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 Merkel cells and Tactile tactile discs corpuscle Free nerve ending Ruffini corpuscle Lamellated corpuscle Root hair plexus Free nerve endings of root hair plexus Sensory nerves
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
Equilibrium and Hearing The Middle Ear consists of: Auditory ossicles Malleus, incus, and stapes Auditory tube
Equilibrium and Hearing
Figure 18.12a Semicircular Canals and Ducts Semicircular ducts Semicircular canal Anterior Lateral Posterior Vestibule KEY Cristae within ampullae Maculae Endolymphatic sac Membranous labyrinth Bony labyrinth Cochlea Utricle Saccule Vestibular duct Cochlear duct Anterior view of the bony labyrinth cut away to show the semicircular canals and the enclosed semicircular ducts of the membranous labyrinth Tympanic duct Organ of Corti
Spiral organ of corti = hearing
Figure 18.17de The Cochlea and Organ of Corti Bony cochlear wall Vestibular duct Vestibular membrane Cochlear duct Tectorial membrane Spiral ganglion Basilar membrane Tympanic duct Organ of Corti Three-dimensional section showing the detail of the cochlear chambers, tectorial membrane, and organ of Corti Cochlear branch of N VIII Tectorial membrane Cochlear duct (scala media) Vestibular membrane Outer hair cell Tectorial membrane Basilar membrane Inner hair cell Nerve fibers Diagrammatic and histological sections through the receptor hair cell complex of the organ of Corti Tympanic duct (scala tympani) Basilar membrane Hair cells of organ of Corti Spiral ganglion cells of cochlear nerve Organ of Corti LM 125
Equilibrium and Hearing
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) Vestibulocochlear nerve (N VIII) External acoustic meatus Bony labyrinth of inner ear Tympanic membrane Elastic cartilage Tympanic cavity Oval window Round window Vestibule Auditory tube Cochlea To nasopharynx
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 Ora serrata Fornix Posterior cavity (Vitreous chamber filled with the vitreous body) Palpebral conjunctiva Ocular conjunctiva Ciliary body Anterior chamber (filled with aqueous humor) Lens Central retinal artery and vein Optic nerve Optic disc Fovea Retina Choroid Pupil Cornea Iris Posterior chamber (filled with aqueous humor) Corneal limbus Suspensory ligaments Sclera Major anatomical landmarks and features in a diagrammatic view of the left eye
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 Parasympathetic stimulation Sympathetic 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 Posterior cavity Choroid Ciliary body Iris Anterior cavity Vascular tunic (uvea) Neural tunic (retina) Neural part Pigmented part Cornea Sclera Fibrous tunic The lens is suspended between the posterior cavity and the posterior chamber of the anterior cavity.
Vision 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 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 Visual axis Anterior cavity Posterior Anterior chamber chamber Edge of pupil Cornea Iris Suspensory ligament of lens Nose Corneal limbus Lacrimal punctum Conjunctiva Lacrimal caruncle Medial canthus Ciliary processes Lens Lower eyelid Lateral canthus Ciliary body Ora serrata Fovea Ethmoidal labyrinth Sclera Choroid Retina Posterior cavity Lateral rectus muscle Medial rectus muscle Optic disc Optic nerve Orbital fat Central artery and vein Sagittal section through the eye
Vision 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
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 Binocular vision Right eye only Optic nerve (N II) Optic chiasm Optic tract Other hypothalamic nuclei, pineal gland, and reticular formation Suprachiasmatic nucleus Lateral geniculate nucleus Superior colliculus Lateral geniculate nucleus Projection fibers (optic radiation) LEFT CEREBRAL HEMISPHERE Visual cortex of cerebral hemispheres RIGHT CEREBRAL HEMISPHERE