1. Unit VIII SPINAL NERVES, RECEPTORS AND REFLEXES
Biology 220
Anatomy & Physiology I
Unit VIII SPINAL NERVES, RECEPTORS AND REFLEXES
Chapter 13
pp ,
E. Gorski/ E. Lathrop-Davis/ S. Kabrhel

2. Spinal Nerves pass through intervertebral foramina (between vertebrae)
all are mixed (sensory/motor)
areas served:
entire body
in head/neck
spinal nerves only provide innervation for sympathetic division of ANS
other head/neck innervation comes from cranial nerves)

3. Number and Naming 31 pairs of nerves cervical -- 8 pair
thoracic pair
lumbar -- 5 pair
sacral -- 5 pair
coccygeal -- 1 pair
cervical (in red) are named after the vertebra (in blue) below them (except C8); others (eg., T1 in orange) named after the vertebra above them C1 C1 T1 C8 T1
Fig , p. 463; see also Fig. 13.5, p. 491

4. Spinal Nerve Composition
Ventral root -- carries motor (efferent = outgoing) fibers (axons) of lower motor neurons and autonomic motor neurons
Dorsal root -- carries afferent (incoming) fibers from sensory neurons
Dorsal root ganglion -- consists of cell bodies of first-order sensory neurons
sensory neurons are unipolar with a peripheral process extending from receptor toward cell body and a central process extending away from cell body and entering spinal cord

5. Spinal Nerve Branches
Spinal nerve formed from union of dorsal and ventral root continues for about 2 cm then branches into rami (mixed fibers):
a. dorsal ramus -- serves skin and muscles of back
b. ventral ramus -- primarily serves skin, muscles of anterior trunk and limbs
c. rami communicantes (singular = ramus communicans) -- carry autonomic motor fibers between ventral ramus and autonomic ganglia

6. Spinal Rami Dorsal Ramus Ventral Ramus Ramus communicans
See Fig. 13.6, p. 492
Fig. 14.6, p. 521

7. Regeneration of Neurons: PNS
cell bodies -- if cell body is damaged, cell dies; neurons “downstream” from damaged neuron may die as well.
peripheral axons (PNS)
portion of axon distal to site of damage is degraded within 1 week
neurolemma usually remains intact (depends on severity of damage; myelin sheath degenerates)
axon regrows at about 1-5 mm per day
Schwann cells that form cord will eventually remyelinate regenerated axon
Research:
macrophages from outside CNS transplanted into CNS aid regeneration by secreting chemicals that inhibit growth-inhibiting proteins secreted by oligodendrites around neighboring axons
fetal astrocytes transplanted into CNS also aid regeneration
mature CNS neurons can be prompted to divide by adding large amounts of growth factors

8. Regeneration of Neurons: PNS
Steps:
1. macrophages enter area and phagocytize debris after Wallerian degeneration of distal axon and myelin sheaths (neurilemma remains intact)
2. Schwann cells proliferate in response to mitosis-stimulating chemicals released by macrophages
3. Schwann cells release nerve growth factor (NGF) and other chemicals that stimulate axonal growth
4. Schwann cells also form cellular cords which axon follows during regeneration

9. Regeneration of Neurons: PNS
Fig. 13.3, p. 482

10. Regeneration of Neurons: CNS
CNS macrophages phagocytize debris more slowly than peripheral macrophages do
normally, neuron dies because oligodendrites (supplying myelin sheath) associated with damaged neuron die, so do not aid regeneration of axon (fiber)
nearby astrocytes may proliferate and produce scarring
myelin sheaths (oligodendrites) of other, nearby axons secrete inhibitory proteins or die (secondary demyelination)
note: in experiments, macrophages transplanted from outside CNS can secrete proteins that inhibit inhibitory proteins
Inhibitory proteins important to normal neural development

11. Nerve Plexuses
Plexuses formed from branching and rejoining of ventral rami in cervical, lumbar and sacral regions (not in thoracic region)
each nerve leaving plexus contains fibers from several spinal nerves
muscles of limbs receive innervation from more than one spinal nerve
Thoracic spinal nerves T2-T12 do not form plexuses; they innervate external and internal intercostal muscles

12. Cervical Plexus Formed from the ventral rami of C1-C5
found in neck region under sternocleidomastoid muscle
sensory innervation from skin on back of neck, ear, over parotid gland, shoulder
motor innervation to lower muscles of neck, deep muscles of neck, trapezius, sternocleidomastoid
Phrenic nerve provides motor (and sensory) innervation to diaphragm
Fig. 13.7, p. 494

13. Brachial Plexus
Formed from ventral rami of C5-T1 (may get fibers from C4 and/or T2)
located in the inferior lateral neck and axillary region
serves arm and lateral shoulder, chest (pectoralis major and minor muscles)
Major nerves:
Median
motor to flexor muscles of forearm (flexor carpi radialis, flexor digitorum superficialis)
sensory from skin of lateral 2/3 of hand
Fig. 13.8, p. 496

14. Brachial Plexus: Major Nerves
Radial
motor to posterior arm extensors (triceps brachii), wrist extensors, supination (supinator), and thumb abduction
sensory from posterior arm
Ulnar
motor to flexor muscles of anterior forearm (flexor carpi ulnaris) intrinsic muscles of hand (other than palm)
sensory from medial 1/3 of hand (anterior and posterior)
Fig. 13.8, p. 496

15. Brachial Plexus: Major Nerves
Axillary
motor to deltoid and teres minor
sensory from skin of shoulder region
Musculocutaneous
motor to flexor muscles of arm (biceps brachii, brachialis)
sensory from anterolateral forearm (varies)
Fig. 13.8, p. 496

16. Lumbar Plexus Formed from ventral rami of L1-L4 Femoral nerve
motor to anterior muscles of thigh (thigh flexion and knee extension: quadriceps femoris)
sensory from medial leg and foot, anterior thigh
Obturator nerve
motor to thigh adductors (adductor magnus, brevis, longus)
sensory from skin of medial thigh
Fig. 13.9, p. 498

17. Sacral Plexus Formed from L4-S4 Sciatic nerve
motor to posterior thigh (hamstrings), posterior and anterior lower leg (gastrocnemius, soleus, tibialis anterior) and adductor magnus (posterior portion)
sensory from posterior skin
Fig , p. 500

18. Sacral Plexus Superior gluteal nerve motor to gluteus medius
Inferior gluteal nerve
motor to gluteus maximus
Fig , p. 500

19. Dermatomes
Area of skin innervated by cutaneous branches of single spinal nerve (all spinal nerves except C1)
dermatomes:
trunk -- C2-T12
arm -- ventral rami of C5-T1 (sometimes T2)
anterior leg -- ventral rami L1-S1
-- overlapping of dermatome nerves provides alternative innervation in the event of nerve damage
Fig , p. 501

20. Dermatomes posterior leg and perineum -- ventral rami of S2-S5
Fig , p. 501

21. General Sensory Receptors
Classification based on:
1. Location
2. Type of stimulus detected
3. Structural complexity

22. 1. Location
exteroceptors -- at body surface (skin); detect pain, temperature, pressure, touch
interoceptors -- internal organs, blood vessels
proprioceptors -- measure degree of stretch in muscles, tendons, joints, ligaments

23. 2. Type of Stimulus Detected
mechanoreceptors -- respond to touch, pressure, vibration, stretch (when they or surrounding tissues are deformed)
thermoreceptors -- respond to heat or cold
photoreceptors -- respond to light energy (light-gated channels)
chemoreceptors -- respond to chemicals in solution (taste, smell, blood chemistry; chemically-gated channels)
nociceptors -- pain (chemicals released during tissue damage)
itch receptors - respond to histamine
NOTE: over stimulation of any receptor causes pain
overstimulation of any receptors causes pain

24. 3. Structural Complexity
Complex receptors
associated with special senses (sight, hearing, smell, taste)
involve more complex structures and tissues
Simple receptors -- most common
modified dendritic endings of sensory neurons associated with general senses
found in epithelia, muscles, and connective tissues
free dendritic endings or encapsulated receptors (mechanoreceptors)

25. Free Dendritic Endings
= naked dendritic endings
most are unmyelinated fibers (type C)
examples:
free nerve endings -- pain, temperature, some mechanoreceptors (Merkel discs)
root hair plexuses -- mechanoreceptors of hair movement

26. Simple Encapsulated Receptors
Meissner’s corpuscles -- fine touch in skin
Krause’s end bulbs -- touch in mucous membrane of mouth and eyes (conjunctiva), lips
Pacinian corpsuscles -- pressure and vibration in dermis, tendons, joint capsules; also found in viscera
Ruffini’s endings -- deep pressure and stretch in dermis, hypodermis, joint capsules

27. Simple Encapsulated Receptors
muscle spindles -- stretch in skeletal muscle
Golgi tendon organs -- stretch in tendons
joint kinesthetic receptors (Pacinian and Ruffini corpuscles, free dendritic endings, Golgi tendon-like organs) -- stretch, joint position, motion
located in joint capsule of synovial joint
important of proprioception

28. Reflexes
Rapid, predictable, unlearned, involuntary motor response to specific stimuli
integrates sensory input with motor output without involving higher centers
used clinically to test condition of nervous system
Fig , p. 502

29. Components of the Reflex Arc
1. receptor -- specialized dendritic end
2. sensory neuron -- unipolar neuron carrying incoming impulse from receptor
3. integrating center within gray matter of spinal cord
single synapse between sensory and motor = monosynaptic
with interneuron = polysynaptic
4. motor neuron -- carries outgoing impulse to effector
5. effector -- muscle or gland that responds to motor impulse

30. Types of Spinal Reflexes
Stretch reflex*
Patellar reflex*
Achilles' reflex*
Flexor reflex
Crossed extensor reflex
Deep tendon reflex
Superficial reflex*
Plantar Reflex* (Babinski sign)
*Lab

31. Flexor Reflex=Withdrawal Reflex
polysynaptic, ipsilateral
initiated by painful stimulus, which results in contraction of flexor to withdraw body part from stimulus
may be intersegmental (may involve more than one spinal cord segment)
may involve more than one muscle group
e.g., moving hand away from flame
Fig , p.507

32. Crossed Extensor Reflex
e.g., arm being grabbed (also stepping on Lego)
two components:
1) ispilateral flexor reflex
afferent fibers (via dorsal root) --> ipsilateral interneuron (gray matter of spinal cord) --> ipsilateral flexor reflex (contraction of flexor on same side)
2) contralateral extensor reflex
same afferent fibers --> interneurons on opposite side (cross through gray commissure; contralateral) --> inhibit motor neuron to flexor on opposite side and stimulate motor neuron to extensor

33. Crossed Extensor Reflex (con’t)
Fig , p.507

34. Deep Tendon Reflex protects against over-stretching of tendon
causes inhibition (relaxation) of contracting muscle; stimulation of ipsilateral antagonist
steps:
> stimulation of Golgi tendon receptors
> afferent impulses to spinal cord enter through dorsal root
> synapse with interneurons to motor neuron of contracting muscle causing inhibition and stimulates motor neuron to antagonist muscle causing contraction

35. Deep Tendon Reflex
Fig , p. 506