Week 6: The Peripheral Nervous System

Question 1

What is the Peripheral Nervous system?

Answer 1

• All nervous tissue outside of the CNS
  
  • Nerves are categorized as either cranial or spinal nerves (depending on where they exit the CNS)
  • Nerves that exit from the brain = cranial nerves
  • Nerves that exit from the spinal cord = spinal nerves
    Anterior rami = spinal nerves

Question 2

Peripheral nerve inside-out?

Answer 2

Neuron/nerve cell (wrapped in endoneurium which is superficial to any myelin present)
Bundled together
Fascicle (wrapped in perineurium)
Bundled together
Nerve (wrapped in epineurium)

Question 3

Cranial nerves?

Answer 3

• There are 12 pairs of cranial nerves represented by roman numerals
  
  • They supply all structures of the head and neck as well as some of the visceral structures of the thorax and abdomen
  • The numbering of the cranial nerves reflects the sequence of their attachment to the brain from forebrain to hindbrain
  • Cranial nerves 1 and 2 are considered part of the CNS not PNS as they are myelinated by oligodendrocytes not Schwann cells

Question 4

Cranial Nerves 1-6:
Name:
Function:

Answer 4

1: Olfactory, Sensory: Nose
2: Optic: Sensory: Eye
3: Oculomotor: Motor: All eye movements except those supplied by 4 and 5
4: Trochlear: Motor: Superior oblique muscle
5: Trigeminal: Both: Sensory (face, sinuses, teeth etc. Motor (muscles of mastication)
6: Abducens: Motor: External rectus muscles

Question 5

Cranial Nerves 7-12:
Name:
Function:

Answer 5

7: Facial: Both: Muscles of the face. Sensory: Middle ear, nasal cavity, soft palate
8: Vestibulocochlear: Sensory: Inner ear
9: Glossopharyngeal: Both: Motor (pharyngeal musculature) Sensory (posterior part of tongue, tonsil, pharynx)
10: Vagus: Both: Motor (heart, lungs, bronchi, GIT) Sensory (heart lungs, bronchi, trachea, larynx, pharynx, GIT, external ear)
11: Accessory: Motor: Sternocleimastoid, trapezius
12: Hypoglossal: Motor: Muscles of the tongue

Question 6

What is a mnemonic to remember cranial nerves and their function?

Answer 6

Name Mnemonic:
Oh, Oh, Oh, To, Touch, And, Feel, Very, Good, Velvet, Ahh, Heaven

Function Mnemonic:
Some, Say, Marry, Money, But, My, Brother, Says, Big, Brains, Matter, Most

Question 7

Structure of spinal nerves:

Answer 7

• Anterior/ventral roots contain motor neurons, info out
  
  • Posterior/dorsal roots contain sensory neurons, sensory info in
    Spinal nerves form where the anterior and posterior rotos come together as they exit the spinal cord, branch into anterior and posterior rami

Question 8

Anterior and posterior rami:

Answer 8

• Posterior ramus supplies the structures of the back
  
  • Anterior ramus supplies the entire anterior surface of the body and limbs (hence larger diameter)
  • Both rami contain both motor and sensory neurons
  • Smaller and skinnier than anterior ramus
  • Supplies tissues at front and limbs which is why it is thicker
    Split for the purposes of innervation

Question 9

Functional classification of neurons:

Answer 9

Motor neurons:
- Efferent (info out)
- Somatic (voluntary e.g. to skeletal muscle) or autonomic/visceral (involuntary e.g. to cardiac and smooth muscle glands)
Sensory neurons:
- Afferent (info in)
- Somatic sensory (skin, muscles, joints etc.) or visceral sensory (internal organs)
Interneurons (only in CNS) relay information

Question 10

What is a somatic nerve plexus?

Answer 10

• A limb has many nerves supplying the different tissues and regions of that limb and these arise from a nerve plexus
  
  • A nerve plexus serves to redistribute nerve fibers from several anterior rami into a number of different peripheral nerves in a limb
  • All anterior rami except T2-T12 are associated with a nerve plexus
  • Nerves supply tissues, muscle, bone
  • Limbs have a plexus that redistributes nerve fibers from the anterior rami to a number of peripheral nerves going to the limb
  • No involvement of thoracic spinal nerves as they only supply the trunk, not limbs

Question 11

Organization of nerve plexus:

Answer 11

• Many anterior rami combine to form a final peripheral nerve that will enter a limb: It allows a limb to have many nerves supplying different tissue regions
  Plexuses begin with roots e.g. the specific anterior rami that merge into trunks, then divisions then cords then finally the peripheral nerve (branches)

Question 12

What are the 4 somatic nerve plexuses?

Answer 12

Cervical, Brachial, Lumbar, Sacral

Question 13

Cervical plexus:

Answer 13

• Involves anterior rami C1-C5
  
  • Supplies skin and muscles of the head and neck and superior parts of the shoulders and chest
    One major peripheral nerves is the phrenic nerve which supplies the diaphragm

Question 14

Brachial plexus:

Answer 14

• Involves anterior rami C5-T1
  
  • Almost entire nerve supply of shoulders and upper limb
  • Examples of peripheral nerves include the media, radial and ulnar nerves
  • Ulnar, medial side of arm, ring finger
    Medial nerve supplies the rest of your hand

Question 15

Lumbar plexus:

Answer 15

• Involves anterior rami L1-L4
  
  • Supplies the anterolateral abdominal wall, external genitals and part of the lower limbs
  • Examples of peripheral nerves include the femoral and obturator nerves
  • Supplies abdominal wall
    Genitals, lower limbs

Question 16

Sacral plexus:

Answer 16

• Involves anterior rami L4-S4
  
  • Supplies the buttocks and perineum and lower limbs
    One major peripheral nerves is the sciatic nerve which is the largest nerve in the body

Question 17

What is an autonomic nerve plexus?

Answer 17

• Visceral structures like your heart, lungs and digestive system are also controlled by nerve plexuses
  
  • Autonomic meaning that we have no voluntary control over these structures
    Unlike the ordered system that make up somatic plexuses, autonomic plexuses are made up of a tangled network of sympathetic and parasympathetic neurons

Question 18

The cardiac plexus:

Answer 18

• The cardiac plexus has a sympathetic and parasympathetic component (each with neurons that form their own plexus)
  Responsible for taking motor information to the heart and takes sensory information back to the CNS

Question 19

Myotomes:

Answer 19

• A myotome is a groups of muscles that are innervated by one single spinal segment (anterior ramus) e.g. segment C1
  
  • An individual muscle may be a part of multiple myotomes
    Its possible to test myotome functionality (and thus find spinal injury) by checking a patients ability to perform specific muscle actions that are known to be controlled by the myotome in question e.g. hip flexion can indicate functionality of spinal nerves L2 and L3

Question 20

What are Myotomes and their vertebrae and tests you can do?

Answer 20

C5: Shoulder abduction
C5, C6: Forearm flexion
C6, C7: Forearm extension
C6, C7: Flexion of Wrist
C5, C6: Extension of wrist
L2, L3: Hip Flexion
L5, S1, S2: Hip Extension
L4, L5: Hip Abduction
L2, L3, L4: Hip Adduction
L3, L4: Knee extension
L5, S1, S2: Knee Flexors
L4, L5: Dorsiflexion of Ankle
L4, S1, S2: Ankle Plantar flexion

Question 21

What are Dermatomes?

Answer 21

• A dermatome is a cutaneous (skin) area that receives most sensory innervation from one single spinal segment (anterior ramus) e.g. segment C1
  
  • Similar to myotomes, its possible to test dermatome functionality (and thus find spinal injury) by checking a patients ability to feel sensation on specific areas of their skin)
    Skin equivalent of dermatomes

Question 22

What is a reflex arc?

Answer 22

• Reflexes carried out via reflex arcs are an essential protective mechanism of the body e.g. removing your hand from something hot
  
  • They involved a rapid motor response to a stimulus that is mediated entirely within the spinal cord
  • Brain still notified by other receptors but not involved in the reflex arc e.g. residual pain from a burnt finger
  • Protective mechanisms of the body
    Keeps us safe

Question 23

What is a reflex arc?
Reflex arc involves 5 functional components

Answer 23

1. Sensor receptor: Receives stimulus
   
   2. Sensory neuron: either somatic or visceral, sends signal to CNS
   3. Integrating center: Relays information from sensory to motor neuron. Polysynaptic: Includes interneuron, Monosynaptic: No interneuron
   4. Motor neuron: Either somatic or visceral, sends signal to PNS effector
2. Effector: Either muscle or gland that carries out response

Question 24

The plantar reflex:

Answer 24

• The polysynaptic reflex arc is a nociceptive (pain-related) spinal reflex that serves to protect the sole of the foot
  
  • Tested by stroking the lateral part of the sole of the foot with a fairly sharp object, which produces plantar flexion of the big toe (there is often also plantar flexion and adduction of the other toes)
  • If this stimulus results in dorsiflexion of the big toes, and often abduction (fanning) of the other toes, this abnormal reflex is termed Babinski’s sign/reflex
  • Indicates metabolic or structural abnormality in the corticospinal system upstream from the segmental reflex

Question 25

Monosynaptic reflex arcs:

Answer 25

- There are few reflexes that are classified as 'monosynaptic meaning that there are only two neurons involved in the entire reflex are e.g. the patellar reflex - Sensory (afferent) neurons detect a stimulus and send a nerve signal into the spinal cord to synapse directly with a motor (efferent) neuron - motor neuron sends signal to effector muscles, completely bypass an interneuron

Question 26

The stretch reflex

Answer 26

- A monosynaptic reflex arc e.g. the patellar reflex (knee jerk reflex) - The function of a stretch reflex is to ensure the muscle stays at its desired length Important for postural muscles e.g. the quadriceps femoris: if we have a sudden change in the length of that muscles that’s probably because were falling over, so the body tries to fix that by tightening that muscle back up quickly

Question 27

Cranial Nerves: Video

Answer 27

- PNS - 12 pairs of cranial nerves, supply head and neck and some visceral structures of thorax and abdomen - Cranial nerves carry a range of nerve fiber types - Contain motor fibers (any) - Stimulate muscles under conscious control - Some stimulate involuntary smooth muscles - Some cranial nerves (cranial nerve number 10) stimulate the heart - Can carry sensory information - touch pain, changing BP, oxygen

Question 28

Where does the spinal cord sit? What is the intervertebral foramen? When do they become spinal nerves?

Answer 28

- The spinal cord sits inside the vertebra canal and is protected by vertebrae - However nerves go through an opening in the vertebral column called the intervertebral foremen - Once it leaves the intervertebral foremen they become spinal nerves Inside the vertebral canal= spinal cord, outside the vertebral canal = spinal nerves and its branches

Question 29

Anterior Median fissure?

Answer 29

Divides the left and the right portions of the grey matter, this is characterized by a wide groove

Question 30

Posterior Median Sulcus?

Answer 30

narrower fissure than the anterior median fissure These landmarks can characterize anterior from posterior or front from back

Question 31

What do Ventral and dorsal roots do?

Answer 31

- The ventral root carries motor information to effector cites Dorsal root carries sensory information back to the spinal cord

Question 32

Dorsal root ganglion?

Answer 32

appears swollen in small region, as the cell bodies of the sensory neurons lie outside the spinal cord in a ganglion, the dorsal root and ventral root combine to make a spinal nerve

Question 33

What happens after a spinal nerve is formed?

Answer 33

- Once there is a spinal nerve it will divide into the dorsal primary branch/ dorsal primary ramus/posterior ramus, ventral primary ramus - One spinal nerve dividing into dorsal and ventral ramus - All spinal nerves divide into posterior and anterior rami, the posterior or dorsal rami supply structures of the back Anterior/ventral primary rami supply all other somatic structures including upper and lower limbs

Question 34

How does sensory information travel?

Answer 34

Sensory comes in through the dorsal root and general somatic afferent nerve fiber e.g. Sensation from the back of the skin, in through the dorsal primary ramus, travel through the dorsal root with cell body in the dorsal root ganglion before it enters the dorsal horn. From there it will be going through an ascending tract pathway through to the CNS

Question 35

Somatic Nerve plexi?

Answer 35

- Somatic nerve plexi: a limb has many nerves supplying the different tissues and regions of that limb these arise from a nerve plexus A nerve plexus serves to redistribute nerve fibers from several anterior rami into a number of different peripheral nerves in a limb

Question 36

Spinal segments and dermatomes:

Answer 36

- Each spinal segment is connected via its GTA fibers to the skin in a specific region of the body - The area of skin innervated by the branches of a single spinal segment is called a dermatome (skin segment) - This can be used to help located the injured region of the spinal cord if it has been damages

Question 37

Spinal segments and myotomes:

Answer 37

- Each spinal segment is connected (via its somatic efferent fibers) to a block of muscle in a specific part of the body - The block of muscle innervated by the branches of a single spinal segment is called a myotome - This can also be used to help locate the injured region of the spinal cord if it has been damages Muscles producing movements at more inferior joints in a limb are innervated by more inferior spinal segments. This is a myotome pattern

Question 38

Nervous reflex arcs:

Answer 38

- Many bodily responses belong to a response known as a reflex - A reflex is a rapid motor response to a stimulus They are generally fast, intrinsic and predictable response to a stimulus

Question 39

Reflex arcs:

Answer 39

- Reflexes can be mediated by the spinal cord (spinal reflex) - They do not require involvement from a higher (brain) centers, however in many cases the brain is advised of any reflex activity/response The neural circuitry involved is referred to as a reflex arc

Question 40

Stretch reflex:

Answer 40

1. Stretching of muscle through tapping of the tendon stimulates sensory receptors known as muscle spindles 2. Nerve impulses travels through sensory OR somatic afferent neuron through the posterior root of spinal nerve into the spinal cord 3. Sensory/somatic afferent neuron synapses directly with a motor neuron/somatic efferent neuron 4. Nerve impulses leaves the spinal cord into the anterior root and to the stimulated muscle Action potentials in the stretched muscle (effector) leads to muscle contraction

Question 41

Stretch reflex-reciprocal innervation:

Answer 41

- A stretch reflex is monosynaptic (just two neurons and one synapse) but polysynaptic reflex arc to the antagonist muscles occurs at the same time - In this pathway 1. Stretching of the muscle stimulates sensory receptors 2. Nerve impulses travels through a sensory OR somatic afferent neuron through the posterior root of spinal nerve into the spinal cord 3. Sensory/somatic afferent neuron synapses with an inhibitory interneuron The inhibitory interneuron synapses with an inhibits a motor neuron/somatic efferent neuron that normally excited the antagonistic muscles

Question 42

Withdrawal reflex - A polysynaptic reflex

Answer 42

1. Pain registered through a sensory receptor 2. Nerve impulse travels through a sensory OR somatic afferent neuron through the posterior root of spinal nerve into the spinal cord 3. Within the spinal cord the sensory/somatic afferent neuron synapses with interneurons which extend through several spinal cord segments 4. The interneurons synapse with motor neurons in several spinal cord segments. Motor neurons/somatic efferent neurons will then leave the spinal cord into the anterior root and to the stimulated muscle Action potentials In the muscle (effector) leads to muscle contraction to move the leg from potentially damaging stimulus

Question 43

Which of the following options correctly states the (A) Cranial nerve responsible, and (B) The respective function of that nerve, that best explains the symptom outlined in the scenario (stroke, trouble looking out her left eye)

Answer 43

(A) Oculomotor nerve (B) Motor

Question 44

Which of the following options correctly lists the cranial nerves that must be affected in order to produce the symptoms outlined in the scenario (trouble with food and speaking)

Answer 44

Glossopharyngeal & Hypoglossal

Question 45

Which of the following options correctly identifies the nerve plexus that would be associated with this anatomical region (forearm)

Answer 45

Brachial plexus