Sense Organs



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Lecture 12


Sense Organs

Receptor Types and the General Senses



  • Classification of Receptors

    • 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

          • 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

    • 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

    • 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

    • Unencapsulated nerve endings – sensory dendrites that lack a connective tissue wrapping

      • Free nerve endings –

        • Three types:

          • Warm receptors respond to rising temperatures

          • Cold receptors respond to falling temperatures

          • 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.

    • 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

      • 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

    • 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

    • 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

    • 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

    • Projection pathways - The pathways followed by sensory signals to their ultimate destinations in the CNS

    • From the receptor to the final destination, most somesthetic signals travel by way of 3 neurons called the first-, second-, and third- order neurons

    • Signals from the head travel through cranial nerves to the brainstem

    • Signals from below the head travel up the spinothalamic tract and other pathways

      • Most of these travel through the thalamus to the cerebral cortex

    • Somethsteic pathways cross in the spinal cord or medulla oblongata to the cerebral hemisphere contralateral to the origin of the stimulus




  • Pain Pathways

    • 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

    • 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

          • Conduct at speeds of 12 to 30 m/sec

          • Produce the sensation of fast pain

            • Sharp, localized, stabbing pain perceived at the time of injury

        • Unmyelinated pain fiber

          • Conduct at speeds of .5 to 2.0 m/sec

          • Produce slow pain

            • Longer-lasting, dull, diffuse feeling

      • Pain in the viscera is often mistakenly thought to come from the surface of the body

        • 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

          • In the case of cardiac pain, spinal cord segments T1 to T5 receive input from the heat as well as the chest and arm

          • 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

          • The brain cannot distinguish which source the arriving signals are coming from

          • 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

    • 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

          • They tiny spikes that are not used for taste

          • They are important to appreciation of the texture of food

          • (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

          • 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

          • Shaped somewhat like mushrooms

          • 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

          • There are only 7 to 12 vallate papillae

          • 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:

          • Taste cells – banana-shaped cells

            • 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

          • Supporting cells – cells that lie between taste cells

            • They have a shape similar to taste cells

            • They lack taste hairs

          • Basal cells – cells that undergo mitosis to produce new taste cells every 7 to 10 days

        • There are five primary taste sensations

          • Sweet – detected primarily at the tip

          • Salty – detected on the sides

          • Sour – detected at the sides

          • Bitter – detected at the back

          • Umami – sensitive to meaty taste stimulated by certain amino acids such as glutamate and aspartate

            • Newly discovered

            • Not well understood

  • Smell

    • 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

      • 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

          • They have a life span of only 60 days

          • Unlike most neurons, they are replaceable

    • 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

      • 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


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