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Sensory Receptor Cells (i.e., Sensors,
Receptors)
Cells that transduce physical or chemical stimuli into signals that are transmittable and interpretable. Most sensory cells are modified neurons , specialized for detecting different kinds of stimuli, such a pressure, heat, or light. Most sensory cells have membrane receptor proteins that detect a stimulus and respond by altering the flow of ions across the plasma membrane. TERM 2
What are the three types of sensory
receptors?
DEFINITION 2 Mechanoreceptors Chemoreceptors Light receptors TERM 3
What steps are needed for a sensory cell to
fire an action potential?
DEFINITION 3 Sensory cells transduce the energy from a stimulus into action potentials. The first step is activation of a receptor protein in the plasma membrane of a sensory cell by a stimulus. The activated protein opens or closes ion channels. The resulting change in membrane potential causes the sensory cell to fire action potentials or to change its secretion of a neurotransmitter onto an associated neuron that fires action potentials. TERM 4
Intensity of the stimulus is encoded in the
______ of the action potential.
DEFINITION 4 frequency TERM 5
Sensory Organs
DEFINITION 5 Congregations of sensory cells and other types of cells, such as eyes, ears, and noses.
Sensory Systems
Include the sensory cells, the associated structures, and the neuronal networks that process the information. TERM 7
How is different sensory data
interpreted?
DEFINITION 7 Although they are simply depolarization events, sensory data are interpreted in different ways according to the different places in the CNS where messages from different kinds of sensory cells arrive. Whether a stimulus is interpreted as one or another sensation depends on which cells of the central nervous system receive the signal. TERM 8
Give an example of sensed information that
we are unconscious of.
DEFINITION 8 The brain receives continuous information about levels of CO2, blood sugar, and O2. Such information is important for the maintenance of homeostasis. TERM 9
Ionotropic Sensory Detection
DEFINITION 9 Where the receptor protein itself is part of the ion channel and, by changing its conformation, opens or closes the channel pore. TERM 10
Metabotropic Sensory Detection
DEFINITION 10 Where the receptor protein is linked to a G protein that activates a cascade of intracellular events that eventually open or close ion channels.
Olfaction
The sense of smell which depends on chemoreceptors. In vertebrates, olfactory sensors areneurons embedded in a layer of epithelial cells at the top of the nasal cavity. The axons of these sensors project to theolfactory bulb of the brain. The dendrites end in olfactory hairs at the surface of the nasal epithelium. Molecules from the environment diffuse through nasal mucus to reach the surface of the olfactory hairs. TERM 17
Give an overview of how the olfactory system
works.
DEFINITION 17 Olfactory cilia have receptors that bind specific odorant molecules. Action potentials generated by odorant binding are transmitted to glomeruli in the olfactory bulb. Neurons in a glomerulus receive input only from receptor cells expressing the same receptor gene. TERM 18
Odorants
DEFINITION 18 Chemicals that bind to olfactory recepto r proteins. Each olfactory receptor protein binds particular odorant molecules, which activates a G protein. The G protein then activates an enzyme that increases the level of the second messenger cAMP. cAMP opens a Ca2+ channel Ca2+ leads to opening of a Cl- channel which depolarizes the membrane and an action potential is fired. TERM 19
Humans have about ________ olfactory
receptor neurons.
DEFINITION 19 20 million TERM 20
How many olfactory receptors does each
olfactory receptor neuron in the nose
express?
DEFINITION 20 Just one.
Humans have about _______ genes for
olfactory receptors.
1000; of these, only ~380 produce functional olfactory receptors; the other genes have mutations and are pseudogenes. TERM 22
The number of __________ greatly exceeds the
number of different receptor proteins.
DEFINITION 22 odorant molecules; each odorant may bind to one or more specific receptor proteins. A specific odorant is distinguished according to the different and unique combination of cells it activates. A higher concentration of odorant molecules produces a higher frequency of action potentials and is perceived as a stronger smell. TERM 23
Describe the combinatorial coding in
olfaction.
DEFINITION 23 Each individual olfactory receptor recognizes a number of similar structures that are part of odorants. Most odorants activate more than one type of olfactory receptor. This aspect provides for the identification of an almost limitless number of different molecules. Like the immune system, this system allows molecules that have never been encountered before to be characterized. TERM 24
Gustation
DEFINITION 24 The sense of taste which depends on clusters of sensory cells called taste buds. TERM 25
Mechanoreceptors
DEFINITION 25 Cells that are sensitive to mechanical forces. They are involved in many sensory systems, including skin sensations and sensing blood pressure. Physical distortion of a mechanoreceptor's plasma membrane causes ion channels to open, which leads to the generation of action potentials. The frequency of the action potentials is related to the strength of the stimulus. Skin is packed with diverse mechanoreceptors that sense various sensations.
Hair Cells
Mechanoreceptors that each contain a set of stereocilia (microvilli- like). When the stereocilia are bent in one direction, receptor potential becomes more negative; when they are bent in another direction, it 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. TERM 32
Vertebrate organs of equilibrium use _________
to detect the position of the body with respect
to gravity.
DEFINITION 32 hair cells; Semicircular canals and the vestibular apparatus in the mammalian inner ear use hair cells to detect position and orientation of the head, as well as acceleration produced by movement. In the semicircular canals, the gelatinous cupulae are pushed one way or the other when the changes in the position of the head causes the fluid in the canals to shift. TERM 33
Hair cells have _________ on their stereocilia.
DEFINITION 33 mechanoreceptors TERM 34
How do hair cells trigger action potentials in
adjacent sensory neurons?
DEFINITION 34 Ion channels open when stereocilia are bent in one direction and close when they are bent in the opposite direction. The stereocilia project into the middle canal, which contains a fluid high in K+ and low in Na+. Thus, when K+ channels open, K+ enters and depolarizes the cell. Membrane depolarization opens voltage-gated Ca2+ channels, causing neurotransmitter release. TERM 35
Otoliths
DEFINITION 35 Or ear stones, are granules of calcium carbonate on top of a gelatinous layer. Due to inertial mass of otoliths, when the head changes position, accelerates, or decelerates, the gelatinous otolithic membrane bends hair cells.
Photosensitivity
The sensitivity to light. It ranges from the ability to orient to the sun to the ability to see. Evolution has conserved molecules used for photosensitivity across the entire range of animal species. These are a family of pigment called rhodopsins. TERM 37
Rhodopsin
DEFINITION 37 The light receptor molecule in animals. Rhodopsin molecules can absorb photons of light and undergo conformational changes. Rhodopsin molecules consist of a protein called opsin and a light absorbing group, 11-cis-retinal. 11-cis-retinal is covalently bound in the center of the opsin molecule. When 11-cis-retinal absorbes a photon, it changes to all- trans-retinal , which changes the conformation of the opsin. This change signals the detection of light. TERM 38
Retinal is derived from
_________.
DEFINITION 38 vitamin A; Vitamin A deficiency is common in developing countries but rarely see in developed countries. Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A. Night blindness is one of the first signs of vitamin A deficiency. TERM 39
Night Blindness
DEFINITION 39 The inability to see well in dim light-is associated with a deficiency of vitamin A. Without adequate amounts of retinal, regeneration of rhodopsin is incomplete and night blindness occurs. Since carrots are a good source of beta-carotene, there is truth in the old belief that carrots help you see better in the dark. TERM 40
Rod Cell
DEFINITION 40 A photoreceptor that is a modified neuron. It releases neurotransmitters that influence other neurons. Rod cells have an outer segment, and inner segment, and a synaptic terminal. The inner segment has the nucleus and many mitochondria.. The outer segment has a stack of discs of plasma membrane densely packed with rhodopsin. The discs function to capture photons. More sensitive to light than cone cells, provide black-and-white vision at low light levels, humans have ~125 mil. rods.
Iris
Located inside the cornea is the pigmented iris , which controls the amount of light that can enter. TERM 47
Pupil
DEFINITION 47 The region of the eye where light enters. TERM 48
Lens
DEFINITION 48 Makes fine adjustments in the focus of images on the photosensitive retina at the back of the eye. Mammals and birds alter the shape of the lens to focus. TERM 49
Fovea
DEFINITION 49 The most sensitive area of the retina. TERM 50
For _____ vision, ciliary muscle are relaxed.
For _____ vision, ciliary muscles contract,
causing the lens to round up.
DEFINITION 50 distant; near
Retina
Includes layers of neurons that process visual information from the photoreceptors and produce an output signal that is transmitted via the optic nerve. Light must pass through all the layer of cells before photons are captured by rhodopsin. TERM 52
There are two types of vertebrate
photoreceptors: "_____" and "_____".
DEFINITION 52 rods; cones TERM 53
Briefly describe the process of vision.
DEFINITION 53 Light travels through layers of transparent neurons--ganglion, amacrine, bipolar, and horizontal cells--and is absorbed by the rods and cones (the photoreceptive layer) at the back of the retina. Visual information is processed through several layers of neurons and finally converges on ganglion cells, which send their axons to the brain. TERM 54
Ganglion Cells (Vision)
DEFINITION 54 Cells at the front of the retina which fire action potentials. Their axons form the optic nerve. TERM 55
Bipolar Cells
DEFINITION 55 Processing cells that connect ganglion cells and photoreceptors. Release of neurotransmitter from the photoreceptor cells in turn causes the rate of neurotransmitter release from the bipolar cells to change. Release of neurotransmitter from bipolar cells causes ganglion cells to fire action potentials.
Blind Spot
There are no photoreceptors where blood vessels and bundles of axons going to the brain pass through the back of the eye. This creates a blind spot on the retina. TERM 62
Age-Related Macular Degeneration (AMD)
DEFINITION 62 Medical condition in which the photoreceptors in the macula malfunction and, over time, degenerate. Leading cause of central vision loss (blindness) in the United States today for those over the age of fifty years. Exact cause unknown but there are many risk factors.
