A & P Chapter 13: The Peripheral Nervous System and Reflex Activity

Comprehensive practice flashcards for Marieb Human Anatomy & Physiology (Twelfth Edition) Chapter 13, covering sensory receptors, cranial and spinal nerves, motor control hierarchy, and reflex activity.

What four parts comprise the peripheral nervous system (PNS)?

Part 1: Sensory Receptors; Part 2: Transmission Lines: Nerves and Their Structure and Repair; Part 3: Motor Endings and Motor Activity; Part 4: Reflex Activity.

How are sensory receptors classified based on stimulus type?

Mechanoreceptors (touch, pressure, vibration, stretch), thermoreceptors (temperature), photoreceptors (light energy), chemoreceptors (chemicals), and nociceptors (pain-causing stimuli).

What are the three classes of sensory receptors based on body location?

Exteroceptors (sensitive to stimuli arising outside the body), interoceptors or visceroceptors (respond to internal viscera and blood vessels), and proprioceptors (respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissues).

What temperature ranges activate cold and heat thermoreceptors, and what occurs outside these ranges?

Cold receptors are activated from $10^\text{o}\text{C}$ to $40^\text{o}\text{C}$ in the superficial dermis; heat receptors are activated from $32^\text{o}\text{C}$ to $48^\text{o}\text{C}$ in the deeper dermis. Outside these ranges, nociceptors are activated and the stimulus is interpreted as pain.

Which key protein in nerve membranes acts as an ion channel opened by heat, low pH, and capsaicin?

The vanilloid receptor.

What are tactile (Merkel) discs, and where are they located?

They are modified free nerve endings (epithelial tactile complexes) that function as slowly adapting light touch receptors located in the basal layer of the epidermis.

What is the structural and functional classification of tactile (Meissner's) corpuscles?

They are encapsulated exteroceptors that act as rapidly adapting mechanoreceptors for light pressure, discriminative touch, and low-frequency vibration, found in the dermal papillae of hairless skin.

What type of stimuli do lamellar (Pacinian) corpuscles detect?

Deep pressure, stretch, and high-frequency vibration when first applied.

What are muscle spindles and what do they monitor?

Spindle-shaped encapsulated proprioceptors containing intrafusal fibers that monitor skeletal muscle stretch and length.

What are the three main levels of neural integration in the somatosensory system?

1. Receptor level (sensory reception and transmission to CNS)
2. Circuit level (processing in ascending pathways)
3. Perceptual level (processing in cortical sensory centers)

What is the difference between phasic receptors and tonic receptors?

Phasic receptors are fast-adapting and send signals at the beginning or end of a stimulus (e.g., receptors for pressure, touch, and smell), whereas tonic receptors adapt slowly or not at all (e.g., nociceptors and most proprioceptors).

According to this referred pain map, pain in the left chest wall and radiating down the left medial arm typically indicates damage to which visceral organ?

The heart.

Why does referred pain occur?

Because visceral and somatic pain fibers travel along the same nerves, leading the brain to assume the pain stimulus originates from the more common somatic region.

Which neurotransmitters are released by pain fibers, and what endogenous chemicals can inhibit pain impulses?

Glutamate and substance P are released to transmit pain impulses, while endogenous opioids like endorphins block pain impulses.

What are the three connective tissue coverings of a peripheral nerve from innermost to outermost?

1. Endoneurium (encloses individual axons and Schwann cells)
2. Perineurium (bundles fibers into fascicles)
3. Epineurium (tough fibrous sheath around all fascicles forming the nerve)

Why can peripheral nervous system (PNS) axons regenerate while central nervous system (CNS) axons typically cannot?

CNS oligodendrocytes produce growth-inhibiting proteins and astrocytes form scar tissue, whereas in the PNS, macrophages clean debris and Schwann cells form a regeneration tube and secrete growth factors.

What is Wallerian degeneration during peripheral nerve repair?

The disintegration and degeneration of the cut axon and its myelin sheath distal to the site of nerve injury.

Which cranial nerves carry parasympathetic fibers?

Oculomotor (cranial nerve III), Facial (cranial nerve VII), Glossopharyngeal (cranial nerve IX), and Vagus (cranial nerve X).

What condition is caused by ethmoid bone fractures or lesions to cranial nerve I (olfactory nerve)?

Anosmia (partial or total loss of smell).

What are the clinical signs of oculomotor nerve (cranial nerve III) paralysis?

The eye rotates laterally at rest (external strabismus), the upper eyelid droops (ptosis), the eye cannot move up, down, or inward, and the patient experiences double vision and trouble focusing on close objects.

Which extrinsic eye muscle is innervated by the trochlear nerve (cranial nerve IV)?

The superior oblique muscle.

What are the three divisions of the trigeminal nerve (cranial nerve V) and their skull exit routes?

Ophthalmic division ($V_1$) exits via superior orbital fissure; Maxillary division ($V_2$) exits via foramen rotundum; Mandibular division ($V_3$) exits via foramen ovale.

What is trigeminal neuralgia (tic douloureux)?

A condition caused by inflammation or vascular compression of the trigeminal nerve near its brain stem exit, resulting in excruciating stabbing facial pain provoked by light sensory stimuli.

What clinical manifestation occurs when the abducens nerve (cranial nerve VI) is paralyzed?

The affected eye cannot be moved laterally, and at rest, the eyeball rotates medially (internal strabismus).

What are the five major motor branches of the facial nerve (cranial nerve VII)?

Temporal, zygomatic, buccal, mandibular, and cervical.

What clinical condition results from swelling of the facial nerve (cranial nerve VII), often linked to herpes simplex 1?

Bell's palsy, characterized by paralysis of facial muscles on the affected side, lower eyelid drooping, corner of the mouth sagging, and partial loss of taste.

Which cranial nerve innervates the carotid sinus baroreceptors and carotid body chemoreceptors?

Glossopharyngeal nerve (cranial nerve IX).

What is unique about the vagus nerve (cranial nerve X) regarding its anatomic distribution?

It is the only cranial nerve that extends beyond the head and neck region, descending into the thorax and abdomen.

Which two muscles are innervated by the accessory nerve (cranial nerve XI)?

The trapezius and sternocleidomastoid muscles.

What clinical sign indicates unilateral damage to the hypoglossal nerve (cranial nerve XII)?

The tongue deviates (points) toward the affected side when protruded, and the paralyzed side eventually atrophies.

Why are there $8$ pairs of cervical spinal nerves despite there being only $7$ cervical vertebrae?

The first $7$ pairs ($C_1$ to $C_7$) exit superior to the vertebra for which they are named, while the eighth pair ($C_8$) exits inferior to the $C_7$ vertebra.

Which spinal nerve roots contribute to the phrenic nerve, and what does it innervate?

Fibers from $C_3$ to $C_5$; it innervates the diaphragm.

Damage to which nerve causes difficulty using the pincer grasp, as commonly seen in carpal tunnel syndrome?

The median nerve.

What deformity is illustrated here, caused by severe damage to the ulnar nerve?

Clawhand (hyperextension of the little and ring fingers at the knuckles and flexion at the distal interphalangeal joints).

What condition resulting from radial nerve compression is shown here?

Wrist drop (inability to extend the hand at the wrist).

Which two major nerves arise from the lumbar plexus ($L_1-L_4$)?

The femoral nerve and obturator nerve.

Which two nerves compose the sciatic nerve, the longest and thickest nerve of the body?

The tibial nerve and the common fibular nerve.

What is a dermatome, and which single spinal nerve does not participate in dermatomes?

A dermatome is an area of skin innervated by cutaneous sensory fibers of a single spinal nerve; $C_1$ does not participate.

What are the three levels of motor control hierarchy, from lowest to highest?

1. Segmental level (spinal cord and central pattern generators [CPGs])
2. Projection level (motor cortex and brain stem nuclei)
3. Precommand level (cerebellum and basal nuclei)

What are the five essential components of a reflex arc depicted in this diagram?

1. Receptor
2. Sensory neuron
3. Integration center
4. Motor neuron
5. Effector

Why is the stretch reflex classified as monosynaptic and ipsilateral?

It is monosynaptic because sensory neurons synapse directly with alpha motor neurons without an intervening interneuron, and ipsilateral because motor activity occurs on the same side of the body as the stimulus.

What is reciprocal inhibition during a stretch reflex?

Afferent sensory fibers synapse with inhibitory interneurons in the spinal cord that inhibit the motor neurons of antagonistic muscles, allowing the prime mover to contract unopposed.

How does the tendon reflex differ in its motor outcome from the stretch reflex?

The tendon reflex causes the contracting muscle under tension to relax and its antagonist to contract (reciprocal activation) to prevent tendon damage, whereas the stretch reflex causes the stretched muscle to contract.

What occurs during the crossed-extensor reflex when stepping barefoot on broken glass?

An ipsilateral flexor (withdrawal) reflex flexes and withdraws the injured limb, while a contralateral extensor reflex activates extensor muscles in the opposite limb to support the shifted body weight.

What spinal cord levels are assessed by the plantar reflex, and what is the normal adult response?

It tests integrity from $L_4$ to $S_2$; the normal response is downward flexion (curling) of the toes when the lateral sole of the foot is stroked.