M06 | Peripheral Nervous System | Part One

What does the Peripheral Nervous System (PNS) link?

It provides links from and to the world outside the body.

What structures are included in the PNS?

All neural structures outside the brain, including sensory receptors, peripheral nerves, associated ganglia, and efferent motor endings.

What is the primary function of sensory (afferent) neurons?

They carry input toward the Central Nervous System (CNS).

What is the role of motor (efferent) neurons?

They carry output away from the Central Nervous System (CNS).

What type of nerves are cranial and spinal nerves primarily?

They are mostly mixed nerves, containing both afferent and efferent axons.

Where are neuron cell bodies located in the PNS?

Neuron cell bodies are located in the CNS or in ganglia, while their extensions (nerves) run outside the CNS.

What is the function of sensory receptors?

They are specialized to respond to changes in the environment (stimuli) and trigger nerve impulses.

What is the difference between sensation and perception?

Sensation is the awareness of a stimulus, while perception is the interpretation of the meaning of that stimulus.

What are the three levels of neural integration in sensory systems?

Receptor level, circuit level, and perceptual level.

What is adaptation in sensory receptors?

Adaptation is a change in sensitivity in the presence of a constant stimulus, where receptor potentials decline in frequency or stop.

What are phasic receptors?

Phasic receptors are fast-adapting receptors that signal the beginning or end of a stimulus.

What are tonic receptors?

Tonic receptors adapt slowly or not at all, such as most proprioceptors and nociceptors.

What is the purpose of nociceptors?

Nociceptors warn of actual or impending tissue damage and continue to report pain.

What factors can stimulate nociceptors?

Extreme pressure, extreme temperature, histamine, potassium (K+), ATP, acids, and bradykinin.

What is pain tolerance?

Pain tolerance varies among individuals, meaning that people may perceive pain at the same stimulus intensity but respond differently.

What is referred pain?

Referred pain is when pain from one body region is perceived in a different region due to shared nerve pathways.

What is the structure of a nerve?

A nerve is a cordlike organ of the PNS, consisting of a bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue.

What is the difference between a neuron and a nerve?

A neuron is a single cell that typically transmits signals in one direction, while a nerve is a bundle of axons from many neurons that can transmit signals in both directions.

What is visceral pain?

Visceral pain is felt as vague aching, gnawing, or burning, activated by tissue stretching, ischemia, chemicals, or muscle spasms.

What is hyperalgesia?

Hyperalgesia is a condition of increased sensitivity to pain, often resulting from long-lasting or intense pain.

What is the role of endogenous opioids in pain perception?

Endogenous opioids, like endorphins, can block some pain impulses.

What is the somatic nervous system responsible for?

The somatic nervous system stimulates voluntary effectors, such as skeletal muscles.

What is the autonomic nervous system responsible for?

The autonomic nervous system stimulates involuntary effectors.

What are the three connective tissue coverings of a nerve?

Endoneurium, Perineurium, and Epineurium.

What is the function of the Endoneurium?

It encloses axons and their myelin sheaths.

What does the Perineurium do?

It bundles fibers into fascicles.

What is the role of the Epineurium?

It serves as a tough fibrous sheath around a nerve.

How are nerves classified based on impulse direction?

Mixed nerves, Sensory (afferent) nerves, and Motor (efferent) nerves.

What are mixed nerves?

Nerves that contain both sensory and motor fibers, transmitting impulses to and from the CNS.

What do sensory (afferent) nerves do?

They transmit impulses only toward the CNS.

What is the function of motor (efferent) nerves?

They transmit impulses only away from the CNS.

What types of fibers are found in mixed nerves?

Somatic afferent, Somatic efferent, Visceral afferent, and Visceral efferent.

What are ganglia?

Clusters of neuron cell bodies associated with nerves in the PNS.

What type of neurons are found in dorsal root ganglia?

Cell bodies of sensory neurons.

What do autonomic ganglia contain?

Autonomic motor neurons.

What is the regeneration capacity of mature neurons?

Mature neurons are amitotic, but peripheral axons may regenerate if the soma is intact.

What happens when a peripheral axon is damaged?

Macrophages clean the dead axon, and axon filaments grow through a regeneration tube formed by Schwann cells.

What inhibits regeneration in CNS fibers?

CNS oligodendrocytes bear growth-inhibiting proteins, and astrocytes form scar tissue that blocks regrowth.

How many pairs of cranial nerves are there?

Twelve pairs.

What is the primary function of the Olfactory Nerves?

They are sensory nerves responsible for the sense of smell.

Where do the receptors for the Olfactory Nerves reside?

In the nasal mucosa.

What is the pathway for the Olfactory Nerves?

Receptors in olfactory epithelium pass through the cribriform plate to the olfactory bulb.

What distinguishes purely sensory nerves from mixed nerves?

Purely sensory nerves transmit only sensory information, while mixed nerves carry both sensory and motor information.

What is the significance of proprioceptive input in motor nerves?

All skeletal muscles send feedback to the CNS, making motor nerves technically mixed.

What is the difference between somatic and visceral fibers?

Somatic fibers relate to the body, while visceral fibers relate to organs.

What is the function of the Olfactory Nerves?

They are responsible for the sense of smell.

What condition is characterized by a partial or total loss of smell?

Anosmia

What is the pathway of the Optic Nerves?

Photoreceptors in the retina pass through optic canals as the Optic Nerve, converge at the optic chiasma, and continue to the thalamus before reaching the visual cortex in the occipital lobe.

What happens if the optic nerve is damaged?

It can cause blindness in the affected eye; damage after the optic chiasma can lead to partial visual loss.

What is the primary function of the Oculomotor Nerves?

They are mostly motor nerves that control eye movement, including raising the eyelid and directing the eyeball.

Which muscles are innervated by the Oculomotor Nerves?

Inferior Oblique, Superior Rectus, Medial Rectus, Inferior Rectus, and the Levator Palpebra Superioris.

What is a clinical sign of Oculomotor nerve paralysis?

External strabismus, where the eye rotates laterally at rest.

What is the function of the Trochlear Nerves?

They are primarily motor nerves that direct the eyeball.

What are the symptoms of damage to the Trochlear Nerve?

Double vision and inability to look down and laterally.

What are the three divisions of the Trigeminal Nerves?

Ophthalmic division, Maxillary division, and Mandibular division.

What is the primary function of the Abducens Nerves?

They are mostly motor nerves that innervate the Lateral Rectus muscle, moving the eye away from the midline.

What is a clinical test for the Abducens Nerves?

The H or Z pattern for eye movement.

What is Bell's Palsy?

A condition that causes rapid onset paralysis of facial muscles, often related to Herpes Simplex 1 Virus.

What are the symptoms of Bell's Palsy?

Partial loss of taste, drooping of the lower eyelid, and difficulty speaking and eating.

What is the role of the Facial Nerves?

They control facial expressions and innervate lacrimal, nasal, and salivary glands.

What is the clinical test for the Trigeminal Nerves?

Corneal reflex test for the ophthalmic division and clenching teeth for the mandibular division.

What is Trigeminal neuralgia?

A condition characterized by excruciating facial pain triggered by mild stimuli.

What is the pathway of the Oculomotor Nerves?

They originate in the gray matter of the midbrain, enter the bony orbit through the superior orbital fissure, and innervate the eye.

What is the effect of damage to the Abducens Nerve?

It causes internal strabismus, where the eye rotates medially.

What are the functions of the autonomic fibers in the Oculomotor Nerves?

They control pupillary constriction and change in lens shape for focusing.

What is the role of the sensory fibers in the Oculomotor Nerves?

They provide proprioception from the four extrinsic eye muscles.

What is the function of the sensory receptors in the Trigeminal Nerves?

They provide sensation from the scalp, upper eyelid, nose, and other facial areas.

What is the clinical significance of the pupillary reflex?

It tests the function of the Oculomotor Nerves, indicating their integrity.

What is the significance of the optic chiasma?

It is where the optic nerves partially cross over before continuing to the thalamus.

What is the primary function of the Vestibulocochlear Nerve (CN VIII)?

It has mostly sensory functions for hearing and balance, with a small motor component for adjusting receptor sensitivity.

What are the two divisions of the Vestibulocochlear Nerve?

The vestibular division for equilibrium and the cochlear division for hearing.

What clinical signs indicate damage to the cochlear nerve?

Central deafness (nerve deafness).

What symptoms may arise from damage to the vestibular nerve?

Dizziness, rapid involuntary eye movements (nystagmus), loss of balance, nausea, and vomiting.

What are the primary functions of the Glossopharyngeal Nerve (CN IX)?

Motor functions for swallowing and innervation of the parotid salivary glands; sensory functions for taste from the pharynx and posterior tongue, and general sensory impulses from the same areas.

What is the significance of chemoreceptors and baroreceptors in the Glossopharyngeal Nerve?

Chemoreceptors are sensitive to changes in blood O2 and CO2 levels, while baroreceptors respond to changes in blood pressure.

What clinical tests are used for the Glossopharyngeal Nerve?

View the uvula while saying 'aah', test the gag reflex with a cotton swab, and check taste on the posterior tongue.

What is the primary role of the Vagus Nerve (CN X)?

It regulates activities of the heart, lungs, and abdominal viscera, primarily through parasympathetic fibers.

What are the sensory functions of the Vagus Nerve?

It carries impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, taste buds of the posterior tongue and pharynx, and proprioceptive fibers from larynx and pharynx muscles.

What clinical signs indicate damage to the Vagus Nerve?

Hoarseness or loss of voice, difficulty swallowing, and changes in gastrointestinal motility.

How do the Accessory Nerves (CN XI) differ from other cranial nerves?

They originate from spinal rootlets in the upper cervical regions rather than cranial rootlets.

What muscles are innervated by the Accessory Nerves?

The trapezius and sternocleidomastoid muscles.

What clinical test is used for the Accessory Nerve?

Muscle testing of the trapezius and sternocleidomastoid against resistance, such as head rotation and shoulder shrugging.

What is the primary function of the Hypoglossal Nerve (CN XII)?

It innervates the extrinsic and intrinsic muscles of the tongue, contributing to swallowing and speech.

What clinical test assesses the Hypoglossal Nerve?

Asking subjects to protrude and retract their tongue and checking for deviation to one side.

What is a common homeostatic imbalance associated with damage to the Hypoglossal Nerve?

Difficulty with speech and swallowing, along with decreased movement or paralysis of the tongue.

What cranial nerve is responsible for smell?

The Olfactory Nerve.

Which cranial nerve is responsible for vision?

The Optic Nerve.

What is the function of the Oculomotor Nerve?

It controls eye muscles.

What muscle does the Trochlear Nerve innervate?

The superior oblique muscle of the eye.

What is the primary function of the Trigeminal Nerve?

It provides sensation to the face and controls jaw movement.

What muscle does the Abducens Nerve innervate?

The lateral rectus muscle of the eye.

What functions does the Facial Nerve serve?

It is responsible for taste and facial expression.