Biology 220
Anatomy & Physiology I
Unit VIII
SPINAL NERVES,
RECEPTORS
AND REFLEXES
Chapter 13
pp. 475-483,
491-507
E. Gorski/ E. Lathrop-Davis/ S. Kabrhel
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)
Number and Naming
•
31 pairs of nerves
° cervical -- 8 pair
° thoracic -- 12 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
C8
T1
T
1
Fig. 12.24, p. 463; see also Fig. 13.5, p. 491
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
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
Spinal Rami
Dorsal
Ramus
Ventral
Ramus
Ramus
communicans
See Fig. 13.6, p. 492
Fig. 14.6, p. 521
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
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 mitosisstimulating 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
Regeneration of Neurons: PNS
Fig. 13.3, p. 482
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
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
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
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
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
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
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
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. 13.10, p. 500
Sacral Plexus
•
•
Superior gluteal nerve
° motor to gluteus
medius
Inferior gluteal nerve
° motor to gluteus
maximus
Fig. 13.10, p. 500
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
Fig. 13.11, p. 501
Dermatomes
°
posterior leg
and perineum -ventral rami of
S2-S5
Fig. 13.11, p. 501
General Sensory Receptors
•
Classification based on:
1. Location
2. Type of stimulus detected
3. Structural complexity
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
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 (lightgated 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
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)
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
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
•
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 tendonlike organs) -- stretch, joint position, motion
° located in joint capsule of synovial joint
° important of proprioception
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. 13.12, p. 502
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
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
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. 13.17, p.507
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
Crossed Extensor Reflex (con’t)
Fig. 13.17,
p.507
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
Deep Tendon Reflex
Fig. 13.16, p. 506