General and Special Senses: chapter 18
Definitions:
Sensory receptor: specialized cell or cell process that monitors conditions in the body or the external environment
Sensation: sensory info arriving at the CNS
Perception: conscious awareness of a sensation
General senses: temperature, pain, touch, pressure, vibration and proprioception
Special senses: smell, taste, equilibrium, hearing and vision
Receptor sensitivity: what the receptor will respond to
Receptive field: The area monitored by a single receptor. The larger the field the less precisely the sensation can be localized.
Tonic receptors: always active (such as the eye)
Phasic receptors: normally off but activate in response to changes in the condition they monitor
Adaptation: reduction in sensitivity in presence of constant stimulation
Peripheral adaptation: adaptation at the sensor or sensory neural level
Central adaptation: adaptation at the CNS (such as sense of smell)
GENERAL SENSES:
Exteroceptors: monitor the outside world
Interoceptors: Monitor the inside world
Proprioceptors: monitor position of joints
Nociceptors: monitor tissue damage
Thermoreceptors: monitor temperature, found in skin, muscle liver and hypothalamus
Mechanoreceptors: monitor contact and pressure, found mostly in the skin
Chemoreceptors: monitor chemical composition of body fluids (O2, CO2, pH etc.)
Baroreceptors: subset of mechanoreceptors, monitor changes in pressure of tubular organs (vessels, ureters, bowel)
Special senses:
OLFACTION:
Olfactory epithelium covers the inferior cribriform plate, superior nasal septum and upper conchae. The nasal Conchae produce turbulent flow which brings inhaled substances into contact with the olfactory epithelium which has olfactory receptor cells (modified neurons).
There are 10-20 million olfactory receptors per 5 sq cm.
Axons in the olfactory epithelium synapse in the olfactory bulb, then the second order neurons proceed to the hypothalamus, limbic system and olfactory cortex via CN I.
Olfaction is the only sensation that reaches the cerebral cortex without synapsing in the thalamus.
GUSTATION: Taste
Gustatory receptors (taste buds) are on the surface of the tongue and adjacent areas of the pharynx and larynx.
Taste buds lie at the sides of epithelial projections called papillae. There are three types of papillae:
Filiform
Fungiform
Circumvallate
Taste buds are monitored by CN’s 7, 9 and 10
Olfactory receptors play an important role in taste as well
Gustatory pathways:
We are more sensitive to things that taste bad than those that taste good.
Equilibrium and hearing:
External: external auditory canal and ear
Middle: that part central to the tympanic membrane that contains the auditory ossicles.
Inner: contains the sensory organs for hearing and equilibrium
MIDDLE EAR:
Contains auditory ossicles: malleus, incus and stapes (hammer, anvil and stirrup)
Malleus: attaches to the TM at three points and responds to its movements
Incus: links malleus to the stapes
Stapes: connects to the oval window
The tympanic membrane is 22 times the size of the oval window which allows for sound amplification. To protect the ear from damage secondary to loud sounds the tensor tympani muscle attaches to the malleus tightening the TM and decreasing it’s response to sound. Also the stapedius muscle attaches to the stapes and reduces its movement at the oval window.
The middle ear communicates with the nasopharynx via the Eustachian tube.
INNER EAR:
The inner ear contains the receptors which supply senses of hearing and equilibrium
The receptors are housed in a fluid filled collection of chambers called the membranous labyrinth. A bony labyrinth surrounds and protects the inner labyrinth. Perilymph is interposed between bony and membranous labyrinth.
The bony labyrinth is divided into the vestibule, cochlea and semicircular canals.
Vestibular complex = vestibule and semicircular canals
The stapes conducts movement to the perilymph through the oval window which is picked up as sound by receptors in the cochlear duct.
The semicircular canals (anterior, lateral, and posterior) detect movement of endolymph via cilia within. The three dimensional distribution of the SCC’s allow complex analysis of head movement.
HEARING:
Sound waves travel through E.A.M. to the tympanic membrane which vibrates in response to wavelengths of ~ 20 to 20 kHz. Stapes movement applies pressure to the perilymph.
Conductive hearing loss = middle ear abnormality such as scaring of the TM and ossicles fusion or destruction.
Table 18.2
VISION:
Humans are visual animals.
Accessory Structures: fig 18.19
Eyelids = palpebrae
Medial and lateral canthus
Palpebral fissure
Conjunctiva: epithelium covering inner surface of eyelids and outer surface of the eyes including the cornea.
Conjunctivitis
Lacrimal apparatus: produces, distributes and removes tears. Superior and inferior lacrimal puncta.
THE EYE
Definitions:
Fibrous tunic: cornea and sclera
Sclera: white of the eye, dense fibrous connective tissue
Cornea: dome shaped widow over anterior eye. Avascular, so it’s clear and derives its nutrients from tears. Is richly innervated and provides 2/3rds of the eyes focusing power.
Iris: pigmented cells with intrinsic muscles that regulate the opening, the pupil.
Lens: Jelly like substance suspended behind the iris by suspensory ligaments attached to the ciliary body which contains ciliary muscles which can alter the shape of the lens.
Neural tunic: (retina)
Composed of neural and pigmented layers
Neural layer holds the photoreceptors
Rods: do not discriminate color, good for night vision
Cones: color vision
Rods are generally distributed around the periphery of the retina
Cones are more centrally located with the highest concentration in the fovea
Ganglion cells converge on the optic disc, penetrate the posterior wall and proceed as optic nerve.
Anterior cavity is divided into anterior and posterior chambers. These chambers are filled with aqueous humor.
The larger posterior cavity is filled with vitreous humor which is more gelatinous and helps maintain the shape of the eye.
In glaucoma, the aqueous humor is not removed via canal of Schlemm resulting increased intraocular pressure and eventual damage to optic nerve.
