Download JF Medicine sense organs

• General properties of sense organs
• sense organs code several aspects of a stimulus
• 1. Modality
• all sense organs detect a specific form of energy of the stimulus
• A. light (photoreceptors)
• B. chemical (chemoreceptors)
• taste (gustation)
• smell (olfaction)
• C. mechanical energy (mechanoreceptors)
• touch and pressure
• muscle stretch
• auditory and balance
• baroreceptors
• D. thermal energy (thermoreceptors)
• hot and cold
• also electro-receptors & magneto-receptors present in certain animals but not humans
2. Sense organs have the property of transduction
•  the energy of stimulus is altered to electrical energy
• energy of stimulus is usually increased (amplified) or reduced during transduction
3. Intensity
amplitude of stimulus is coded by the frequency of action potentials (APs)
(Weber –Fechner law)
500
AP
frequency
(Hz)
0
stimulus intensity (log scale)
4. Location
-location of stimulus is detected by the receptive field of the sensory neuron
-the area of the sensory surface of the body, eg, skin, that when stimulated
increases the AP frequency of that neuron
receptive
field (RF)
eg, in skin
RF
of
SN1
RF
of
SN2
sensory neuron 1
sensory neuron 2
(SN1)
(SN2)
duration of
stimulus
Sensory receptor
potential
-40 mV
sensory
potential
(mv)
-70 mV
time
Generation of sensory potential
• the sensory stimulus generates a sensory receptor potential in the sensory cell
• usually a depolarization
• amplitude is proportional to stimulus
• generated by Na influx via channels of specific sensory receptor protein channels
• sensory potential may decline if stimulus is prolonged (called sensory adaptation)
Sensory adaptation
• sensory receptors vary in degree of adaptation
• tonic receptors are non or slowly adapting
• phasic receptors are rapidly adapting
on
stimulus
off
tonic receptors
phasic receptors
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action potentials
generated in sensory
axon
tonic receptor
stimulus
200
Increase in AP freqn
Is maintained
AP frequency
(Hz)
0
Time
phasic receptor
stimulus
Increase in AP freqn
is not maintained
-only lasts a brief period
AP frequency
Time
Somatic sensory organs (somatosensory)
• sensory organs in skin, muscles and tendons (exteroceptors, detect external stimulus)
and in internal organs (interoceptors, detect internal stimulus)
• the sensory endings have proteins embedded in membrane which detect a specific type
of sensory stimulus
A) Touch & pressure receptors
• class Aβ axons
• myelinated.
• large diameter (10 µm).
• conduction velocity - 50 m/s
• include sensory axons with free nerve endings and those with a terminal accessory
organ or corpuscle
i) rapidly adapting
• detect light touch on smooth skin and low frequency vibrations
ii) slowly adapting
-detect prolonged touch and pressure
B) Nociception (pain perception)
-nociceptors are specialized receptors sensitive to noxious stimuli (unpleasant, aversive,
potentially tissue damaging)
-pain is the conscious experience of noxious stimuli
-the receptors are free nerve endings devoid of myelin sheath
-the ending have specific proteins sensitive to noxious stimuli
Two major types of nociceptors
1. Aδ (Adelta) fibres (fast pain system)
-axons are medium diameter (2 µm), thinly myelinated, 15 m/s conduction velocity
-sensitive to abnormally high mechanical , heat & chemical stimuli
-rapidly adapting
-well localised pain
-sensation of sharp pain, eg, pinprick, and initial response to noxious heat
2. C fibres (slow pain system)
-axons are non-myelinated, <1µm diameter, 0.5m/s velocity
-sensitive to high mechanical, heat & cold & chemical stimuli
-non-adapting
-continuous throbbing pain
-not well localised pain
-very heterogeneous
-nociceptors release transmitters in CNS & also many neuropeptides
peripherally which can induce vasodilation and have other actions
-both peripheral and central terminals respond to a variety of chemicals
& pH which regulate their sensitivity
Hyperalgesia (sensitization) to pain
-caused by prolonged severe tissue damage (burns, arthritis)
-an increased sensitivity (lowering of the threshold) of C fibre nociceptors,
either at site of injury (primary) or at adjacent sites (secondary),
Itch receptors
-specialised free nerve ending
Temperature receptors
-further specialised free nerve endings for hot and cold
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Proprioceptors
detect position of body in space, particularly position of body limbs
gives rise to the kinaesthetic or "6th sense"
(an automatic non-conscious awareness of body and limb position)
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two main proprioceptive sense organs:
i) Golgi organs
-located on tendons
-free nerve endings
-detect length and movement of joints
Golgi tendon organ
Iii) Muscle spindle organs
•  detect length and movement of muscles
•  composed of 3-12 modified muscle fibres called the intrafusal muscle fibres
•  the two end regions of the intrafusal muscle fibres have contractile filaments
•  innervated by gamma (γ) motor (efferent) axons
•  (normal contractile fibres are called extrafusal fibres, and innervated by
alpha (α) motor axons)
•  the central region of each intrafusal muscle fibre does not contain
contractile filaments, but has a swollen sensory region innervated by
sensory (afferent) axons
•  sensory endings on the muscle spindles are activated by stretch
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(2 types of muscle spindles, nuclear bag and nuclear chain)
Properties of sensory axons to muscle spindles
type
diameter
condn vel.
endings
adaptation
group I axons
Aα
20µm
100m/s
primary
phasic
group II axons
Aβ
10µm
50m/s
secondary
tonic
Primary endings (group 1 axons)
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resting discharge rate of firing of AP in the sensory axons is ~10 / sec
increase in AP discharge when muscle spindle is stretched, ie, when
muscle is stretched
increase in AP frequency is proportional to rate of stretch - detect velocity of
muscle stretch
AP frequency rises as high as 500/sec during very rapid stretch
are phasic receptors, ie, rapidly adapt in response to stretch of muscle and
muscle spindle
500
Rapid adaptation
AP freqn
10
stretch
time
Secondary endings (group II axons)
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increase in AP discharge when muscle is stretched - from resting frequency of 10 /
sec
increase in AP frequency is proportional to increased length of muscle spindle
AP frequency rises only to a maximum of ~ 50/sec during stretch
are tonic receptors, ie, only slow adaption in response to stretch of muscle and
muscle spindle
detect absolute of muscle length
50
AP freqn
10
stretch
time
•  Role of intrafusal muscle fibres
•  Contraction of intrafusal muscle fibres maintains sensitivity of the
muscle spindles when a muscle contracts
Vision
-waves of photons
-visible light is wavelength 400 (violet) to 700 nm (red) of electromagnetic spectrum
The eye
-light enters eye via pupil
-size of pupil is controlled by the iris, which contains two types of muscle
-pupil dilates or contracts to control amount of light entering eye
In dim light
-contraction of radial (dilator) muscle of iris via sympathetic stimulation
-size of pupil increases
In bright light
-contraction of circular (constrictor) muscle of iris via parasympathetic stimulation
-size of pupil decreases
Focusing of light
-light is focused on retina by cornea and lens
-adjustment of focusing is by changing shape of lens
Distant sight
-ciliary muscles relax (sympathetic stimulation)
-suspensory ligaments become taut
-lens becomes thin, flattened
Close sight
-ciliary muscles contract (parasympathetic stimulation)
-suspensory ligaments relax
-lens thick, rounded (accomodated)
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Retina
-contains light sensitive cells, the photoreceptors called rods and cones
-photoreceptors contain a pigment(s) which absorbs light
-photoreceptors are stimulated by intensity of light (luminance)
Rods
contain photopigment rhodopsin
sensitive to all wavelengths
high sensitivity to light (even as low as a single photon)
night sight
found over all retina except fovea
100 million in total
Cones
contain photopigments cone opsins (several different types)
different absorption spectra for red, green or blue
low sensitivity to light (>100 photons)
daylight sight
located mainly in central retina called the macula, and especially the central area of
the macula, the fovea
6 million in total
Myopia
Nearsightedness
distant objects cannot be focused
eyeball grows too long
correction - concave lens
Visual disorders
Hyperopia
-farsightedness - near objects cannot be focused
-lens cannot accommodate
-cells of lens die with age, & lens loses elasticity (presbyopia)
-correction - convex lens
normalised
with convex lense
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Cataract
causes blindness
loss of transparency of lens (cells become opaque with ageing)
diagnosed as a grey patch in pupil
correction - lens removed & replaced with prosthetic (artificial) lens
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Glaucoma
increase in pressure in anterior chamber of eye (aqueous humor)
due to narrowing of a small canal in cornea near edge of iris
initially blurring of peripheral vision
later full blindness as optic nerve is damaged
correction - relief of pressure by opening canal
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Macular degeneration
blurred/distorted central vision
degeneration of retinal cells in macula
dry (atrophic) (90% of cases) - block of blood flow to macula
wet (exudative) (10%) - new weak blood vessels grow in retina which leak fluid
Floaters
•  deposits in the vitreous humour, which is normally transparent.
•  visible because of the shadows they cast on the retina, appearing as spots,
or fragments of cobwebs, which float slowly before the observer's eyes
•  due to degenerative changes, mainly in elderly
•  shrinkage of the vitreous humour, causing collagen in the humour to break
down into fibrils, or detachment of the vitreous humour from surrounding
tissue, which may sometimes cause retinal detachment
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Mechanism of hearing
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Hearing of sound amplitude
sound waves oscillate the eardrum
pressure waves transmitted through ear bones to a fluid filled chamber, the cochlea
duct (scala media)
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sensory cells are called hair cells
hair cells generate sensory potentials in response to sound
depolarization evokes release of vesicles containing neurotransmitter (L-glutamate)
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neurotransmitter evokes APs in afferent sensory axon of the auditory nerve
NB, the hair cells are also innervated by an efferent nerve
Hearing
-propogated sound is oscillations of air pressure
-the ear detect changes in the air pressure
amplitude of
air pressure
(intensity)
increase
decrease
time
-amplitude is expressed in db (log scale)
-0 db is the auditory threshold
-30-40db whisper,50-60db conversation,100-120db rock concert (ear damage!)
-frequency (pitch) is measured in Hz (cycles / sec)
-humans detect 20-20,000 Hz
-auditory threshold varies with frequency
-most sensitive at 500-4000Hz (speech frequency)
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Mechanism of hearing
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Hearing of sound amplitude
sound waves oscillate the eardrum
pressure waves transmitted through ear bones to a fluid filled chamber, the cochlea
duct (scala media)
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sensory cells are called hair cells, located in the organ of Corti (the organ in the
inner ear that contains the auditory sensory cells)
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hair cells generate sensory potentials in response to sound
depolarization evokes release of vesicles containing neurotransmitter (L-glutamate)
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neurotransmitter evokes APs in afferent sensory axon of the auditory nerve
NB, the hair cells are also innervated by an efferent nerve
kinocilium
vesicles containing
transmitter
L-glutamate
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Hair cells
these have ~100 hairs (stereocilia) projecting upwards and one kinocilium
with no sound, cilia of hair cells project directly upwards
-auditory (cochlear) axons have a “resting” frequency of APs at ~20 Hz
sound waves cause oscillation of basilar membrane which moves up and
down, and cilia deflected from side to side
upward movement of basilar membrane, stereo cilia deflected one way,
causes a depolarizing sensory receptor potential and an increase in AP
frequency
downward movement of basilar membrane, stereocilia deflected other way,
causes a hyperpolarizing sensory receptor potential and a decrease in AP
frequency
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Frequency discrimination
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individual frequencies are detected by different part of cochlea
end nearest oval window is narrow and stiff
vibrates and detects high freqn
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end nearest tip (helicotrema) is wide and flexible
vibrates and detects low frequency
~20,000 hair cells located along cochlea, each innervated by a sensory
axon in cochlea nerve
each hair cell & axon responds to a different frequency of sound
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NB, two types of hair cells are present
i) inner – carry out the signalling to CNS as described
ii) outer – do not signal. Sound alters their membrane potential, which in
turn causes a length change of the hair cell (shorten or lengthen – called
electromotility)
-the change in length of the hair cell amplifies basilar membrane movement
•  Deafness
•  most common physical disability (#10% popn)
Conductive deafness
•  sound waves inadequately conducted through external and middle ear
•  several causes
•  1. deposition of calcium salts at joints between ear bones and therefore loss of
mobility
•  occurs gradually with age
•  corrected with a hearing aid, which amplifies sound
•  2. blocking of ear canal with earwax (glue ear)
•  3. rupture of eardrum
•  4. middle ear infections
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Sensory-neural deafness
-defect in hair cells / auditory nerve
eg, neural presbycusis, age related neurodegeneration of hair cells, in elderly especially affects detecting high frequency sound.
-can also occur in young with hereditary defects
Correction
cochlear implant. Electrodes surgically implanted which transduce sound to electrical
signals & stimulate auditory nerve directly
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Further Disorder of Hearing
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Tinnitus (meaning “ringing") is the perception of sound within the human ear in the
absence of external sound.
-range of causes, eg, hearing impairment caused by loud noise (especially) &
during ageing, ear infections, foreign objects or wax in the ear, nose allergies that
prevent fluid drain, wax build-up, withdrawal from a certain drugs, eg,
benzodiazapine addiction
very common - one in five people between 55 and 65 years old report tinnitus
symptoms
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Sense of equilibrium, balance
detected by the vestibular organs
semicircular canals and otolith organs - detect changes in position and motion of the
head
1. Semicircular canals
• detect rotation of head
• three fluid filled canals (bones) at right angles to each other
• bulbous expansion at base called ampulla
• ampullae contains hair cells with cilia projecting into a gelatinous mass, the cupula
which protrudes into the fluid (endolymph) in the ampulla
(cilia)
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hair cells have high resting discharge of APs of ~100 / sec
hair cells have 1 large kinocilium & 30 stereocilium
any rotation of head causes fluid movement in at least one canal
inertia of fluid (endolymph) causes cilia to be deflected to side
bending of cilia in one direction causes depolarization & increase in AP
frequency of vestibular axons from resting 100 Hz to a higher frequency
bending of cilia in other direction – hyperpolarization & decrease in AP
frequency from 100 / sec to a lower frequency
the three canals give information about rotation in all directions
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output via vestibular nerve to brain
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2. Otolith organs
detect position of head in space, relative to gravity, ie, head tilt
two organs, the utricle and saccule
both are sac-like structures in a bony chamber between semicircular canals
and cochlea
contain hair cells protruding into a gelatinous mass containing otoliths (small
crystals of calcium carbonate)
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Detection of head tilt:
a) standing (head upright)
cilia project upwards in utricle
head tilt from vertical causes cilia to bend in directection of tilt (stereocilia
towards kinocilium) by gravitational force
depolarization & increase in freqn of APs
head tilt back - cilia are bent in opposite direction – hyperpolarization &
decrease in AP freqn
b) lying down
cilia project upwards in saccule
head tilt from horizontal causes cilia to bend in direction of tilt
increase in freqn of APs
Disorders of Balance
•  Vertigo - loss of balance & sometimes nausea
-often caused by problems of the inner ear.
i) labyrinthitis (inflammation within the inner ear) & vestibular neuritis
(inflammation of the vestibular nerve)
-characterized by the sudden onset of vertigo and may be associated with hearing loss
-viral or bacterial infection
ii)Benign paroxysmal positional vertigo (BPPV) - the most common
-characterized by the sensation of motion initiated by head movements
-the crystals in the otolith organs become dislodged and move into the semicircular
canals, which become sensitive to head tilt movements (abnormal)
iii) Meniere’s disease
-dysfunction of the semicircular canals
-an increased pressure of the endolymph fluid in the canals, due to increased
secretion, or blockage of drainage, and dilated membranous sacs
-symptoms include severe vertigo, tinnitis (ringing in the ears) and hearing loss.