Chapter 13: The Peripheral Nervous System and Reflex Activity Flashcards
5 sensory receptors classified by stimulus type
- Mechanoreceptors
- Thermoreceptors
- Photoreceptors
- Chemoreceptors
- Nociceptors
Mechanoreceptors
respond to touch, pressure, vibration, and stretch
Thermoreceptors
sensitive to changes in temperature
Photoreceptors
respond to light energy, light sensitive (e.g., retina)
Chemoreceptors
respond to chemicals (e.g., smell, taste, changes in blood chemistry)
Nociceptors
sensitive to paint-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals)
1st step in regeneration of a PNS axon
- the axon fragments
- cut axon ends seal themselves off
- axon transport interrupted, causing cut ends to swell
- without access to cell body, axon and its myelin sheath begins to disintegrate distal to the injury
- degeneration of distal end of cut axon, called Wallerian degeneration, spreads down axon
2nd step in regeneration of a PNS axon
- Schwann cells and macrophages clean out dead axon distal to injury
- surviving Schwann cells engulf myelin fragments and secrete chemicals that recruit macrophages
- macrophages help dispose of debris and release chemicals that stimulate Schwann cells to divide
3rd step in regeneration of a PNS axon
- axon filaments grow through a regeneration tube
- Schwann cells release growth factors and express cell adhesion molecules (CAMs) that encourage axon growth
- Schwann cells line up along the tube of remaining endoneurium, forming a regeneration tube that guides the regenerating axon “sprouts” across the gap to their original contact
4th step in regeneration of a PNS axon
- the axon regenerates and a new myelin sheath forms
- Schwann cells protect and support the regeneration axon and ultimately produce a new myelin sheath
Cranial nerves (I-XII)
I. Olfactory
II. Optic
III. Oculomotor
IV. Trochlear
V. Trigeminal
VI. Abducens
VII. Facial
VIII. Vestibulocochlear
IX. Glossopharyngeal
X. Vagus
XI. Accessory
XII. Hypoglossal
Sensory cranial nerves
I. Olfactory: smell
II. Optic: vision
VIII. hearing and balance: cochlea (hearing receptors) and vestibule of inner ear (equilibrium receptors)
Motor cranial nerves
III. Oculomotor
IV. Trochlear
VI. Abducens
XI. Accessory
XII. Hypoglossal
Motor cranial nerves: motor nerves for eye movement
III. Oculomotor: innervates 4/6 extrinsic eye muscles: inferior oblique, superior rectus, inferior rectus, medial rectus
IV. Trochlear: innervates 1/6 extrinsic eye muscles: superior oblique muscle
VI. Abducens: innervates 1/6 extrinsic eye muscles: lateral rectus muscle
Other motor cranial nerves
XI. Accessory: move head and neck innervates sternocleidomastoid and trapezius
XII. Hypoglossal: swallowing and speech; innervates all extrinsic and intrinsic muscles of tongue except palatoglossus
Sensory and motor cranial nerves
V. Trigeminal- sensory: general sensation; motor: mandibular branch, muscle of mastication (chewing)
VII. Facial- sensory: taste; motor: chief motor nerve of face- facial expression, lacrimal glands, salivary glands
IX. Glossopharyngeal- sensory: taste; motor: tongue and pharynx for swallowing, parotid salivary glands
X. Vagus- sensory: taste; motor: heart, lung and abdominal vescera (regulate heart rate, breathing, digestive system activity), pharynx and larynx (swallowing)
Largest cranial nerve
Trigeminal (V): actually 3 nerves
2 cranial nerves with multiple branches and names of the branches
Facial (VII): 5 branches- temporal, zygomatic, buccal, mandibular, and cervical
Trigeminal (V): 3 branches- ophthalmic, maxillary, and mandibular
Tic Douloureux - cause & cranial nerve associated
Cause: compression of trigeminal nerve near brain stem
What is it: inflammation of trigeminal nerve, produces excruciating pain
* treated with meds, if unresponsive –> surgery to relieve compression or destroy nerve –> loss of sensation on side of face with destroyed nerve
Bell’s palsy: cause & cranial nerve associated
cause: inflamed, swollen facial nerve, possibly due to herpes simplex 1 viral infection
What is it: paralysis of facial muscles on affected side with partial loss of taste
* treatment w/ corticosteroids
Nerve Plexuses
- interweaving network of nerves formed by branching and interconnection of ventral rami
- each branch contains fibers from several different spinal nerves
What is the benefit of arranging nerves in plexuses
- damage to one branch does not cause paralysis because there are other branches involved in the plexus
- allows for a more organized distribution of nerve signals to different body parts
How might the major nerve of the sacral plexus be damaged
- sciatica: stabbing pain in sciatic nerve
- causes: fall, disc herniation, improperly placed injection buttocks
- if nerve cut leg nearly useless- cannot flex leg, foot drop (plantar flexion bc foot and ankle cannot move)
- recovery from sciatic nerve injury is slow and incomplete (deficits remain)
Nerve plexuses and their major nerves
- cervical; major nerve: phrenic
- brachial; major nerves: axillary, musculocutaneous, median, radial, ulnar
- lumbar; major nerves: femoral, obturator
- sacral; major nerves: sciatic (composed of tibial and common fibular)
Brachial plexus- how subdivisions are oriented from medial to lateral (4)
- Roots: five ventral rami (C5 to T1 which unite to form…
- Trunks: upper, middle, and lower, which unite to form…
- Divisions: anterior and posterior, which unite to form…
- Cords: lateral, medial, and posterior
Intrinsic (inborn) reflexes
- rapid, predictable, involuntary motor movements in response to some stimuli
- not learned, not premeditated, subconscious (happen without thinking)
- regulated by brain stem and spinal cord
- ex. posture, control of visceral activities
Learned (acquired) reflexes
- acquired, result from practice or repetitions
- ex. riding a bike, driving a car
- with practice these activities become largely automatic bc we have put a lot of time and effort into developing the skill
Dermatomes - what are they
- area of skin innervated by cutaneous branches of a single spinal nerve (all spinal nerves except C1)
- areas of skin that send signals to the brain through spinal nerves
Dermatomes - how are they useful to clinicians
- can help healthcare provider detect and diagnose conditions or problems affecting spine, spinal cord, or spinal nerves
- extent of spinal cord injuries ascertained by affected dermatomes
Stretch reflexes (4)
- plantar
- crossed extensor
- knee-jerk
- tendon reflex
Stretch reflexes purpose
- maintain posture and muscle tone in postural muscles
- muscles contract if stretched too far
Can you artificially trigger any of the stretch reflexes? If so, how would you trigger that stretch reflex to assess it in a clinical setting?
Stretch reflexes can be used for clinical testing, such as by tapping the patellar tendon, to stimulate the knee-jerk reflex, which slightly stretches the quadriceps muscle
Babinski’s sign- what is it
damage to primary motor cortex or corticospinal tracts causes abnormal response called Babinski’s sign- big toe dorsiflexes, smaller toes fan out laterally
* normal response: downward curling (flexion) of toes
What reflex is this associated with
plantar (babinski) reflex
When is it normal vs abnormal to see babinski’s sign
normal: infants (1-1.5 years old) due to incomplete myelination of nerve fibers
abnormal: children older than 2
