The Somatic Nervous System
Chapter 14
Chapter Objectives
▪ Describe the components of the somatic nervous system
▪ Name the modalities and submodalities of the sensory
systems
▪ Distinguish between general and special senses
▪ Describe regions of the central nervous system that
contribute to somatic functions
▪ Explain the stimulus-response motor pathway
Sensory Perception
14.1
Sensory Perception
• A component of the somatic nervous system.
• The somatic nervous system is responsible for our conscious perception of
the environment and our voluntary responses to that perception by means of
skeletal muscles.
• Sensory receptors detect environmental stimuli and relay information
through sensory neurons to the brain for processing.
This Photo by Unknown Author is licensed under CC BY
General Properties of Receptors
• Sensation—a subjective awareness of the stimulus
▫ Most sensory signals delivered to the CNS produce no
conscious sensation !!!!
 Filtered out in the brainstem before reaching the cortex, thus
preventing information overload
 Some signals do not require conscious awareness like pH and
body temperature
Receptor Classification By Structure
Receptor Classification By Location of Stimulus
▪Exteroceptor
▪ Receives external stimuli from environment
▪Interoceptor
▪ Receives internal stimuli from visceral organs
▪Proprioceptor
▪ Receives stimuli about body position and movement
Receptor Classification By Function
▪Chemoreceptor (responds to chemical stimuli)
▪Nociceptor (detects pain)
▪Mechanoreceptor (responds to physical stimuli –
pressure, vibration, & body position)
▪Thermoreceptor (sensitive to temperature)
Sensory Modalities
• General sense
▫ Distributed throughout body
• Special sense
▫ Specific organ dedicated to it
• Modality
▫ Refers to how information is
encoded
Somatosensation (General Sense)
▪ A group of sensory modalities that are associated with touch, proprioception,
and interoception. The receptors are spread throughout the body.
Mechanoreceptors of Somatosensation
Name
Historical Name
Location(s)
Stimuli
Free nerve endings
*
Dermis, cornea,
tongue, joint
capsules, visceral
organs
Pain, temperature,
mechanical
deformation
Mechanoreceptors
Merkel’s discs
Epidermal–dermal
junction, mucosal
membranes
Low frequency
vibration (5–15 Hz)
Somatosensation (General Sense)
Mechanoreceptors of Somatosensation
Name
Historical Name
Location(s)
Stimuli
Tactile corpuscle
Meissner’s corpuscle Papillary dermis,
especially in the
fingertips and lips
Lamellated
corpuscle
Pacinian corpuscle
Deep dermis,
Deep pressure, highsubcutaneous tissue frequency vibration
(around 250 Hz)
Hair follicle plexus
*
Wrapped around
hair follicles in the
dermis
Light touch,
vibrations below 50
Hz
Movement of hair
Somatosensation (General Sense)
Mechanoreceptors of Somatosensation
Name
Historical Name
Location(s)
Muscle spindle
*
In line with skeletal Muscle contraction
muscle fibers
and stretch
Tendon stretch
organ
Golgi tendon organ In line with tendons Stretch of tendons
Bulbous corpuscle
Ruffini’s corpuscle
Dermis, joint
capsules
Stimuli
Stretch
Gustation
▪ Sense of taste is associated with the tongue and provides
information about the foods and liquids we consume.
▪ Basic Taste Sensations: sweet, salty, sour, bitter, umami, and
fats/lipids
▪ Water receptors in taste buds located in the pharynx
▪ Taste buds are found in the papillae of the tongue
▪ Composed of chemoreceptors
▪ Chemicals must be dissolved in the saliva to taste
▪ Gustation decrease dramatically with age
▪ Ageusia: loss of the sense of smell
Lingual Papillae
Taste Buds
Gustatory
Pathway
This Photo by Unknown Author is licensed under CC BY
Connection of Taste and Smell
• Both taste (gustation) and smell (olfaction) are
chemical senses
• Sense of taste is heavily connected to sense of smell
• Without smell, many foods cannot be properly
tasted or distinguished (often severe cold congestion
that blocks up the nasal cavities affects our sense of
taste for this reason)
17
Olfaction
• Olfactory cells
Olfaction
▫ Are neurons
▫ Head bears 10 to 20 cilia called olfactory hairs
▫ Have binding sites for odorant molecules and are
nonmotile
▫ Axons collect into small fascicles and leave cranial
cavity through the cribriform foramina in the
ethmoid bone
▫ cranial nerve I
Olfactory Pathway
▪ The olfactory
tracts reach
the olfactory
cortex located
within the
temporal lobe
of the
cerebrum.
Anosmia – loss
of the sense of
smell
Hearing and Equilibrium
• Hearing—a response to vibrating air molecules
• Equilibrium—the sense of motion, body orientation,
and balance
• Both senses reside in the inner ear, a maze of fluidfilled passages and sensory cells
• Fluid is set in motion and the sensory cells convert
this motion into an informative pattern of action
potentials (mechanoreceptors!!)
Audition (Sense of Hearing)
▪ Anatomy of the Ear
Audition (Sense of Hearing)
▪ Anatomy of the Ear
This Photo by Unknown Author is licensed under CC BY
• Ear has three sections: outer, middle, and inner ear
▫ First two are concerned only with the transmission of sound to the inner ear
▫ Inner ear: vibrations converted to nerve signals
Outer Ear
• Outer ear—a funnel for conducting vibrations to the
tympanic membrane (eardrum)
▫ Auricle (pinna) directs sound down the auditory canal
 Shaped and supported by elastic cartilage
▫ Auditory canal (external acoustic meatus): passage leading
through temporal bone to tympanic membrane
 Guard hairs protect outer end of canal
 Cerumen (earwax)—mixture of secretions of ceruminous and
sebaceous glands and dead skin cells
Middle Ear
• Middle ear—located in the air-filled tympanic cavity in
temporal bone
▫ Tympanic membrane (eardrum) closes the inner end of
the auditory canal (separates it from middle ear)
 Suspended in a ring-shaped groove in the temporal bone
 Vibrates freely in response to sound
Middle Ear
▫ Auditory (eustachian) tube connects middle-ear to
nasopharynx
 Equalizes air pressure on both sides of tympanic membrane
 Normally closed, but swallowing or yawning open it
 Allows throat infections to spread to middle ear
▫ Auditory ossicles
 Malleus: has long handle attached to inner surface of
tympanic membrane
 Incus: articulates with malleus and stapes
 Stapes: shaped like a stirrup; footplate rests on oval window—
where inner ear begins
Middle-Ear Infection
• Otitis media (middle-ear infection) is common in children
▫ Auditory tube is short and horizontal allows for URI to spread
to middle ear
▫ Infections easily spread from throat to tympanic cavity and
mastoid air cells
• Symptoms
▫ Fluid accumulates in tympanic cavity producing pressure, pain,
and impaired hearing
▫ Can spread, leading to meningitis
▫ Can cause fusion of ear ossicles and hearing loss
• Tympanostomy—lancing tympanic membrane and draining
fluid from tympanic cavity
Inner Ear
• Two major functions: hearing and equilibrium
• Consists of a labyrinth: bony passageways lined with
membranous tubes
▫ Includes: vestibule, three semicircular ducts, and
cochlea
▫ Endolymph: fills the inside of the tubes
▫ Perilymph: between bone and membranous tubes
• Cochlea: organ of hearing
29
Inner Ear
▪ Cochlea: hearing
▪ Semicircular
canals (ducts):
dynamic
equilibrium
▪ Vestibule (utricle
& saccule): static
equilibrium
Perilymph
Endolymph
This Photo by Unknown Author is licensed under CC BY-SA-NC
Inner Ear: Cochlea
• Cochlea has three fluid-filled chambers separated by membranes
▫ Scala vestibuli: superior chamber
 Filled with perilymph
 Begins at oval window and spirals to apex
▫ Scala tympani: inferior chamber
 Filled with perilymph
 Begins at apex and ends at round window
▫ Scala media (cochlear duct): middle chamber
Filled with endolymph
 Separated from:
 Scala vestibuli by vestibular membrane
 Scala tympani by thicker basilar membrane
 Contains—organ of Corti: acoustic organ that converts
vibrations into nerve impulses
16-31
Anatomy of the Cochlea
Auditory Pathway
Stimulation of Cochlear Hair Cells
• Tympanic membrane vibrates in response to sound and pushes on the
ossicles
• Ossicles stapes pushes on oval window
• Causes movement of the perilymph in the inner ear and movement of
basilar membrane under cochlear hair cells
▫ As often as 20,000 times per second
• Movement of hair cells with basilar membrane stimulates the cells
and results in generation of a signal in the cochlear nerve CNVIII
34
Cochlea (Organ of Corti)
Deafness
• Deafness—hearing loss
▫ Conductive deafness: conditions interfere with
transmission of vibrations to inner ear
 Damaged tympanic membrane, otitis media, blockage of
auditory canal, and otosclerosis
 Otosclerosis: fusion of auditory ossicles that prevents their free
vibration
▫ Sensorineural (nerve) deafness: death of hair cells or any
nervous system elements concerned with hearing
 Factory workers, musicians, construction workers
Equilibrium
• Equilibrium—coordination, balance, and orientation
in three-dimensional space
• Vestibule receptors for equilibrium
Three semicircular ducts
Two chambers
saccule and utricle
Equilibrium
Head position is sensed by…utricle and saccule
composed of Macula tissue (hair cells with support cells)
• Head movement sensed by semicircular canals
Static Equilibrium
▪ The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head
starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolith in which they
are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.
Dynamic Equilibrium Semicircular canals (ducts)
• Rotary movements detected by the three semicircular ducts
• Bony semicircular canals of temporal bone hold membranous
semicircular ducts
• Each duct is filled with endolymph and opens up as a dilated sac
ampulla
• Each ampulla contains crista ampullaris—mound of hair cells and
supporting cells
Dynamic Equilibrium
▪ Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the
canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and
stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the
head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.
Disorders of the Ear
▪ Otitis media infection middle ear
▪ Otitis externa infec external auditory canal
▪ Otalgia earache
▪ Presbycusis loss of the ability to hear high-pitched
sounds
▪ Deafness:
▪ Conduction deafness
▪ Sensorineural deafness
▪ Vertigo loss of equilibrium
▪ Tinnitus ringing/clicking sounds in ear
• https://www.youtube.com/watch?v=bwCz3Q8y-PM
• balance---- https://www.youtube.com/watch?v=-jc4gkN8w7o
• More Hearing and balance
hearing and balance
Vision
▪ Accessory Structures of the Eye
Lacrimal Apparatus
Source: https://commons.wikimedia.org/wiki/File:Tear_system-pt.svg
• Lacrimal apparatus makes, distributes and drains tears.
• Tears from lacrimal gland wash and lubricate eye, deliver O2 and
nutrients, and prevent infection with a bactericidal lysozyme
• Tears flow through lacrimal punctum (opening on eyelid edge) to
lacrimal sac, then into nasolacrimal duct emptying into nasal cavity
Structures of the Eye
▪ The wall of the eye is composed of three layers: the fibrous tunic,
vascular tunic, and neural tunic.
The Tunics
• 1) Tunica fibrosa—outer fibrous layer
▫ Sclera: dense, collagenous white of the eye
▫ Cornea: transparent region of modified sclera in front of eye that
admits light
• 2) Tunica vasculosa (uvea)—middle vascular layer
▫ Choroid: highly vascular, deeply pigmented layer behind retina
▫ Ciliary body: extension of choroid; a muscular ring around lens
 Supports lens and iris
 Secretes aqueous humor
▫ Iris: colored diaphragm controlling size of pupil (opening)
 If there is a lot of melanin in chromatophores (cells) of iris—brown or black
eye color
 If there is reduced melanin—blue, green, or gray eye color
• 3) Tunica interna—retina and beginning of optic nerve
The Optical Components
• Optical components—transparent elements that admit
light, refract light rays, and focus images on retina:
cornea, aqueous humor, lens, vitreous body
▫ Cornea: transparent anterior cover
▫ Aqueous humor
 Serous fluid secreted by ciliary body into posterior chamber—
posterior to cornea, anterior to lens
 Then moves through pupil to anterior chamber
 Reabsorbed by scleral venous sinus at same rate it is secreted
The Optical Components
▫ Lens
 Lens fibers—flattened, tightly compressed, transparent cells
that form lens
 Suspended by suspensory ligaments from ciliary body
 Changes shape to help focus light
 Rounded with no tension or flattened with pull of suspensory
ligaments
▫ Vitreous body (humor) fills vitreous chamber
 Jelly fills space between lens and retina
The Neural Components
• Include retina and optic nerve
• Retina
▫ Pressed against rear of eyeball by vitreous humor
▫ Detached retina causes blurry areas of vision and can lead to
blindness
• Macula lutea—patch of retina on visual axis of eye (3 mm diameter)
▫ Fovea centralis: center of macula; finely detailed images due to packed
receptor cells
• Optic disc—blind spot
▫ Optic nerve exits retina and there are
▫
no receptors there !!!!!!!!!!
• Blind spot—use test illustration above (IN LAB)
▫ Close right eye, stare at X and red dot disappears
Neural Tunic
(Retina)
▪ Photoreceptors – sensitive
to light
▪ Cones – for bright light &
high acuity color vision
▪ Rods – for dim light &
peripheral vision
▪ Fovea centralis: site of
highest visual acuity; has
only cones
Formation of an Image
• Light passes through lens to form tiny inverted image
on retina
• Iris diameter controlled by two sets of contractile
elements
▫ Pupillary constrictor: smooth muscle encircling pupil
 Parasympathetic stimulation narrows pupil
▫ Pupillary dilator: spoke-like myoepithelial cells
 Sympathetic stimulation widens pupil
Refraction of Light
▪ The bending of light rays by the anterior surface of the cornea,
the lens, and the internal eye fluids (aqueous & vitreous
humor) to focus directly on the retina.
▪ It produces an inverted image on the retina. Eventually, the
brain will revert the image to its original orientation.
Disorders of the Eye
▪ Astigmatism unequal curvature of the lens or cornea
▪ Cataracts clouding of the lens
▪ Glaucoma increased intraocular pressure which may increase and
compress on the retina and optic nerve.
▪ Retinal detachment the retina detaches from the vascular layers
and allow the jellylike vitreous humor to seep between them and
can cause permanent blindness.
▪ Diplopia double vision
▪ Strabismus cross eyed
▪ Colorblindness
▪ Nyctalopia “night blindness” that is usually the result of a vitamin
A deficiency
• Hyperopia farsighted can see far can't see close well
• Myopia near sighted can see near not far
• Presbyopia reduced ability to accommodate for near vision WITH AGE
Central Processing
14.2
Sensory
Pathways
▪ Ascending
Pathway (Tract)
▪ carries
peripheral
sensations to
the brain
Sensory
Homunculus
▪ A map of the location
of the somatosensory
receptors in the body
onto the
somatosensory cortex.
Referred Pain
• Referred pain: pain in viscera often mistakenly thought to come from
the skin or other superficial site
▫ Results from convergence of neural pathways in CNS
▫ Brain interprets that visceral pain is coming from skin and cannot
distinguish the source
• Example: heart pain felt in shoulder or arm because both areas send
pain input to spinal cord segments T1 to T5
• Very important to keep in mind while diagnosing pts
60
Referred pain
Motor Responses
14.3
Spinothalamic tract is part of the anterolateral system that passes up the anterior and lateral
columns of the spinal cord
It carries signals for pain, pressure, temperature, light touch, tickle, and itch
Spinoreticular tract travels up anterolateral system
Carries pain signals!!resulting from tissue injury
emotional aspect of pain and is important point in pain control
Spinocerebellar Tracts Carry proprioceptive signals from limbs and trunk up to the cerebellum
Motor Pathways
Descending (motor) tracts
• Descending tracts—carry motor signals down brainstem and spinal
cord!!
• Involve two motor neurons
▫ Upper motor neuron originates in cerebral cortex or brainstem and
terminates on a lower motor neuron
▫ Lower motor neuron soma is in brainstem or spinal cord
 Axon of lower motor neuron leads to muscle or other target organ
Corticospinal Tracts
• Corticospinal tracts carry MOTOR signals from cerebral cortex for
precise, finely coordinated movements
• Tectospinal tract—begins in midbrain region (tectum)
▫ Reflex turning of head in response to sights and sounds
• Lateral and medial vestibulospinal tracts
▫ Begin in brainstem vestibular nuclei
▫ Receive impulses for balance from inner ear
▫ Control extensor muscles of limbs for balance control
▫ Reticulospinal tracts very involved with pain
modulation/control
“ descending analgesic pathways” which naturally reduce transmission of
pain signals to the brain
Reflex Arc
▪ Reflexes are rapid, automatic responses to specific stimuli.
▪ Five Components of a Typical Reflex Arc:
▪Receptor cell
▪Sensory neuron
▪Interneurons (association neurons)
▪Motor neuron
▪Effector cell
Classification of Reflexes
▪ Development
▪ Innate reflexes
▪ Acquired reflexes
▪ The Nature of the Response
▪ Somatic reflexes
▪ Visceral reflexes
▪ Polysynaptic reflexes
▪ Monosynaptic reflexes
▪ The Processing Site
▪ Spinal reflexes
▪ Cranial reflexes
Patellar
Reflex
(Somatic)
This Photo by Unknown Author is licensed under CC BY-NC-ND
Withdrawal
Reflex
(Somatic)
This Photo by Unknown Author is licensed under CC BY-NC-ND