Chapter 13 - The Peripheral Nervous System

What does the PNS link?

The CNS to the outside world

Nerves extend from every area of the body →

provide the brain with sensory input and allows for motor output to reach effector organs

General structure of the PNS

• Any nervous system tissue that isn’t the brain or spinal cord, including:

  • Sensory receptors

  • Afferent fibers

  • Efferent fibers

Sensory receptors

responding to specific type of stimuli

Afferent fibers

carry info from body tissues to CNS

Efferent fibers

carry motor output from CNS out to rest of the body

Afferent division

carry impulses from the body to the central nervous system

What type of information is carried by the afferent division?

CNS receives sensory info from the body to carry out integration function figuring out what that sensory info means

What is the function of the afferent division?

impulses allow CNS to interpret information and send out response

Efferent division

carry impulses from CNS to the effector organs

What type of information is carried by the Efferent division?

passing info from the CNS to the rest of the body, motor output

What is the function of the efferent division?

impulses activate muscle or glands to carry out response

Somatic division

skeletal muscle tissue is the effector

Autonomic division

cardiac muscle, smooth muscle, and glands are effectors

What is the function of sensory receptors and sensation?

specialized structures that respond to changes in the enviornment (internal or external) called stimuli

What are the three ways sensory receptors can be classified? Explain each.

1. Stimulus type → what change in the environment activates the receptor

2. Location → either location in the body or the location of the source of the stimulus (inside or outside the body)

3. Receptor structure → nonencapsulated vs encapsulated (capsule around end of sensory portion)

Classification of sensory receptors by stimulus type

1. Mechanoreceptors

2. Thermoreceptors

3. Photoreceptors

4. Chemoreceptors

5. Nociceptors

Mechanoreceptors

respond to mechanical force

• Ex: touch, pressure, vibration, stretch

• activated by the physical deformation of the receptor by the stimulus

Thermoreceptors

respond to temperature changes

Photoreceptors

respond to changing intensities to light

Chemoreceptors

respond to chemicals in solution

• Ex: sense of smell/taste

• Allows body to respond to changes in blood composition

Nociceptors

• respond to damaging stimuli

• extreme stimulus

What sensation do nociceptors provide when they are activated?

• pain sensations

• respond to nauseous (harmful) stimuli

• Ex: bright lights

Classification of sensory receptors by location

1. Exteroceptor

2. Interoceptor

3. Proprioceptors

Exteroceptor

• mostly found at or near the body surface

• receptor is sensitive to stimuli that arise outside the body

Interoceptor

• found deeper in body

• receptor responds to stimuli that arise deep in the body

• Ex: chemoreceptor in body, esp those that measure blood composition

Proprioceptors

• found in skeletal muscle, tendon, joints, ligaments

• receptor responds to changes in body movement/position

• location is more restricted than the interoceptor

• activated by changing the degree of stretch/pressure in the organ that’s being moved

Classification of sensory receptors by receptor structure

Receptors of the general (touch, pain, pressure, vibration) senses

• composed mostly of nerve endings → modified dendritic endings of sensory neurons

• Two types of nerve endings:

  • nonencapsulated (free) nerve ending

  • encapsulated never ending

Non-encapsulated (free) nerve endings

• dendritic ends of sensory receptors have no capsule or covering = directly exposed to stimulis → accurate reading of pain and temperature

• mostly abundant in epithelia and connective tissue

Non-encapsulated (free) nerve endings Function

respond to pain (nociceptors) and temperature (thermoreceptors)

Encapsulated nerve endings

• dendritic endings are enclosed in a capsule

• most mechanoreceptors are encapsulated

  • Ex: muscle spindles, tendon organs

• adapt to sensation responding to until sensation changes

  • Ex: 1 coin on skin and then another

Why is it important to feel pain?

Indicates damage to the body, warning system to stop doing what they’re doing in order to prevent damage

How is pain measured? Why is it difficult (or impossible) to measure pain?

No objective way because everyone feels pain differently.

• Ex: medication doses are different

• There is a pain threshold where many people feel the same pain at a certain level

What triggers pain?

• Excessive temperature and pressure

• Chemicals released by damaged cells or injured tissue → inflammatory chemicals

• Histamine and K⁺ → large amounts activate nociceptors

• ATP → binds to receptor molecules found on free nerve endings that will activate them —> sensation of pain

• Acids → certain acids like hydrochloric acid cause sensation of pain

• Bradykinin → inflammatory mediator

What are the two types of pain?

1. Sharp pain

2. Burning pain

Sharp pain

• usually occurs at onset of injury

• impulses carried by small, myelinated fibers

• sends info faster to the CNS

• Ex: papercut

Burning pain

• occurs after sharp pain

• Lasts longer

• sends info slower up to brain

• impulses carried by small, nonmyelinated fibers

When does suppression of pain perception occur?

Most often during times of stress → coping/escape mechanism

• in emergency or stressful situations

• Ex: running from a bear and falling but you ignore the pain because a bear eating you is more painful than falling

What suppresses feelings of pain?

Endorphins and enkephalins

What pain arises from inside of the body (internal organs)?

1. Visceral pain

2. Referred pain

Visceral pain

• noxious stimulation of receptors of the thorax and abdominal cavity

• stimuli that result in visceral pain:

  • extreme stretching of tissue → overfilled bladder, eating too much

  • schemia → low blood flow to part of the body, not getting O₂

  • muscle spasms/cramps

  • irritating pain → eating spicy foods

Referred pain

pain stimuli arising from one part of the body are percieved as coming from another part of the body

Why does Referred pain occur?

Visceral pain afferents travel along similar route as some somatic pain fibers (misinterpretation)

• Brain is “confused: by the signal → can’t determine exact location of source

• Ex: people having a heart attack feel pain in their left arm. No actual harm to left arm. Happens because same spinal cord segments innervate heart and left arm. Brain interprets input source as somatic pathway in the left arm than the visceral pathway from the heart

Nerve

cordike organ that is part of the periperial nervous system consisiting of parallel bundles of peripheral axons enclosed by connective tissue wrappings

What are the classification of nerves?

• Type of information: sensory or motor

• Direction in which the information is carried:

  • Afferent nerves: carry info toward CNS (sensory)

  • Efferent: carry info away from CNS (motor)

  • Mixed nerves: carry info to and from the CNS (sensory and motor)

Nerves are bundles of

axons enclosed by connective tissue wrappings

A single axon is surrounded by

endoneurium - connective tissue that wraps around 1 individual axon = covers 1 individual neuron along with it’s myelin sheath ; isolated individual neurons from one another

Groups of axons are bundled together by

Perineurium

• forms a fascicle (bundles of axons)

What encloses all other structures?

Epineurium

• Bundles of fascicles form the actual nerve

• Also has blood vessels and lympathic drainage

What don’t mature neurons do?

Divide

In the CNS →

severed/damaged neurons don’t regenerate

• when damaged the neuron dies

• neurons are more sensitive but have more protection

In the PNS →

severed/damaged axons are capable of regeneration

What must happen for regeneration to occur in the PNS?

1. Cell body must remain undamaged

2. The distance between severed ends must be short

   1. Greater distance less likely to regenerate

Regenerated axons don’t behave as before the injury. What does this mean?

• Not functionally identical as before

• Explains physical therapy to get function back

1st step in Axon regeneration in PNS

Injury occurs

• The axon is damaged/torn during the injury

• Proximal portion of axon will seal itself off to prevent any more damage from occurring/exposure

• The distal portion of the cell will degenerate (break down) because no longer attached to the cell body so, not alive

2nd step in Axon regeneration in PNS

Clean up

• Macrophages invade area of damaged axon

  • Dead/dying portion of neuron is destroyed

  • Phagocitizing damaged pieces because it would get in the way of regeneration (takes up space)

3rd step in Axon regeneration in PNS

Axon regeneration begins

• Schwann cells release growth factor to stimulate axon growth

  • Proximal end of axon grows filaments

• Schwann cells form a “tube” that forces filaments to go in the correct direction

4th step in Axon regeneration in PNS

Completion of regeneration

• Axon filaments continue to grow → forms complete axon

• Schwann cells form new myelin sheath around new axon

• Regenerated axon has smaller diameter than original axon so info is sent slower than how it used to be

Where do cranial nerves extend from?

The brain

Cranial nerves serve the

head and neck structures

What is the exception in cranial nerves?

Vagus nerve (X) extends into the abdomen

Olfactory nerve (I)

• Function: nerves associated with olfaction (smell)

• Sensory nerve

• Damaged: partial/complete loss of sense of smell

• Branch from the brain

Optic nerve (II)

• Function: nerves associated with vision

• Sensory nerve

• Damage: different effects (partial/complete blindness), blurry vision, black spots in vision, increased sensitivity to light (pressure)

• Branch from brain

Oculomotor nerve (III)

• Function: supplies four of the six extrinsic eye muscles that move the eyeball

• Motor never

• Damaged: eye misalignment (eyes dont point in the same direction)

Trochlear nerve (IV)

• Function: innervates extrinsic eye muscle that depresses eye and turns it laterally (called the superior oblique; helps look straight down)

• Motor never

Trigerminal nerve (V)

• Function: supplies sensory fibers to the face and motor fibers to the chewing muscles (in cheeks)

• Mixed

Abducens nerve (VI)

• Function: controls extrinsic eye muscle that abducts the eye (the lateral rectus muscle)

• Motor nerve

Facial nerve (VII)

• Function: innervates muscle for facial expression, contributes to taste

• Mixed

Vestibulocochlear nerve (VIII)

• Function: contributes to hearing and balance

• Sensory nerve

Glossopharyngeal nerve (IX)

• Function: innervates tounge for taste and general senses, innervates pharynx for general senses, innervates muscles pf pharynx for swallowing

• Mixed

Vagus nerve (X)

• Function: fibers extend to and supply sensory and motor fibers to the organs of the thorax and abdomen

• Mixed

Spinal Accessory Nerve (XI)

• Function: supplied motor fibers to muscles that move the head and neck

• Motor nerve

Hypoglossal nerve (XII)

• Function: innervates muscles of the tongue and under the tongue to allow movement of tongue for chewing, speech, and swallowing

• Motor nerve

How many pairs of spinal nerves branch from the spinal cord?

31

What is the function of spinal nerves?

supply all parts of the body not served by the cranial nerves

How are spinal nerves connected?

connect to spinal cord via dorsal (sensory) and ventral (motor) roots

Are spinal nerves motor, sensory, or mixed in function?

Mixed

Spinal nerves divide to form

dorsal ramus and ventral ramus

• each ramus provides fibers to specific parts of the body

Dorsal ramus

Function: provides sensory and motor fibers to the skin and muscles of the back

Ventral ramus (ventral rami)

• Function: provides sensory and motor fibers to the lateral and ventral body walls and to the upper and lower limbs

• Ventral rami branch to form nerve networks called nerve plexuses (more complex because of interweaving)

How is a nerve plexus formed?

• As ventral rami extend away from the spinal cord, they branch several times

• Branches from neighboring rami join one another to form a “weaving network” of nerves

Effects of nerve plexus formation?

1. Each branch of the plexus contains fibers from multiple spinal nerves

2. Fibers from each ramus travel to body via several routes

What is the advantage of nerve plexus formation?

If one is damaged, have 2 more for motor and sensory input of body. Won’t completely lose function of the body

Cervical plexus

• Function: cutaneous nerves that supply neck, ear, back of head, and shoulders

• Mostly responsible for sending sensory information

  • Few innervate muscle on anterior side of neck

What are the major branches of the cervical plexus?

• Phrenic nerve: motor and sensory fibers to diaphragm

  • Irriated → hiccups

  • Can result in complete respiratory failure

• Other branches serve skin to provide sensory information for the head, neck, shoulder, and clavicle region

Brachial plexus

Function: provides fiber sthat supply the upper body limbs

Some major branches of the brachial plexus

1. Median

2. Ulnar

3. Radial

4. Other branches supply muscle in chest, shoulder, and back for movement of arms and sensory information for same areas

Median

• Motor function to the arm, hand, wrist

• Sensory information from the lower arm and parts of the hand

Ulnar

• Motor function for hand muscles, flexion of wrist and fingers

• Sensory information from the pinky and half of ring finger, medial side of hand and forearm

Radial

• Motor function for extension of wrist and phalanges

• Sensory information from back of arm, hand, and fingers

Lumbosacral plexus

Lumbar plexus and sacral plexus have large degree of overlap

Lumbar plexus

Function: innervates parts of the abdominal wall muscle, major branches innervate anterior and medial thigh

Some major branches of the lumbar plexus

1. Femoral

   1. Motor function and sensory function for anterior and medial portion of leg

2. Obturator

   1. Motor function and sensory function for medial portion of leg

Lumbosacral Plexus: Sacral plexus

Function: innervates buttocks and posterior aspect of lower limbs, pelvic structures and perineum

Some major branches of the sacral plexus

• Sciatic

• Branches in lower leg to form 2 nerves: tibial and common fibular

Sciatic

serves posterior and lateral portion of thigh for sensory and motor function (longest nerve in human body)

Tibial

• Motor and sensory function to posterior portion of leg and foot

• Responsible for flexion at knee and dorsiflexion (feet down)

Common fibular

• Motor and sensory function anterior and lateral portion of lower leg and foot (serves below the knee)

• Put foot upwards

Ventral Rami in Anterolateral Thorax and abdominal wall

• DO NOT FORM plexuses, but are arranged in segmental pattern (go straight out on either side)

• Function: serve intercostal muscles between ribs, skin of anterolateral thorax, and most of abdominal wall

Doral rami

• Innervate posterior body trunk in segmental pattern

• Function: each dorsal ramus innervates narrow strips of muscle and skin at same area where it emerges from spinal column