Regulation and Integration of the Body
well as receptors in skeletal muscles and tendons and in the
visceral organs. Sense organs contain sensory receptors and other
cells that serve the special senses (vision, hearing, equilibrium,
smell, and taste).
Receptors are classiﬁed according to stimulus detected
as mechanoreceptors, thermoreceptors, photoreceptors,
chemoreceptors, and nociceptors, and according to location as
exteroceptors, interoceptors, and proprioceptors.
Te general sensory receptors are classiﬁed structurally as free
or encapsulated nerve endings of sensory neurons. Te free
endings are mainly receptors for temperature and pain, although
two are for light touch (tactile discs and hair follicle receptors).
Te encapsulated endings, which are mechanoreceptors, include
tactile corpuscles, lamellar corpuscles, bulbous corpuscles, muscle
spindles, tendon organs, and joint kinesthetic receptors.
Sensory Integration: From Sensation to Perception
Sensation is awareness of internal and external stimuli.
Perception is conscious interpretation of those stimuli.
Te three levels of sensory integration are the receptor, circuit, and
perceptual levels. Tese levels are functions of the sensory receptors,
the ascending pathways, and the cerebral cortex, respectively.
Sensory receptors transduce (convert) stimulus energy via
receptor or generator potentials into action potentials. Stimulus
strength is frequency coded. Adaptation (decreased response
to a continuous or unchanging stimulus) occurs in all general
receptors except pain and proprioceptors.
Te circuit level consists of the ascending pathways—the axons of
the ﬁrst-, second-, and third-order sensory neurons. Tese were
discussed in Chapter 12.
Perception—the internal, conscious image of the stimulus
that serves as the basis for response—is the result of cortical
Te main features of sensory perception are perceptual detection,
magnitude estimation, spatial discrimination, feature abstraction,
quality discrimination, and pattern recognition.
Nerves and Their Structure and Repair
Nerves and Associated Ganglia
A nerve is a bundle of axons in the PNS. Each ﬁber is enclosed
by an endoneurium, fascicles of ﬁbers are wrapped by a
perineurium, and the whole nerve is bundled by the epineurium.
Nerves are classiﬁed according to the direction of impulse
conduction as sensory, motor, or mixed; most nerves are mixed.
Eﬀerent ﬁbers may be somatic or autonomic.
Ganglia are collections of neuron cell bodies associated with
nerves in the PNS. Examples are the dorsal root (sensory) ganglia
and autonomic (motor) ganglia.
Injured PNS ﬁbers may regenerate if macrophages enter the area,
phagocytize the debris, and promote the proliferation of Schwann
cells. Schwann cells then form a channel and secrete chemicals to
guide axon sprouts to their original contacts. Fibers in the CNS
do not normally regenerate because the oligodendrocytes inhibit
axonal sprouting and regrowth.
±welve pairs of cranial nerves issue through the skull to innervate
the head and neck. Only the vagus nerves extend into the
thoracic and abdominal cavities. All but the accessory nerve
originate from the brain.
Cranial nerves are (generally) numbered from rostral to caudal in
order of emergence from the brain. Teir names reﬂect structures
served or function or both. Te cranial nerves and their numbers
Olfactory nerves: purely sensory. Concerned with the
sense of smell.
Optic nerves: purely sensory. ±ransmit visual impulses
from the retina to the thalamus.
Oculomotor nerves: primarily motor. Emerge from the
midbrain and serve four extrinsic eye muscles, the levator
palpebrae superioris of the eyelid, and the intrinsic ciliary
muscle of the eye and constrictor ﬁbers of the iris. Also carry
proprioceptive impulses from the skeletal muscles served.
±rochlear nerves: primarily motor. Issue from the dorsal
midbrain and carry motor and proprioceptor impulses to
and from superior oblique muscles of the eyeballs.
±rigeminal nerves: mixed nerves. Emerge from the lateral
pons as the main general sensory nerves of the face. Each
has three sensory divisions: ophthalmic, maxillary, and
mandibular. Te mandibular branch also contains motor
ﬁbers that innervate the chewing muscles.
Abducens nerves: primarily motor. Emerge from the pons
and serve the motor and proprioceptive functions of the
lateral rectus muscles of the eyeballs.
Facial nerves: mixed nerves. Emerge from the pons as the
major motor nerves of the face. Also carry sensory impulses
from the taste buds of anterior two-thirds of the tongue.
Vestibulocochlear nerves: mostly sensory. ±ransmit
impulses from the hearing and equilibrium receptors of
the inner ears.
Glossopharyngeal nerves: mixed nerves. Issue from the
medulla. ±ransmit sensory impulses from the taste buds
of the posterior tongue, the pharynx, and chemo- and
baroreceptors of the carotid bodies and sinuses. Innervate
some pharyngeal muscles and parotid glands.
Vagus nerves: mixed nerves. Arise from the medulla.
Almost all motor ﬁbers are autonomic parasympathetic
ﬁbers; motor eﬀerents to, and sensory ﬁbers from, the
pharynx, larynx, and visceral organs of the thoracic and
Accessory nerves: primarily motor. Arise as spinal rootlets
from the cervical spinal cord and enter the foramen
magnum. Supply somatic eﬀerents to the trapezius and
sternocleidomastoid muscles of the neck and carry
proprioceptor aﬀerents from the same muscles.
Hypoglossal nerves: primarily motor. Issue from the
medulla and carry somatic motor eﬀerents to, and
proprioceptive ﬁbers from, the tongue muscles.
Te 31 pairs of spinal nerves (all mixed nerves) are numbered
successively according to the region of the spinal cord from
which they issue.
Dorsal and ventral roots of the spinal cord fuse to form spinal
nerves. Spinal nerves are short, conﬁned to the intervertebral
Branches of each spinal nerve include dorsal and ventral
rami, a meningeal branch, and in the thoracic region, rami
communicantes (ANS branches).
Ventral rami, except ±
, form plexuses that serve the limbs.