Week 6: The Peripheral Nervous System Flashcards
What is the Peripheral Nervous system?
- 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
Peripheral nerve inside-out?
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)
Cranial nerves?
- 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
Cranial Nerves 1-6:
Name:
Function:
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
Cranial Nerves 7-12:
Name:
Function:
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
What is a mnemonic to remember cranial nerves and their function?
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
Structure of spinal nerves:
- 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
- Posterior/dorsal roots contain sensory neurons, sensory info in
Anterior and posterior rami:
- 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
Functional classification of neurons:
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
What is a somatic nerve plexus?
- 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
Organization of nerve plexus:
- 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)
What are the 4 somatic nerve plexuses?
Cervical, Brachial, Lumbar, Sacral
Cervical plexus:
- 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
- Supplies skin and muscles of the head and neck and superior parts of the shoulders and chest
Brachial plexus:
- 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
Lumbar plexus:
- 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
Sacral plexus:
- 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
- Supplies the buttocks and perineum and lower limbs
What is an autonomic nerve plexus?
- 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
- Autonomic meaning that we have no voluntary control over these structures
The cardiac plexus:
- 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
Myotomes:
- 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
- An individual muscle may be a part of multiple myotomes
What are Myotomes and their vertebrae and tests you can do?
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
What are Dermatomes?
- 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
- 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)
What is a reflex arc?
- 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
What is a reflex arc?
Reflex arc involves 5 functional components
- Sensor receptor: Receives stimulus
- Sensory neuron: either somatic or visceral, sends signal to CNS
- Integrating center: Relays information from sensory to motor neuron. Polysynaptic: Includes interneuron, Monosynaptic: No interneuron
- Motor neuron: Either somatic or visceral, sends signal to PNS effector
- Effector: Either muscle or gland that carries out response
The plantar reflex:
- 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
Monosynaptic reflex arcs:
- 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
The stretch reflex
- 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
- The function of a stretch reflex is to ensure the muscle stays at its desired length
Cranial Nerves: Video
- 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
Where does the spinal cord sit?
What is the intervertebral foramen?
When do they become spinal nerves?
- 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
Anterior Median fissure?
Divides the left and the right portions of the grey matter, this is characterized by a wide groove
Posterior Median Sulcus?
narrower fissure than the anterior median fissure
These landmarks can characterize anterior from posterior or front from back
What do Ventral and dorsal roots do?
- The ventral root carries motor information to effector cites
Dorsal root carries sensory information back to the spinal cord
Dorsal root ganglion?
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
What happens after a spinal nerve is formed?
- 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
How does sensory information travel?
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
Somatic Nerve plexi?
- 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
Spinal segments and dermatomes:
- 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
Spinal segments and myotomes:
- 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
Nervous reflex arcs:
- 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
- A reflex is a rapid motor response to a stimulus
Reflex arcs:
- 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
- They do not require involvement from a higher (brain) centers, however in many cases the brain is advised of any reflex activity/response
Stretch reflex:
- Stretching of muscle through tapping of the tendon stimulates sensory receptors known as muscle spindles
- Nerve impulses travels through sensory OR somatic afferent neuron through the posterior root of spinal nerve into the spinal cord
- Sensory/somatic afferent neuron synapses directly with a motor neuron/somatic efferent neuron
- 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
Stretch reflex-reciprocal innervation:
- 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
- In this pathway
Withdrawal reflex - A polysynaptic reflex
- Pain registered through a sensory receptor
- Nerve impulse travels through a sensory OR somatic afferent neuron through the posterior root of spinal nerve into the spinal cord
- Within the spinal cord the sensory/somatic afferent neuron synapses with interneurons which extend through several spinal cord segments
- 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
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)
(A) Oculomotor nerve (B) Motor
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)
Glossopharyngeal & Hypoglossal
Which of the following options correctly identifies the nerve plexus that would be associated with this anatomical region (forearm)
Brachial plexus
