The Nervous System: General and Special...

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18 The Nervous System: General and Special Senses


Introduction

• Sensory information arrives at the CNS

• Information is “picked up” by sensory

receptors

• Sensory receptors are the interface between

the nervous system and the internal and

external environment

• General senses

• Refers to temperature, pain, touch, pressure,

vibration, and proprioception

• Special senses

• Refers to smell, taste, balance, hearing, and vision


Receptors

• Receptors

• Free nerve endings are the simplest receptors

• These respond to a variety of stimuli

• Receptors of the retina (ex. rod and cone

cells) are very specific and only respond to

light


The General Senses

• Classification of the General Senses

• Nociceptors: respond to the sensation of

pain

• Thermoreceptors: respond to changes in

temperature

• Mechanoreceptors: activated by physical

distortion of cell membranes

• Chemoreceptors: monitor the chemical

composition of body fluids


Figure 18.3c Tactile Receptors in the Skin

Hair

Root hair plexus

Lamellated corpuscle

Ruffini corpuscle

Merkel cells and

tactile discs

Tactile

corpuscle

Free nerve

ending

Sensory

nerves

Free nerve endings

of root hair plexus


The General Senses

• Nociceptors

• Known as pain receptors

• Associated with free nerve endings

• difficult to “pinpoint” the location of the origin of

the pain


The General Senses

• Nociceptors

• Referred pain: perceived

sensation from visceral

organs as if from other

locations

• Ex. Pain from heart,

sensations from skin

• may reach the spinal

cord via the same

dorsal root

• Phantom pain:

experienced by amputees

• Severed sensory

neurons heal and

transmit impulse


The General Senses

• Thermoreceptors

• Found in the dermis, skeletal muscles, liver,

and hypothalamus

• Cold receptors are more numerous than hot

receptors

• Exist as free nerve endings


The General Senses

• Mechanoreceptors

• Receptors that are sensitive to stretch,

compression, twisting, or distortion of the

plasmalemmae

• There are three types

• Tactile receptors- Provide sensations of touch,

pressure, and vibrations

• Baroreceptors- Stretch receptors that monitor

changes in the stretch of organs

• Proprioceptors- Monitor the position of joints,

tension in the tendons and ligaments, and the

length of muscle fibers upon contraction


The General Senses

• Chemoreceptors

• Detect small changes in the concentration of

chemicals

• Respond to water-soluble or lipid-soluble compounds


The Special Senses

• The special senses include:

• Olfaction (smell)

• Gustation (taste)

• Equilibrium

• Hearing

• Vision


Olfaction (Smell)

• The olfactory

epithelium consists of: •Olfactory receptors

•Supporting cells

•Basal cells


Pathway for Olfaction:


Gustation (Taste)

• The tongue consists of

papillae

• Papillae consist of

taste buds

• Taste buds consist of

gustatory cells


Equilibrium and Hearing • Structures of the ear are involved in balance and

hearing

• The ear is subdivided into three regions

• External ear

• Middle ear

• Inner ear


Equilibrium and Hearing

• The External Ear consists of: • Auricle

• External acoustic meatus

• Tympanic membrane



• The Middle Ear

consists of: • Auditory ossicles

• Malleus, incus, and

stapes

• Auditory tube


Figure 18.12a Semicircular Canals and Ducts

Anterior view of the bony

labyrinth cut away to show the

semicircular canals and the

enclosed semicircular ducts of

the membranous labyrinth

Cochlear duct

Vestibular duct

Saccule

Utricle

Tympanic

duct

Organ of

Corti

Cochlea

Endolymphatic sac

Maculae

Cristae within ampullae

Bony labyrinth

Membranous

labyrinth

KEY

Vestibule

Anterior

Lateral

Posterior

Semicircular

canal

Semicircular

ducts


Spiral organ of corti = hearing


Figure 18.17de The Cochlea and Organ of Corti

Diagrammatic and histological sections through the

receptor hair cell complex of the organ of Corti

Three-dimensional section

showing the detail of the cochlear

chambers, tectorial membrane,

and organ of Corti

Bony cochlear wall

Vestibular duct

Vestibular membrane

Cochlear duct

Tectorial membrane

Basilar membrane

Tympanic duct

Organ of Corti

Spiral

ganglion

Cochlear branch

of N VIII

Cochlear duct (scala media)

Vestibular membrane

Tectorial membrane

Organ of Corti LM 125

Tympanic duct

(scala tympani)

Basilar

membrane

Hair cells

of organ

of Corti

Spiral ganglion

cells of

cochlear nerve

Tectorial membrane

Outer

hair cell

Basilar membrane Inner hair cell Nerve fibers


Mechanics of Equilibrium

• Complex process

• Requires input from sensory neurons of the

vestibular organ of both ears

• Other sources of equilibrium information

are:

• Tactile receptors

• Proprioceptors of tendons, muscles, joints


Figure 18.9 Anatomy of the Ear

EXTERNAL EAR MIDDLE EAR INNER EAR

Auricle

Auditory ossicles Semicircular canals

Petrous part of temporal

bone

Facial nerve (N VII)

External acoustic meatus

Elastic cartilage

Tympanic membrane

Tympanic cavity

Oval window

Round window

Vestibule

Auditory tube

Cochlea

To nasopharynx

Bony labyrinth of inner ear

Vestibulocochlear nerve (N VIII)


Vision

• Accessory structures

of the eye • Palpebrae (eyelids)

• Eyelashes

• Conjunctiva (epithelial

lining of the eyelids)

• Extrinsic and intrinsic

eye muscles

• lacrimal gland (produce

tears)


Vision

• Accessory Structures

• Tears are produced by

the lacrimal glands

• Flow over the ocular

surface

• Flow into the

nasolacrimal duct

• Empties into nasal

cavity


Vision

• The Eyes consist of:

• Anterior cavity

• Posterior cavity

• Three tunics:

• (1) fibrous tunic

• (2) vascular tunic

• (3) neural tunic


Functions of the Eyeball

• Transmission of light rays

• Refraction of light rays (bending)

• Accommodation of the lens (to focus)

• Constriction of the pupil (regulate light)

• Convergence of the eyeballs (depth

perception)


Figure 18.21b Sectional Anatomy of the Eye

Major anatomical landmarks and features

in a diagrammatic view of the left eye

Central retinal

artery and vein

Optic nerve

Optic disc

Fovea

Retina

Choroid

Sclera

Posterior cavity

(Vitreous chamber filled

with the vitreous body)

Ora serrata Fornix

Palpebral conjunctiva

Ocular conjunctiva

Ciliary body

Anterior chamber

(filled with aqueous

humor)

Lens

Pupil

Cornea

Iris

Posterior chamber

(filled with aqueous

humor)

Corneal limbus

Suspensory

ligaments


Vision • Cavities and Chambers

Anterior cavity (Anterior + Posterior chamber)

• Filled with fluid called aqueous fluid

• If this fluid cannot drain through the canal of Schlemm, pressure builds up

• This is glaucoma

• Posterior cavity

• Filled with fluid called vitreous fluid

• If this fluid is not of the right consistency, the pressure is reduced against the retina

• The retina may detach from the posterior wall (detached retina)


Vision

• The Eyes

• The Fibrous Tunic

(outer layer)

• Makes up the sclera

and cornea

• Provides some degree

of protection

• Provides attachment

sites for extra-ocular

muscles


Vision

• The Eyes

• The Vascular Tunic

(middle layer)

• Consists of blood

vessels, lymphatics,

and intrinsic eye

muscles

• Regulates the amount

of light entering the eye

• Secretes and reabsorbs

aqueous humor

• Controls the shape of

the lens

• Includes the iris, ciliary

body, and the choroid


Vision • The Vascular Tunic

• The iris • Consists of blood vessels,

pigment, and smooth muscles

• The pigment creates the color of the eye

• The smooth muscles contract to change the diameter of the pupil (regulate light exposure of the retina)

• The ciliary body • The ciliary bodies consist

of ciliary muscles connected to suspensory ligaments, which are connected to the lens

• The choroid • Highly vascularized

• Part of the choroid attaches to portion of the retina

Sympathetic stimulation

Parasympathetic stimulation


Vision

• The Eyes • The Neural Tunic

(inner layer) • Also called the retina

• Innermost layer of the eye

• Made of two layers: (pigmented layer – outer layer) and (neural layer – inner layer)

• Retina cells: rods (night vision) and cones (color vision)


Figure 18.22a The Lens and Chambers of the Eye

The lens is suspended between the posterior cavity

and the posterior chamber of the anterior cavity.

Pigmented part

Neural part Neural

tunic

(retina)

Posterior

cavity

Choroid

Ciliary body

Iris

Vascular

tunic

(uvea)

Anterior

cavity

Cornea

Sclera Fibrous

tunic


• The Neural Tunic (Retina)

• Outer pigmented layer

• Inner nervous layer (visual portion) • Photoreceptors (rods and cones)

• Specialized cells for responding to light

• Bipolar neurons

• Ganglion neurons

• Axons leave the eye as the optic nerve

Vision


Vision

• Visual Pathways Light waves pass through the

cornea (refracts light)

Aqueous humor

Through the lens

Vitreous humor

The lens focuses the image on the retina

(This creates a depolarization of the neural cells)

Signal is transmitted to the brain via CN II


Figure 18.21e Sectional Anatomy of the Eye

Sagittal section through the eye

Orbital fat Central artery

and vein

Medial rectus

muscle

Ethmoidal

labyrinth

Optic nerve

Optic disc

Fovea

Ora serrata

Ciliary body

Lens Ciliary

processes

Medial canthus

Lacrimal caruncle

Lacrimal punctum

Nose

Anterior cavity

Posterior

chamber

Anterior

chamber

Edge of

pupil

Visual

axis

Cornea

Iris

Suspensory ligament of lens

Corneal limbus

Conjunctiva

Lower eyelid

Lateral canthus

Sclera

Choroid

Retina

Posterior cavity

Lateral rectus muscle


• Rod Cells • Dim light

• Do not distinguish color

• Low resolution

• Cone Cells • Bright light

• High resolution

• Color detection

• Optic Disc = Blind spot (0% rods and cones) • An area lacking photoreceptors

• If an object is focused on this area, vision does not occur

• Optic nerve is attached to eyeball at this location

• Fovea Centralis = 100% cones • The best color vision is when an object is focused on the fovea

centralis

Vision


Visual Field and Visual Pathway

• Optic nerves converge at optic chiasma • half of the retina signal

crosses to opposite side

• Optic tract continues beyond chiasma • Composed of fibers from

both retinas

• Terminate at superior colliculi & thalamus

• Transmits impulses to visual cortex (occipital lobe)


Figure 18.26 Anatomy of the Visual Pathways, Part II LEFT SIDE RIGHT SIDE

Left eye

only Right eye

only

Binocular vision

Optic nerve (N II)

Optic chiasm

Optic tract Other hypothalamic

nuclei, pineal gland,

and reticular

formation

Suprachiasmatic

nucleus

Superior

colliculus

Lateral

geniculate

nucleus

Projection

fibers (optic

radiation)

Lateral

geniculate

nucleus

RIGHT CEREBRAL

HEMISPHERE

LEFT CEREBRAL

HEMISPHERE

Visual cortex of

cerebral hemispheres

The Nervous System: General and Special...

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