Detecting Stimuli that Distort Membranes

Mechanoreceptors are cells that are sensitive to mechanical forces. Physical distortion of a mechanoreceptor's plasma membrane causes ion channels to open, altering the membrane potential of the cell, which in turn leads to the generation of action potentials. The rate of action potentials tells the CNS the strength of the stimulus exciting the mechanorecep-tor. Mechanoreceptor cells are involved in many sensory systems, ranging from skin sensations to sensing blood pressure.

Microvilli Taste pore

Taste pore sàww

Taste Bud Sensory System

Taste bud sensory cell

Supporting cell

Cell body of sensory neuron

Axon to central nervous system

| Sensory cells use neurotransmitters to depolarize the dendrites of sensory neurons.

Taste bud sensory cell

Supporting cell

Cell body of sensory neuron

Axon to central nervous system

| Sensory cells use neurotransmitters to depolarize the dendrites of sensory neurons.

Many different sensory cells respond to touch and pressure

Objects touching the skin generate varied sensations because skin is packed with diverse mechanoreceptors (Figure 45.6). The most important tactile receptors found in both hairy and non-hairy skin are Merkel's discs, which adapt rather slowly and provide continuous information about things touching the skin. Other mechanoreceptors, called Meissner's corpuscles, found primarily in non-hairy skin, are very sensitive, but adapt rapidly, so they provide information about changes in things touching the skin. The rapid adaptation of these tactile sensors is why you roll a small object between your fingers, rather than holding it still, to discern its shape and texture. As you roll it, you continue to stimulate Meissner's corpuscles anew.

Two other kinds of mechanoreceptors are found deeper in the skin. Ruffini endings, which are rather slowly adapting, are good at providing information about vibrating stimuli of low frequencies. Pacinian corpuscles, which are rapidly adapting, are good at providing information about vibrating stimuli of higher frequencies. Even deeper in the skin, dendrites of sensory neurons wrap around hair follicles. When the hairs are displaced, those neurons are stimulated.

The density of tactile mechanoreceptors varies across the surface of the body. A two-point spatial discrimination test demonstrates this fact. If you lightly touch someone's skin with two toothpicks, you can determine how far apart the two stimuli have to be before the person can distinguish whether he or she was touched with one or two toothpicks. On the back, the stimuli have to be rather far apart. The same test applied to the person's lips or fingertips reveals finer spatial discrimination; that is, the person can identify as separate two stimuli that are close together.

45.6 The Skin Feels Many Sensations Even a very small patch of skin contains a variety of sensory cells.

Meissner's corpuscle

Sensitive touch

Meissner's corpuscle

Sensitive touch

Ruffini's corpuscles

Touch, pressure

Nerves

Merkel's disks

Touch

Pacinian corpuscle

Pressure

Stretch receptors are found in muscles, tendons, and ligaments

An animal receives information from stretch receptors about the position of its limbs and the stresses on its muscles and joints. These mechanoreceptors are activated by being stretched. The information they feed continuously to the CNS is essential for the coordination of movements.

The stretch receptors found in skeletal muscle are called muscle spindles. These receptors, which are embedded in connective tissue within muscles, consist of modified muscle cells that are innervated in the center by extensions of sensory neurons. Whenever the muscle stretches, muscle spindles are also stretched, and the neurons transmit action potentials to the CNS (Figure 45.7a). The CNS uses this information to maintain muscle tone, keeping muscles taut and ready for action. Earlier in this chapter, we saw how crayfish stretch receptors transduce physical force into action potentials (see Figure 45.2). The actions of muscle spindles are similar.

Another type of stretch receptor, the Golgi tendon organ, is found in tendons and ligaments. It provides information about the force generated by a contracting muscle. When a contraction becomes too forceful, the information from the Golgi tendon organ feeds into the spinal cord, inhibits the motor neurons innervating the muscle, and causes the contracting muscle to relax, thus protecting the muscle from tearing (Figure 45.7b).

Merkel's disks

Touch

Pacinian corpuscle

Pressure

Pacinian Receptors

Epidermis

Free nerve endings

Pain, itch, temperature

Epidermis

45.7 Stretch Receptors Are Activated when Limbs Are Stretched

Stretch receptors provide information about the stresses on muscles andjoints in an animal's limbs. (a) Signals from muscle spindles to the CNS initiate muscle contraction. (b) Golgi tendon organs in tendons and ligaments inhibit a contraction that becomes too forceful, triggering relaxation and protecting the muscle from tearing.

Free nerve endings

Pain, itch, temperature

Hair cells provide information about balance, orientation in space, and motion

Hair cells are also mechanoreceptors. Projecting from the surface of each hair cell is a set of stereocilia, which looks like a set of organ pipes. When these stereocilia (which are really mi-crovilli) are bent, they alter ionotropic receptor proteins in the hair cell's plasma membrane. These receptors are gated by the

Muscle spindles

Muscle

Golgi Tendon Organ

(b) Golgi tendon organs

Muscle spindles

Muscle

Crayfish Stretch Receptor
Muscle spindles are stretch receptors. When muscle spindles are stretched.

stretch_

Time

(b) Golgi tendon organs

^ .sensory neurons associated with them transmit action potentials to the CNS. These signals stimulate motor neurons that initiate muscle contraction.

Golgi Tendon Organ

Sensory neuron

Golgi tendon organs sense load and measure the force of muscle contraction. When contraction becomes too forceful.

Golgi tendon organs sense load and measure the force of muscle contraction. When contraction becomes too forceful.

Golgi Tendon Organ

Sensory neuron

^~the sensory neurons send action potentials to the CNS that inhibit motor neurons, and the muscle relaxes.

movement of the stereocilia. When the stereocilia of some hair cells are bent in one direction, the channel pores close, and the membrane potential becomes more negative; when they are bent in the opposite direction, the channel pores open, and the membrane potential becomes more positive. When the membrane potential becomes more positive, the hair cell releases a neurotransmitter to the sensory neuron associated with it, and the sensory neuron sends action potentials to the CNS.

Hair cells are found in the lateral line sensory system of fishes. The lateral line consists of a canal just under the surface of the skin that runs down each side of the fish (Figure 45.8). The lateral line system provides information about the fish's movements through the water, as well as about moving objects, such as predators or prey, that cause pressure waves in the water.

Vertebrate organs of equilibrium use hair cells to detect the position of the body with respect to gravity. Within the mammalian inner ear, three semicircular canals at right angles to one another sense the position and orientation of the head. The vestibular apparatus has two chambers that sense the static position of the head as well as linear acceleration produced by movement. The structure and function of these organs are described in Figure 45.9.

Lateral canal

Lateral canal

Cupula And Sterocilia

A lateral line canal liesjust below the skin surface.

Lateral line Lateral line organ nerve / Cupula---

Gelatinous material

A lateral line canal liesjust below the skin surface.

[2| Structures called cupulae project into the canal. As the fish moves through the water, fluid in the canal pushes against the cupulae.

Lateral line Lateral line organ nerve / Cupula---

Stereocilia

Gelatinous material

Stereocilia

Canal Associated Neurons
Stereocilia on hair cells in the cupula bend, creating a signal that causes depolarization of the dendrites of associated neurons.

45.8 The Lateral Line System Contains Mechanoreceptors Hair cells in the lateral line of a fish detect movement of the water around the animal, giving the fish information about its own movements and the movements of objects nearby.

45.9 Organs in the Inner Ear of Mammals Provide the Sense of Equilibrium The bony inner ear includes organs of equilibrium—three semicircular canals and two vestibular organs—as well as the snail-shaped cochlea, which is part of the auditory system.

In a semicircular canal

Semicircular, canals

In the semicircular canals, the gelatinous cupulae are pushed one way or the other when changes in the position of the head causes the fluid in the canals to shift.

Vestibule

45.9 Organs in the Inner Ear of Mammals Provide the Sense of Equilibrium The bony inner ear includes organs of equilibrium—three semicircular canals and two vestibular organs—as well as the snail-shaped cochlea, which is part of the auditory system.

In a semicircular canal

Semicircular, canals

In the semicircular canals, the gelatinous cupulae are pushed one way or the other when changes in the position of the head causes the fluid in the canals to shift.

Vestibule

Detecting Stimuli

Hair cell

Dendrites of sensory neurons

Support cell

Due to inertial mass of otholiths, when head changes position, accelerates, or decelerates, the gelatinous otholithic membrane bends hair cells.

Hair cell

Dendrites of sensory neurons

Support cell

Due to inertial mass of otholiths, when head changes position, accelerates, or decelerates, the gelatinous otholithic membrane bends hair cells.

Transduction Sound Auditory Nerve
Auditory nerve

Cross section of cochlea

Auditory systems use hair cells to sense n sound waves

The stimuli that animals perceive as sounds are pressure waves. Auditory systems use mechanoreceptors to convert pressure waves into receptor potentials. Auditory systems include special structures that gather sound waves, direct them to the sensory organ, and amplify their effect on the mechanoreceptors.

Human hearing provides a good example of an auditory system. The organs of hearing are the ears. The two prominent structures on the sides of our heads usually thought of as ears are the pinnae. The pinna of an ear collects sound waves and directs them into the auditory canal, which leads to the actual hearing apparatus in the middle ear and the inner ear (Figure 45.10). If you have ever watched a rabbit, a horse, or a cat change the orientation of its ear pinnae to focus on a particular sound, then you have witnessed the role of pinnae in hearing.

The eardrum, or tympanic membrane, covers the end of the auditory canal. The tympanic membrane vibrates in response to pressure waves traveling down the auditory canal. The middle ear, an air-filled cavity, lies on the other side of the tympanic membrane.

The middle ear is open to the throat at the back of the mouth through the eustachian tube. Because the eustachian tube is also filled with air, pressure equilibrates between the middle ear and the outside world. When you have a cold or allergy, the tube can become blocked by mucus or by tissue swelling, so you have difficulty "clearing your ears," or equilibrating the pressure in the middle ear with the outside air pressure. As a result, the flexible tympanic membrane bulges in or out, dampening your hearing and sometimes causing earaches.

The middle ear contains three delicate bones called the ossicles, individually named the malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles transmit the vibra tions of the tympanic membrane to another flexible membrane called the oval window. The leverlike action of the ossicles amplifies the force of the vibrations about 20-fold. Behind the oval window lies the fluid-filled inner ear. Movements of the oval window result in pressure changes in the inner ear. These pressure waves are transduced into action potentials.

The inner ear is a long, tapered, coiled chamber called the cochlea (from Latin and Greek words for "snail" or "shell"). Across section of this chamber reveals that it is composed of three parallel canals separated by two membranes: Reiss-ner's membrane and the basilar membrane (see Figure 45.10). Sitting on the basilar membrane is the organ of Corti, the apparatus that transduces pressure waves into action potentials. The organ of Corti contains hair cells whose stere-ocilia are in contact with an overhanging, rigid shelf called the tectorial membrane. Hair cells are not neurons and do not fire action potentials. However, they form synapses with associated sensory neurons, whose axons make up the auditory nerve. When the basilar membrane flexes, the tectorial membrane bends the hair cell stereocilia, changing the rate at which they release a neurotransmitter onto the sensory neurons. As a result, there are changes in the rates of action potentials traveling to the brain in the auditory nerve.

What causes the basilar membrane to flex, and how does this mechanism distinguish sounds of different frequencies? In Figure 45.11, the cochlea is shown uncoiled to make it easier to understand its structure and function. To simplify matters, we have left out Reissner's membrane, thus combining the upper and the middle canals into one upper canal. (The purpose of Reissner's membrane is to contain a specific aqueous environment for the organ of Corti separate from the aqueous environment in the rest of the cochlea.)

The simplified diagram of the cochlea shown in Figure 45.11 reveals two additional features that are important to its function. First, the upper and lower canals separated by the basilar membrane are joined at the distal end of the cochlea (the end farthest from the oval window), making one continuous canal that turns back on itself. Second, just as the oval window is a flexible membrane at the beginning of the cochlea, the round window is a flexible membrane at the end of the long cochlear canal.

Air is highly compressible, but fluids are not. Therefore, a pressure wave can travel through air without much displacement of the air, but a pressure wave in fluid causes displacement of the fluid. When the stapes pushes the oval window in, the fluid in the upper canal of the cochlea is displaced. The cochlear fluid pressure wave travels down the upper canal, around the bend, and back through the lower canal. At the end of the lower canal, the displacement pressure is dissipated by the outward bulging of the round window.

Hypothetical uncoiling of jtH

cochlea

, Vibrations from the tympanic membrane

Oval window (under stapes)

45.11 Sensing Pressure Waves in the Inner Ear For simplicity, this diagram illustrates the cochlea as uncoiled, and leaves out Reissner's membrane. Pressure waves of different frequencies flex the basilar membrane at different locations. Information about sound frequency is specified by which hair cells are activated.

Auditory nerve fibers

, Vibrations from the tympanic membrane

Oval window (under stapes)

Auditory nerve fibers

Basilar Membrane Semicircular Canals

Round window

Lower canal

Basilar membrane

Round window

Lower canal

Basilar membrane

400 Hz

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Responses

  • alfrida
    Why are meissner's corpuscles not found in hairy skin?
    8 years ago
  • tomacca
    Why do cell membrane detect stimuli?
    8 years ago
  • Mantissa
    When the hairs on hair cells are bent plasma membrane?
    7 years ago
  • christopher
    How far apart to notice 2 stimuli on body?
    6 years ago
  • Bartolomeo
    Can round window displace pressure dissipated by outward bulging?
    5 years ago
  • venla
    When the fluid in the blank is pushed against the cupula chemoreceptors, mechanoreceptors?
    5 years ago
  • corey
    Are mechanoreceptors in mammals present in reissners membrane?
    3 years ago
  • benvenuto
    Does the plasma membrance repond to stimuli?
    3 years ago

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