Download Sense Organs - human anatomy

Lecture 12
Sense Organs
Receptor Types and the General Senses
 Classification of Receptors
o Receptors can be classified by Modality
 Chemoreceptors respond to chemicals, including odors, tastes, and
composition of body fluids
 Thermoreceptors respond to heat and Cold
 Nociceptors are pain receptors
 They respond to tissue damage from trauma
 They respond to ischemia
o State in which the blood flow to a tissue is
inadequate to meet is metabolic needs
 They respond to excessive stimulation from heat or
chemicals
 Mechanoreceptors respond to physical forces on cells caused by
touch, pressure, stretch, tension, or vibration
 They include the organs of hearing and balance
 They include many receptors of the skin, viscera, and joints
o Receptors can be classified by the distribution of receptors in the body
 General senses employ receptors that are widely distributed in the
skin, muscles, tendons, joint capsules and viscera
 They detect touch, pressure, stretch, heat, cold, and pain
 They also detect stimuli such as blood pressure and blood
chemistry, which we do not perceive consciously
 Special senses employ relatively complex sense organs of the head,
innervated by cranial nerves.
 They include vision, hearing, equilibrium, taste, and smell
o Receptors can be classified by the origins of the stimuli
 Interoceptors detect stimulin in the internal organs and produce
feelings of visceral pain, nausea, stretch, and pressure
 Proprioceptors sense the position and movements of the body or its
parts
 They occur in muscles, tendons, and joint capsules
 Exteroceptors sense stimuli external to the body
 They include the receptors for vision, hearing, taste, smell,
and cutaneous senses
 The General Senses
o Unencapsulated nerve endings – sensory dendrites that lack a connective
tissue wrapping
 Free nerve endings –
 Three types:
o Warm receptors respond to rising temperatures
o Cold receptors respond to falling temperatures
o Nociceptors respond to pain
 Bare dendrites with no special association with any specific
accessory cells or tissues
 Most abundant in connective tissues and epithelia
 Tactile discs (Merkel discs)
 Receptors for light touch and pressure on the skin
 Found at the base of the epidermis
 Hair receptors
 Nerve fibers entwined around a hair follicle that monitor
movements of hair.
o Encapsulated nerve endings – dendrites wrapped in glial cells or
connective tissue
 Tactile corpuscles (Meissner’s corpuscles) –
 Receptors for light tough, texture, and low-frequency
vibration
 Occur in dermal papillae of the skin, especially in sensitive
hairless areas
 Tall, ovoid to pear-shaped, and consist of 2 or 3 nerve
fibers within a mass of connective tissue
 Krause end bulbs
 Resemble tactile corpuscles in structure and function, but
occur in mucous membranes, rather than in skin
 Ruffini corpuscles
 Receptors for constant heavy pressure and joint movements
 Flattened, elongated capsules containing a few nerve fibers
 Located in the dermis, subcutaneous tissues, and joint
capsules
 Lamellated corpuscles (Pacinian corpuscles)
 Receptorss for deep pressure, stretch, and high-frequency
vibration
 Consist of numerous concentric layers of Schwann cells
surrounding a core of one to several sensory nerve fibers
 Occur in the pancreas, mesenteries, and other viscera, and
deep in the dermis of the hands, feet, breasts, and genitals
 Muscle spindles
 Receptors that detect stretch in a muscle and trigger a
variety of skeletal reflexes
 Consists of an elongated fibrous capsule with a fusiform
shape
 Contains 3 to 12 modifies muscle fibers call intrafusal
fibers.
 Different types of sensory nerve fibers twine around the
middle of the intrafusal fibers or have flowerlike endings
that contact the ends of the muscle fibers
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Golgi tendon organs
 Receptors that detect stretch in a tendon and trigger a reflex
that inhibits muscle contraction to avoid muscle or tendon
injury
 Consists of a tangle of knobby nerve endings squeezed into
the spaces between the collagen fibers of the tendon
The Receptive Field – the area monitored by a single sensory neuron
o Any information arriving at the CNS by way of that neuron is interpreted
as coming from that sensory field, no matter where in the field the
stimulus is applied
o If two stimuli are simultaneously applied within the same field, the brain
cannot perceive them as separate, because the input is received through the
same nerve fiber
o A separation of 47 mm is needed for two points of contact to fall in
separate receptive fields and to be felt separately
Somesthetic Projection Pathways
o Projection pathways - The pathways followed by sensory signals to their
ultimate destinations in the CNS
o From the receptor to the final destination, most somesthetic signals travel
by way of 3 neurons called the first-, second-, and third- order neurons
o Signals from the head travel through cranial nerves to the brainstem
o Signals from below the head travel up the spinothalamic tract and other
pathways
 Most of these travel through the thalamus to the cerebral cortex
o Somethsteic pathways cross in the spinal cord or medulla oblongata to the
cerebral hemisphere contralateral to the origin of the stimulus
Pain Pathways
o Pain makes us conscious of potential injury or actual injury, allowing us to
avoid injury or to favor an injured region so that it may heal
o Nociceptors are specialized pain receptors
 They are widespread
 especially dense in the skin and mucous membranes
 They occur in virtually all organs, but not in the brain
 They occur in two types
 Myelinated pain fibers
o Conduct at speeds of 12 to 30 m/sec
o Produce the sensation of fast pain
 Sharp, localized, stabbing pain perceived at
the time of injury
 Unmyelinated pain fiber
o Conduct at speeds of .5 to 2.0 m/sec
o Produce slow pain
 Longer-lasting, dull, diffuse feeling
 Pain in the viscera is often mistakenly thought to come from the
surface of the body
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Example: Pain of a heart attack is felt “radiating” along the
left shoulder and medial side of the arm
Referred pain results from the convergence of neuronal
pathways in the CNS
o In the case of cardiac pain, spinal cord segments T1
to T5 receive input from the heat as well as the
chest and arm
o Pain fibers from the heart and skin converge on the
same spinal interneurons, then follow the same
pathway from there to the thalamus and cerebral
cortex
o The brain cannot distinguish which source the
arriving signals are coming from
o It acts as if it assumes that signals arriving by this
pain are most likely coming from the skin, since it
has more receptors and suffers injury more often
The Chemical Senses
 Taste
o Gustation results from the actions of chemicals on the taste buds
 The tongue has four types of surface projections called lingual
papillae
 Filiform papillae are the most abundant papillae on the
human tongue
o They tiny spikes that are not used for taste
o They are important to appreciation of the texture of
food
o (They may hold food particles on the tongue to
allow other papillae to taste the food)
 Foliate papillae form parallel ridges on the sides of the
tongue about two-thirds of the way back from the tip
o Most of their taste buds degenerate by the age of 2
or 3 years
 Fungiform papillae are widely distributed but are especially
concentrated on the tip and sides of the tongue
o Shaped somewhat like mushrooms
o Each has about three taste buds, located mainly on
its apex
 Vallate papillae are large papillae arranged in a “V” shape
on the back of the tongue
o There are only 7 to 12 vallate papillae
o They contain about half of the taste buds – about
250 each
 Regardless of location, all taste buds look alike
 They are lemon-shaped groups of 40 to 60 cells
 The cells are of 3 kinds:
o Taste cells – banana-shaped cells
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Have taste hairs – microvilli that act as
receptor surfaces for taste molecules
 The hairs project into a pit called a taste
pore on the epithelial surface of the tongue
o Supporting cells – cells that lie between taste cells
 They have a shape similar to taste cells
 They lack taste hairs
o Basal cells – cells that undergo mitosis to produce
new taste cells every 7 to 10 days
There are five primary taste sensations
o Sweet – detected primarily at the tip
o Salty – detected on the sides
o Sour – detected at the sides
o Bitter – detected at the back
o Umami – sensitive to meaty taste stimulated by
certain amino acids such as glutamate and aspartate
 Newly discovered
 Not well understood
Smell
o Olfaction resides in a patch of epithelium called the olfactory mucosa
 Location of the olfactory mucosa
 found on the roof of the nasal cavity
 It covers about 5 cm2 of the superior concha and nasal
septum
 This location is close to the brain, but poorly ventilated –
sniffing may be necessary
 Effectiveness of the mucosa
 Most people can distinguish between 2000 to 4000
different odors
 Women are more sensitive than men
 Structure of the mucosa
 10 to 20 million olfactory neurons as well as epithelial
supporting cells and basal cells
 Has a yellowish tint due to lipofiscin in the supporting cells
 Olfactory cells are the only neurons in the body directly
exposed to the external environment
o They have a life span of only 60 days
o Unlike most neurons, they are replaceable
o Structure of olfactory cells
 Shaped like a bowling pin
 Wider part is the soma, containing the nucleus
 Neck and head of the cell are modified dendrites with a swollen tip
bearing cilia called olfactory hairs
 The cilia are binding sites for odor molecules
 The cilia are embedded in a thin layer of mucus on the
epithelial surface
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The basal end of the cell tapers to become an axon
 The axons collect into small fascicles that leave the nasal
cavity through olfactory foramina in the cribiform plate of
the ethmoid bone
 On the superior side of the cribiform plate, the olfactory
fibers enter the olfactory bulbs
 In the olfactory bulbs, the olfactory fibers follow olfactory
tract which reach the cerebral cortex without passing
through the thalamus – unlike other sensory input
The Ear
 Anatomy of the Ear
 Auditory Function
 The Auditory Projection Pathway
 The Vestibular Apparatus
 Vestibular Projection Pathways
The Eye
 Accessory Structures of the Orbit
 Anatomy of the Eyeball
 Formation of an Image
 Structure and Function of the Retina
 The Visual Projection Pathway
Developmental and Clinical Perspectives
 Disorders of the Sense Organs