sensory systems systems.pdf3 7 mechanoreceptors proprioceptors monitor muscle length and tension...

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Sensory Systems

Chapter 45

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Overview of Sensory Receptors

Sensory receptors provide information from our internal and external environments that is crucial for survival and success -Exteroceptors sense external stimuli -Some function well on land but not in

water, and vice versa -Interoceptors sense internal stimuli -Usually simpler than exteroceptors

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Overview of Sensory Receptors

Receptors can be grouped into three classes

1. Mechanoreceptors are stimulated by mechanical forces such as pressure 2. Chemoreceptors detect chemicals or chemical changes 3. Energy-detecting receptors react to electromagnetic and thermal energy

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Mechanoreceptors

Cutaneous receptors respond to stimuli in between external and internal environments

Nociceptors transmit impulses perceived as

pain -Most consist of free nerve endings located throughout the body, especially where damage is most likely to occur

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Mechanoreceptors

Thermoreceptors are naked dendritic endings of sensory neurons that are sensitive to changes in temperature

-Contain TRP ion channels that are responsive to hot and cold -Cold receptors are located higher in

the skin, and are much more numerous, than warm receptors

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Mechanoreceptors

Several types of mechanoreceptors in the skin detect the sense of touch

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Mechanoreceptors

Proprioceptors monitor muscle length and tension -Provide information about the relative position or movement of animal’s body parts -Examples: -Muscle spindles -Golgi tendon organs

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Mechanoreceptors

Baroreceptors monitor blood pressure -A highly branched network of afferent neurons located in the carotid sinus and aortic arch -Detect tension or stretch in the walls of these blood vessels -When blood pressure decreases, the

frequency of impulses produced by baroreceptors decreases

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Hearing

Hearing is the detection of sound waves -Sound is the result of vibration, or waves, traveling through a medium

Auditory stimuli travel farther and more quickly

than chemical ones Auditory receptors provide better directional

information than chemoreceptors

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Lateral Line System in Fish

Consists of canals running the length of the fish’s body beneath the skin surface -Innervated by sensory neurons that

transmit impulses to the brain

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Hearing Structure in Fish

Hearing structures in fish are called otoliths -Composed of calcium carbonate crystals -Contained in the otoliths organs of the membranous labyrinth

Otoliths vibrate against stereocilia-containing

hair cells -Thus producing action potentials

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Ear Structure of Land Vertebrates

Air vibrations are channeled through the auditory canal of the outer ear

-Vibrations reach the tympanic membrane causing movement of three small bones (ossicles) in the middle ear -Malleus (hammer), incus (anvil) and

stapes (stirrup) -The stapes vibrates against the oval window, which leads into the inner ear

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The inner ear consists of the cochlea, a bony structure containing part of the cochlear duct

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The organ of Corti, which transduces sound in the cochlea, consists of: -Basilar membrane: Bottom of cochlear duct -Hair cells with associated sensory neurons -Tectorial membrane: Overhanging, gelatinous membrane

Stereocilia of hair cells bend in response to vibrations of the basilar membrane -Send nerve impulses to brain, where they are interpreted as sound

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Navigation by Sound

A few mammals have the ability to perceive presence and distance of objects by sound -Bats, shrews, whales, dolphins -They emit sounds and then determine

the time it takes these sounds to return -This process is called echolocation

The invention of sonar (sound navigation and ranging) relied on echolocation principles

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Detection of Body Position

Most invertebrates can orient themselves with respect to gravity due to a sensory structure called a statocyst -Consists of ciliated hair cells embedded in calcium carbonate stones called statoliths

In vertebrates, the gravity receptors consist of two chambers in the membranous labyrinth -Utricle and saccule

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Within the utricle and saccule are hair cells with stereocilia and a kinocilium -Embedded in the calcium carbonate-rich otolith membrane

Utricle more sensitive to horizontal acceleration

Saccule more sensitive to vertical acceleration -Both types of accelerations cause cilia to bend, thus producing an action potential in an associated sensory neuron

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The utricle and saccule are continuous with three semicircular canals that detect angular acceleration in any direction

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When the head rotates, the semicircular canal fluid pushes against the cupula, causing the cilia to bend -Bending in the direction of the kinocilium causes a receptor potential -Stimulates an action potential in the

associated sensory neuron Saccule, utricle and semicircular canals are

collectively called the vestibular apparatus

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Chemoreceptors

Chemoreceptors can bind to particular chemicals in the extracellular fluid -Membrane of sensory neuron becomes depolarized and produces action potentials

Chemoreceptors are used in the senses of taste and smell -Are also important in monitoring the chemical composition of blood

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Taste

The perception of taste (gustation) is a mixture of physical & psychological factors -Broken down into five categories: -Sweet, sour, salty, bitter, and umami

(perception of glutamate and other amino acids)

Taste buds are collections of chemosensitive cells associated with afferent neurons

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Taste In fish, taste buds are scattered all over the

body surface In land vertebrates, taste buds are located in

the epithelium of the tongue and oral cavity within raised areas called papillae

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Taste

Many arthropods have taste chemoreceptors -Flies have them in sensory hairs located on their feet

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Smell

In land vertebrates, the sense of smell (olfaction) involves neurons located in the upper portion of the nasal passages -These neurons transmit impulses directly to the brain via the olfactory nerve

Humans can discern thousands of different

smells

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Vision

Vision begins with the capture of light energy by photoreceptors -Visual information is used to determine both the direction and distance of an object

Invertebrates have simple visual systems with photoreceptors clustered in an eyespot -Flatworms can perceive the direction of light but cannot construct a visual image

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Vision

The members of four phyla have evolved well-developed, image-forming eyes -Annelids, mollusks, arthropods, and chordates

Although these eyes are similar in structure,

they have evolved independently -An example of convergent evolution

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Vision

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Structure of the Vertebrate Eye

Sclera = White portion of the eye, formed of tough connective tissue

Cornea = Transparent portion through which light enters; begins to focus light

Iris = Colored portion of the eye -Contraction of iris muscles in bright light decreases the size of its opening, the pupil

Lens = A transparent structure that completes focusing of light onto the retina

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The lens is attached to the ciliary muscles by the suspensory ligament

-In near vision, ciliary muscles contract, -Lens becomes more rounded and

bends light more strongly -In distance vision, ciliary muscles relax -Lens becomes more flattened and

bends light less

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The vertebrate retina contains two types of photoreceptors: -Rods = Responsible for black-and-white vision when illumination is dim -Cones = Responsible for color vision and high visual acuity (sharpness) -Most are located in the central region of

the retina known as the fovea -Sharpest image is formed

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The retina consists of three layers of cells -External layer contains the rods and cones -Middle layer contain bipolar cells -Layer closest to eye cavity contains ganglion cells

Once photoreceptors are activated, they stimulate bipolar cells, which in turn stimulate ganglion cells -Transmit impulses to brain via optic nerve

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Visual Processing

Primates and most predators have two eyes, one located on each side of the face -The two fields of vision overlap, permitting binocular vision -Ability to perceive 3-D images and depth

In contrast, prey animals generally have eyes located to the sides of the head -This prevents binocular vision, but enlarges the overall receptive field

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Diversity of Sensory Experiences

The only vertebrates that can sense infrared radiation are pit viper snakes -Have a pair of heat-detecting pit organs on either side of the head between the eye and nostril -Locate heat sources in the

environment, including prey in darkness

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The elasmobranchs (sharks, rays and skates) have electroreceptors called the ampullae of Lorenzini -Can sense electrical currents generated by the muscle contractions of their prey

Eels, sharks, bees and many birds appear to

navigate along the magnetic field lines of the Earth

sensory systems systems.pdf3 7 mechanoreceptors proprioceptors monitor muscle length and tension...

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