Components of PNS

Question 1

Functional organization of the Peripheral Nervous System

Answer 1

1. Somatic- conscious afferent and efferent neurons, voluntary motor control
2. Autonomic- unconscious efferent, involuntary motor control of internal organs to maintain homeostasis
   - Sympathetic (fight or flight)
   - Parasympathetic (rest and digest)

Question 2

Where do Somatic Afferents come from?

Answer 2

Sensory fibers from skin, muscle, joints, tendons

Question 3

Where do Visceral Afferents come from?

Answer 3

Sensory fibers from visceral organs; some result in conscious sensations, but others do not. They are not considered part of the autonomic nervous system

Question 4

Does the Autonomic nervous system have efferent or afferent neurons?

Answer 4

Entirely efferent

Question 5

Components of the PNS

Answer 5

1. Nerves (bundles of nerve fibers)
2. Nerve Endings
3. Ganglia (cell bodies): sensory vs. autonomic ganglia

Question 6

Myelination’s role in Nerves

Answer 6

Speeds up the conduction of electrical impulses along axons. Nerve impulse jumps from node to node.

Question 7

Myelination in axons in the CNS vs PNS

Answer 7

• CNS: myelinated by oligodendrocytes
• PNS: myelinated by Schwann cells (or neurolemmocytes)

Question 8

Are all nerves myelinated?

Answer 8

• No, if the axon does not require speedy transport then they will be unmyelinated (use less energy because no need to form myelination)
• Less speed needed in smaller nerves or where the function of the nerve does not require a quick response

Question 9

Myelinated Nerve Fiber Formation Steps

Answer 9

1. Neurolemmocyte starts to wrap around a portion of an axon
2. Neurolemmocyte cytoplasm and plasma membrane begin to form layers around axon
3. The overlapping inner layers of the neurolemmocyte plasma membrane form the myelin sheath
4. Eventually, the neurolemmocyte nucleus and cytoplasm are pushed to the periphery of the cell as the myelin sheath is formed

Question 10

Unmyelinated nerve fiber formation steps

Answer 10

1. Neurolemmocyte starts to envelope multiple axons
2. The unmyelinated axons are enveloped by the neurolemmocyte, but there are no myelin sheath wraps around each axon

Question 11

Components of a Myelinated Nerve

Answer 11

• Nodes of Ranvier
• Internode
• Paranode

Question 12

Nodes of Ranvier

Answer 12

Areas of the myelinated axon that are not covered by the myelin sheath

Question 13

Internode

Answer 13

Area between two nodes

Question 14

Paranode

Answer 14

Transitional zone of the myelin sheath

Question 15

What does each Schwann cell myelinate?

Answer 15

Each schwann cell myelinates a single internode

Question 16

Components of PNS nerve fiber bundle

Answer 16

1. Endoneurium- surrounds axons
2. Perineurium- surrounds axon fascicles. Often a barrier between blood and nerve.
3. Epineurium- surrounds the entire nerve bundle

Question 17

PNS axon types

Answer 17

Most nerves in the PNS contain a mixture of myelinated and unmyelinated axons. Different functions will have different speeds and myelination needs.

• Sensory and motor pathways that require rapid signal transmission are usually myelinated
• Unmyelinated axons are more commonly found in autonomic nervous system

Question 18

Perineurium cells and tight junctions

Answer 18

Tight junctions between the perineurium cells form the blood-nerve barrier

Occasionally there is even a blood vessel within the fascicle and perineurium will surround it.

Question 19

Where are myelinated fibers commonly found?

Answer 19

They facilitate fast signals and are commonly found in the somatic nervous system

Question 20

Where are unmyelinated fibers commonly found?

Answer 20

They facilitate slow signals and are typically found within the autonomic nervous system

Question 21

Nerve Endings

Answer 21

Two types: afferent nerve endings and efferent nerve endings

Question 22

Afferent Nerve Endings

Answer 22

Contain sensory receptors

Two types:
1. Free nerve endings
2. Encapsulated nerve endings

Question 23

Free nerve endings

Answer 23

• Type of afferent nerve ending
• Most common type

Eg. Somatic and visceral sense of pain (nociceptors) and temperature (thermoreceptors)

Question 24

Encapsulated nerve endings

Answer 24

• Type of afferent nerve ending
• Ensheathed by a connective tissue capsule

Eg. Mechanoreceptors in the inner dermis

Eg. Proprioceptors in skeletal muscle

Question 25

Efferent Nerve endings

Answer 25

• Innervate target organs

Eg. Neuromuscular junction- has a motor endplate at the connection between motor neuron and the muscle

Question 26

Ganglia (Ganglion- singular)

Answer 26

• Part of the PNS
• Relay station to transmit nerve impulses
• Two types: sensory or autonomic ganglion
• Has ovoid structures containing neuronal cell bodies and glial cells called satellite cells supported by connective tissues

Question 27

Sensory Ganglion

Answer 27

• cell bodies of sensory neurons (afferent)
• Located in cranial and spinal nerves
• Impulse travels towards CNS

Question 28

Autonomic Ganglion

Answer 28

• cell bodies of efferent neurons
• Controls involuntary movement
• Two neuron circuits of the sympathetic and parasympathetic nervous systems
• Impulse travels away from CNS

Question 29

Satellite cells

Answer 29

• Ovoid structures containing neuronal cell bodies and glial
• Part of the ganglion

Question 30

Histological differences between autonomic and sensory ganglion

Answer 30

1. Location of nucleus: Autonomic (on the side), Sensory (center)
2. Size of the neuron: Autonomic (smaller), Sensory (bigger)
3. Distribution of Nissl substances: Autonomic (edge of cell), Sensory (evenly distributed)