Test 3 - Peripheral Nervous System

Peripheral Nervous System

• Any part of the nervous system that isn’t the brain or spinal cord

• Primarily responsible for getting the sensory information into the CNS so it can interpret our surroundings and come up with responses

• Also carries the responses from the CNS to the effectors

Sensory Receptors

• 5 types of sensory receptors:

  • Mechanoreceptors

  • Thermoreceptors

  • Photoreceptors

  • Chemoreceptors

  • Nociceptors

• Can be classified into:

  • Exteroreceptors

  • Interoreceptors

  • Proprioceptors

Mechanoreceptors

• Respond to physical forces

• i.e. Touch, pressures, vibrations, stretch, and itch

Thermoreceptors

Respond to temperature changes

Photoreceptors

Respond to light

Chemoreceptors

Respond to chemicals in solutions

Nociceptors

Respond to harmful things that cause pain

Exteroreceptors

Monitor outside the body

Interoreceptors

Monitor internal conditions

Proprioceptors

Monitor body position

Cranial Nerves

• Can be classified as afferent or efferent

• Afferent = carries sensory information to the CNS

• Efferent = carries motor information away from the CNS

• There are 12 pairs of Cranial Nerves

• On Old Olympus Towering Top, A Finn And German View A Hop

Cranial Nerve I

• The Olfactory Nerve

• Composed of bipolar sensory neurons that act as chemoreceptors

• Sensory only and sends impulses to the ipsilateral and contralateral hemispheres via the anterior commissure

• Damage to this nerve results in the loss of smell

Cranial Nerve II

• The Optic Nerve

• Conducts impulses from photoreceptors (rods and cones) from the retina of the eye

• Carry impulses that cross over to the ipsilateral and contralateral hemispheres at the optic chiasma

• After crossing over, the optic tracts lead to the thalamus where synapses are made and the signals are passed on to the visual cortexes in the occipital lobes

• Lesions here will depend on where the actual damage is:

  • Damage to the Optic Nerve: results in loss of vision of that eye only

  • Damage to the Optic Chiasma: results in loss of peripheral vision

  • Damage to an optic tract: results in loss of the contralateral visual field

Cranial Nerve III

• The Oculomotor Nerve

• Primarily a motor nerve arising from the midbrain to have certain intrinsic and extrinsic eyeball movements

• Crosses over

• Has two branches:

  • Superior Branch

  • Inferior Branch

• There is also autonomic innervation to allow for dilation and lens accommodation

• Damage to this nerve results in a drooping upper eyelid called ptosis, and we can’t move the eyeball according to the four muscles innervated

Intrinsic eyeball movements

Pupillary contractions and other internal movements

Extrinsic eyeball movements

Orbit of eyeball to eye-motor looking movements

Superior Branch of Cranial Nerve III

• Controls:

  • Superior rectus (superior gaze of the eyeball, aka looking up)

  • The Levator Palpabrae Superioris muscle (moves upper eyelid)

Inferior Branch of Cranial Nerve III

• Controls:

  • Medial Rectus (medial gaze)

  • Inferior Rectus (inferior gaze)

  • Inferior Oblique (superior and lateral gaze)

Cranial Nerve IV

• The Trochlear Nerve

• Primarily a motor nerve

• Innervates the superior Oblique muscle of the eye to allow for downward and lateral gazeing

• Damage to this nerve prevents downward and lateral motion of the eyeball

Cranial Nerve V

• The Trigeminal Nerve

• Mixed nerve that contains motor functions and sensory functions

• Sensory Portion has 3 branches that all deal with the sense of touch, temperature, and pain:

  • Opthalmic Branch

  • Maxiallary Branch

  • Mandibular Branch

• Damage to these sensory nerves results in loss of sensations in certain areas of the face

• Motor root innervates the muscles of mastication and muscles on the floor of the mouth

• Damage to the motor branch results in impaired chewing

Opthalmic branch

• Part of Cranial Nerve V

• Handles touch, temperature, and pain from the anterior half of the scalp, forehead, upper eyelid, surface of the eyeball, tear glands, lateral nose, and upper mucosa of the nasal cavity

Maxillary Branch

• Part of Cranial Nerve V

• Gets lower eyelid, later and inferior mucosal of the nasal cavity, palate, and portions of the pharynx, teeth, gums of upper jaw, lip, and skin of the cheek

• Dentists numb this nerve to work on the upper teeth - can be called a second division block

Mandibular Branch

• Part of Cranial Nerve V

• Gets the lower jaw, anterior 60% of the tongue, muscoa of mouth, auricle of ear, and lower part of the face

• Dentist will anesthetize this never to work on lower teeth - called a third division block

Cranial Nerve VI

• The Abducens Nerve

• Primarily motor function

• Innervates the lateral rectus muscle of the eye, resulting in lateral movement of the eyeball

• Damage to this nerve results in loss of lateral gaze of the eye, and inward gaze due to lack of tonic stimulation

Cranial Nerve VII

• The Facial Nerve

• Mixed nerve

• Carries impulses for motor control of the digastric muscles and muscles of facial expression, including scalp and platysma muscles

• Also has autonomic innervation to the submandibular and sublingual salivary glands

• Sensory portion gets input from the taste buds on the anterior portion of the tongue

• Impulses travel from the taste buds (chemoreceptors) to the medulla through the thalamus to the taste area of the cerebral cortex located in the insula

• Damage to this nerve is fairly common (Bell’s Palsy) and results in a loss of motor control of the face, may also result in a loss of taste

Cranial Nerve VIII

• The Vestibolocochlear Nerve

• Used to be called the auditory nerve as well

• Sensory nerve only

• Comprise of two branches:

  • Choclear branch

  • Vestibular branch

• Damage to this nerve causes deafness and dizziness and poor balance

Choclear branch of Cranial Nerve VIII

• Arises from the organ of Corti

• Allows for our sense of hearing

Vestibular branch of Cranial Nerve VIII

• Arises from the vestibule and semicircular canals

• Involved in body equilibrium and balance

Cranial Nerve IX

• The Glossopharyngeal Nerve

• Mixed nerve

• Innervates portion of the tongue and pharynx

• Motor fibers innervate the pharynx and parotid gland to stimulate swallowing reflex and salivation

• Sensory fibers arise from the pharyngeal region, parotid gland, middle ear cavity, and taste buds - there are also fibers from the carotid sinus of the neck involved in blood pressure and maintenance

• Damage to this nerve results in the loss of bitter and sour taste and aberrations in blood pressure regulation, and if the motor portion is damaged, swallowing is impaired

Cranial Nerve X

• The Vagus Nerve

• Mixed Nerve

• Innervates the visceral organs of the thoracic and abdominal cavities

• Damage to both Vagus nerves results in death

• If only one Vagus nerve is damaged, then vocal problems will be manifest among swallowing and other problems

Motor Portion of Cranial Nerve X

Innervates the Pharynx, Larynx (allows for speech), has parasympathetic output to the respiratory tract, lungs, heart, esophagus, and abdominal viscera except the large intestine

Sensory fibers of Cranial Nerve X

• Come from essentially the same organs and areas as the motor portion

• Gives us information on hunger and intestinal problems

Cranial Nerve XI

• Accessory Nerve

• Principally a motor Nerve

• Unique in that it arises from both brain and spinal cord

• Has two components:

  • Cranial motor Component

  • Spinal Motor Component

• Damage to this nerve makes it difficult to swallow and to move the head/shrug the shoulders

Cranial Motor Component of Cranial Nerve XI

• Arises from the medulla

• Innervates Skeletal muscles of the soft palate, pharynx, and larynx

Spinal Motor Component of Cranial Nerve XI

• Arises from the first 5 segments of the spinal cord

• Innervates the sternocleidomastoid (turns our head) and the trapezius (shrugs our shoulders) muscles

Cranial Nerve XII

• Hypoglossal Nerve

• Mixed Nerve

• Arises from the medulla

• Innervates the tongue muscles

• Motor portion: moves tongue, allowing food manipulation, swallowing, and speech

• Sensory portion: Allows for proprioception of the tongue

• Damage to this nerve makes it difficult to swallow, speak, and protrude the tongue

Reflexes

• Unlearned, predictable responses to a stimulus

• Many have protective functions and allow for a faster response than if the brain had to process the stimulus

• Mediated by the spinal cord to provide rapid, automatic responses to things like burning pain

• All reflexes have the same basic components which make up the reflex arc

The Reflex Arc consists of:

• Receptor

• Sensory Neuron

• Integration Center - can be either by monosynaptic or polysynaptic

• Motor Neuron

• Effector

Stretch Reflex

• Example: Patellar Reflex

• Receptors: The Muscle Spinder Fibers and golgi tendon organs - these are structures in the muscles and tendons that sense the degree and rate of stretch of the muscle - when stimulated by large amounts of stretching or very rapid stretching, they will increase the frequency of their action potentials

• Afferent Neurons from the stretch receptors carry information to the spinal cord

• In the spinal cord, there are synapses with alpha neurons that lead back to the stretched muscle and tell it to contract - this contraction would stop the rapid or large stretch and prevent the muscle from being torn

• Monosynaptic

Flexor Reflex

• Protective reflex

• triggered by pain

• results in the withdrawal of the threatened body part

• Reflex Arc:

  1. Stimulus is a painful event that’s detected by nociceptors

  2. These impulses travel to the spinal cord via the afferent neuron and then synapse with interneurons in the spinal cord

  3. Interneurons then synapse to send output to flexor muscles of affected area causing withdrawal of the limb, as well as impulses to antagonist muscles relaxing them - allowing for a smooth and rapid withdrawal of the limb

  4. Interneurons will ALSO send impulses up the spinal cord to the brain - this information will allow the brain to be aware of the pain

• Polysynaptic

Crossed Extensor Reflex

• Complex reflex

• Consists of an ipsilateral flexor reflex coupled with a contralateral extensor reflex

• Can be triggered by pain

• will result in the withdrawal of the affected limb, and the extension of the opposite limb

• Reflex arc:

  1. Receptors can be touch or pain receptors

  2. Afferent neurons carrying pain information will synapse with interneurons controlling ipsilateral flexor reflex as well as interneurons controlling contralateral extensor muscles - this part of the reflex will cause the opposite, unaffected limb to extend itself to help you catch yourself - there will also be synapses leading up to the brain

  3. Results in the withdrawal of the affected limb, the extension of the unaffected limb, and it notifies your brain why all these things are happening

Reflexes might be tested because:

• Stretch reflexes:

  • Stretch reflexes are absent when there is peripheral nerve damage or damage to the ventral horn at the levels of innervation for that muscle

  • These are the lower motor neurons and a lack of a reflex response would let the doctor know there is damage along here at some point

  • Diabetes, Coma, and Neurosyphilis can also cause these reflexes to be absent

• If there is damage to higher centers of the brain or spinal cord, there there are hyperactive reflexes (spastic) that lack inhibition form the cerebellum - this is referred to as damage to the upper motor neurons

• Polio and stroke patients can exhibit this type of abnormal reflexes