal. Dura is continuous with epineurium, outside nerve sheath. Fascicles covered with perineurium. Single fibres covered with endoneurium

Question 7

Where are cell bodies of sensory nerves found?

What are sensory nerve fibre also used to describe?

Answer 7

• Dorsal root ganglion - they enter the dorsal root of spinal cord
• Anatomical axons not always taking action potential to the synapse

Question 8

Where are cell bodies of motor nerves found?

Answer 8

Ventral horn of spinal cord and exit through the ventral root and then through the intervertebral foramina

Question 9

What do the upper motor neurons connect?

Answer 9

Brain to the ventral cord of spinal cord

Question 10

What do the lower motor neurons connect?

Answer 10

Ventral root to skeletal muscle by travelling through a peripheral nerve. LMN also have a single axon emerging froma cell body/. The cell body has a large number of dendrites.

Question 11

Define Myelination

Answer 11

Myelination is the process of coating the axon of each neuron with a fatty coating called myelin, which protects the neuron and helps it conduct signals more efficiently. Myelination begins in the brainstem and cerebellum before birth, but is not completed in the frontal cortex until late in adolescence.

Question 12

What are peripheral myelin formed by?

Answer 12

Schwann Cells

Question 13

What are the gaps in myelination called?

Answer 13

Nodes of Ranvier

Question 14

What damages the myelin sheath and what happens to the action potential if this happens?

Answer 14

Demyelinating diseases of peripheral nerves damage the myelin sheath and block conduction of action potentials.

Question 15

Why does the nerve blood supply need oxygen?

Answer 15

Need oxygen to maintain sodium pumps

Question 16

What runs within a peripheral nerve?

Answer 16

Arteries and veins and the arteries branch off spinal cord arteries

Question 17

How many spinal arteries run through ventral surfaces and dorsal surfaces?

Answer 17

Ventral surface: 1 ventral spinal artery, Dorsal surface: 2 dorsal spinal arteries

Question 18

What is meant by a dermatome?

Answer 18

Dermatomes = the area of skin innervated by a spinal nerve (or the corresponding dorsal root)

Question 19

Name the dermatomes: C4, C6, C7. C8. T4, T10, L1, L3, L5, S1, S3 and S5.

Answer 19

C4 - Shoulder
C6 - Thumb
C7 - Middle Finger
C8 - Little Finger
T4 - Nipples
T10 - Umbilicus
L1 - Inguinal
L3 - Medial Knee
L5 - Big Toe
S1 - Little toe
S3 - Gluteal
S5 - Anal Canal

Question 20

Describe the essential differences between afferent and efferent nerves.

Answer 20

Afferent neurons carry signals to the brain and spinal cord as sensory data, and efferent neurons send signals from the brain to the muscles, glands and organs of the body in response to sensory input. The main cell bodies of afferent neurons are located near the brain and spinal column, which comprise the central nervous system.

** See google docs for more info (TABLE)

Question 21

Describe the 6 steps of action potentials which arise from generator (receptor) potentials

Answer 21

1. Sensory nerve action potentials start in the axon endings in the receptors.
2. Sensory nerves endings = non-voltage-gated sodium channels that are opened by mechanical bending.
3. This produces depolarisation called a receptor potential.
4. This causes a current to flow along the inside of the nerve fibre to the first node.
5. At the node the current raises the potential inside the nerve so the node becomes depolarised.
6. When there is sufficient depolarisation an action potential is initiated and then travels up to the spinal cord.
   The receptor potential generates action potentials in the axon at the point where regenerative sodium channels start to occur normally at the first node of Ranvier.

Question 22

What does higher intensity of stimulus equal to?

Answer 22

Higher intensity of stimulus = higher frequency of action potentials

Question 23

What are the rapidly and slowly adapting receptors?

Answer 23

Rapidly adapting receptors: Pacinian corpuscles, Meissner’s corpuscles.
Slowly adapting receptors: Ruffini endings, Merkel’s receptors (disks)

Question 24

What are Meissner’s corpuscles?

Answer 24

nerve ending in the skin that detects light touch. In particular, they have highest sensitivity (lowest threshold) when sensing vibrations between 10 and 50 Hz. They are rapidly adapting receptors. They are most concentrated in thick hairless skin, especially at the finger pads.

Question 25

What are Merkel receptors?

Answer 25

mechanoreceptors found in the basal epidermis and hair follicles. They are slowly adapting mechanoreceptors. They provide information on light pressure, position, and may be used to detect features such as shapes and edges.

Question 26

What are Pacinian Corpuscles?

Answer 26

detect high frequency vibration (>50 Hz) of the skin. They are rapidly adapting mechanoreceptors.

Question 27

What are Ruffini Corpuscles?

Answer 27

detect heavy pressure on the skin. They are slowly adapting receptors with a higher threshold than Merkel receptors.

Question 28

Define sensory receptive field

Answer 28

The area of skin that can activate a single sensory nerve fibres

Question 29

Where are sensory receptive field found?

Answer 29

Receptive fields vary in size depending on where they are on the body surface. Receptive fields are smallest on the parts of the body used for sensory discrimination, i.e. the glabrous skin of fingers and hand. Receptive fields are larger on the axial parts of the body, eg the abdomen and back.

Question 30

Every piece of skin is part of how many receptive fields?

Answer 30

Receptive fields of individual nerve fibres overlap. Every piece of skin is part of at least two receptive fields, (often more) so that damage to a single fibre does not leave any region of skin anaesthetic.

Question 31

What test can be used to measure the size and density of receptive fields

Answer 31

Smaller receptive fields means better localisation of stimuli on the skin. This can be tested by the two point discrimination test = measure of the size and density of receptive fields.

Question 32

What does ‘free’ sensory nerve endings mean? And name their characteristics.

Answer 32

Some sensory nerve endings do not have capsules around their endings and are ‘free’
They often arise from C type endings
They are ubiquitous
Nociceptors activity = pain as detect tissue damage
Can also detect chemicals = chemoreceptors

Question 33

What are free sensory nerve endings sensitive to?

Answer 33

Temperature?

Question 34

Name the main types of primary afferent and efferent fibres found in mammalian peripheral nerves.

Answer 34

Type Ia sensory fiber = primary afferent ⇒ muscle spindle which measure the change in muscle length and monitors velocity of stretch
Type Ib sensory fiber = afferent ⇒ Golgi Tendon Organ which measure the change in muscle tension
Gamma motor neurons = primary motor efferent ⇒ used by nervous system to modify spindle’s sensitivity

Question 35

How is the conduction velocity measured in myelinated fibres?

How is the conduction velocity measured in unmyelinated fibres?

How are different sizes of peripheral nerve fibre classified as?

Answer 35

- 10 µm diameter = 60 m/s conduction velocity
Conduction velocity (m/s) = 6 x diameter in um 

• Conduction velocity (m/s) = diameter in um
• Different sizes of peripheral nerve fibre are classified with capital letter and greek letters = size is related to function

Question 36

Axon classification - name examples of myelinated (type A) and unmyelinated (type C)

Answer 36

Myelinated = type A
A alpha – motor to skeletal muscle, sensory from muscle spindle, sensory from golgi tendon organ
A beta – general sensory afferents – from skin, viscera
A gamma – motor to muscle spindles – adjusts sensitivity of the muscle – does not cause contraction
A delta – nociceptor or thermoreceptor – fast pain
Unmyelinated = type C
Nociceptor or thermoreceptor – slow pain

Question 37

What does FFR stand for?
Medium fibres for touch are mostly what?
What are the 2 smallest fibers?

Answer 37

• Fast Fibres Reflexes
• Mostly encapsulated
• Pain and temperature

Question 38

Describe the 7 steps of a peripheral nerve damage.

Answer 38

1. Distal part is disconnected from the cell body and degenerates
2. Distal Schwann cells then unwrap themselves
3. Schwann cells form a continuous line of cell lining the distal endoneurial sheaths
4. Proximal cut end begins to form a growth cone
5. As the regenerating fibres grow into the distal sheaths, Schwann cells proliferate + begin to wrap around the fibre
6. Suturing forms a physical conduit for the re-growing axons
7. Growth cone can progress all the way along to the end

Question 39

Describe the 7 steps of motor neuron action potential initiation

Answer 39

1. Synapses project onto dendrites of the motor neurones
2. Generate an excitatory postsynaptic potential (EPSP)
3. Need to have at least 2 synapses stimulated simultaneously to generate an action potential as EPSPs are sub-threshold in voltage = spatial summation
4. If one input fires two action potentials close together in time this can generate an action potential = temporal summation
5. Dendrites act as integrators
6. EPSPs can all add together and generate an action potential at the axon hillock
7. Central excitatory synapses use the neurotransmitter glutamate