Biology - Sensory Systems

Front 1

What is a sensory cell?

Back 1

Human sensory cells convert (or transduce) physical and chemical stimuli from the environment into signals that are transmitted to other parts of the nervous system.

Different types of sensory cells are specialized for detecting different types of stimuli, such as sound (in hearing), odor (in smell), and light (in vision).

Most are modified neurons, but some are other types of cells closely associated with neurons.

Front 2

What is a sensory organ?

Back 2

It is an assembly of sensory cells and other types of cells (e.g. ears, noses, and eyes).

The sensory organs enhance the ability of the sensory cells to collect and amplify stimuli from the environment.

Front 3

What is the function of receptor proteins in sensory cells and where are they located?

Back 3

They open or close ion channels in the plasma membrane by one of several mechanisms when activated by a specific stimulus.

They are located in the plasma membranes of sensory cells.

Front 4

What are mechanoreceptors, chemoreceptors, and
photoreceptors?

Back 4

They are types of receptor proteins.

1. Mechanoreceptors: Respond to changes in pressure
- Part of an ion channel, typically open or close the channel directly by changes in its confirmation (or 3D shape)

2. Chemoreceptors: Respond to chemical stimuli (smell)
3. Photoreceptors: Respond to light stimuli (visual)
- Typically act indirectly by initiating a signal transduction cascade that controls the opening or closing of an associated ion channel in the plasma membrane

Front 5

What is a receptor potential?

Back 5

The opening or closing of ion channels by the activated receptor proteins alters the membrane potential of the sensory cell. This alteration results in a receptor potential.

* Must be converted into an action potential in order for the signal from the sensory cell to be propagated in the nervous system

Front 6

Are all sensory cells capable of generating and propagating action potentials?

Back 6

No, only some are.

Those that are capable, the receptor potential spreads to a region of the plasma membrane that contains voltage-gated sodium channels. Opening of the voltage-gated sodium channels causes an action potential to be generated and propagated by the membrane of the sensory cell.

In those that are NOT capable, the receptor potential spreads to a presynaptic region of the plasma membrane and induces the release of a neurotransmitter. Release of the neurotransmitter may then cause an action potential to be generated and propagated by the postsynaptic neuron.

Front 7

What is the auditory system?

Back 7

How we sense sound

Key Sensory Organ: Ear

Front 8

What is the main functions of the ear pinna?

Back 8

It collects pressure waves (or sound waves) and directs them into the auditory canal.

It is the prominent structure on the side of our heads.

Front 9

What is the main functions of the auditory canal?

Back 9

Pressure (sound) waves travel down the canal.

Front 10

What is the main functions of the tympanic membrane?

Back 10

It covers the end of the auditory canal and vibrates in response to pressure waves traveling down the canal.

aka eardrum

Front 11

What is the main functions of the ossicles?

Back 11

They are the three specialized bones in the middle ear.

They amplify (20-fold) and transmit the vibrations of the tympanic membrane to the oval window, a flexible membrane that separate the middle ear from the fluid-filled inner ear.

Front 12

What is the main functions of the oval window?

Back 12

Vibrations of the oval window result in pressure waves that are ultimately detected by mechanoreceptors.

Front 13

What is the main functions of the cochlea?

Back 13

It is a long, tapered, coiled chamber.

It contains three parallel canals separated by two membranes, one of which is the basilar membrane.

Front 14

What is the main functions of the basilar membrane?

Back 14

The organ of Corti is located on the basilar membrane (contain sensory cells).

Front 15

What is the main functions of the organ of Corti?

Back 15

They contain sensory cells known as hair cells.

Front 16

What is the main functions of the tectorial membrane?

Back 16

A

Front 17

What are hair cells and stereocilia?

Back 17

The hair cells are mechanoreceptors. Projecting from the surface of the hair cells are stereocilia.

The stereocilia are in contact with a rigid shelf that overhangs the organ of Corti, known as the tectorial membrane.

Front 18

How are pressure (or sound) waves converted into electrical signals by the auditory system?

Back 18

Pressure waves in the cochlea resulting from vibrations of the oval window cause the basilar membrane to flex. When the membrane flexes, the stereocilia press against the rigid tectorial membrane and bend. As the stereocilia bend, they activate receptor proteins in the plasma membranes of the hair cells.

If the plasma membranes are depolarized by the activated receptor proteins, the hair cells release a neurotransmitter that causes action potential to be generated and propagated by associated neurons.

The latter neurons form part of the auditory nerve that transmits information to the brain.

Front 19

What are “conduction deafness” and how does it arise?

Back 19

It is caused by the loss of function of the tympanic membrane or the ossicles of the middle ear.

Scarring of the tympanic membrane or stiffening of the connections between the ossicles may occur because of repeated infections of the middle ear. In addition, progressive stiffening of the ossicle connections may happen with increasing age.

The effect is less efficient conduction of pressure (or sound)waves from the tympanic membrane to the oval window.

Front 20

What is “nerve deafness” and how does it arise?

Back 20

It is caused by damage to the inner ear or to the auditory nerve.

For example, if the hair cells of the organ of Corti are damaged, they fail to release neurotransmitters in response to pressure waves in the cochlea.

A common cause of damage to the hair cells is exposure to very loud sounds, such as those of jet engines or highly amplified music. Such damage is cumulative and permanent.

Damage to the auditory nerve itself may result from viral infections, among other causes.

Front 21

What is the olfactory system?

Back 21

Sense odor

Key Sensor: Nose

Front 22

Where are the sensory cells of the olfactory system located?

Back 22

They are embedded in a layer of epithelial cells at the top of the nasal cavity.

The dendrites of these sensory cells end in olfactory cilia at the surface of the nasal epithelium.

The axons of the sensory cells project into the olfactory bulb (in the frontal lobe) of the brain.

Front 23

What is the sequence of events that follows the binding of an odorant molecule to a receptor protein in the olfactory system?

Back 23

1. An ordor molecule (or odorant) binds to a specific receptor protein in the olfactory cilia of the sensory cell.
2. Activation of the receptor protein causes an adjacent G protein in the plasma membrane to become activated.
3. In turn, the G protein activates the enzyme adendylyl cyclase, which catalyzes the synthesis of cyclic AMP.
4. Cyclic AMP, which serves as a so-called "second messenger," binds to sodium channels in the plasma membrane and causes them to open.
5. The resulting depolarization of the plasma membrane leads to the generation and propagation of an action potential in the sensory cell.

Front 24

What contributes to our ability to discriminate among many different kinds of odorant molecules?

Back 24

Diversity of receptor proteins in the olfactory system is large

Functional olfactory receptor proteins are encoded by a family of more than 300 different genes in humans.

Front 25

What is the visual system?

Back 25

Sense light

Key Organ: Eyes
(Fluid-filled structure covered by a layer of tough connective tissue)

Front 26

What is the cornea, and what is its function?

Back 26

It is at the front of the eye. It is a transparent, connective tissue.

Light passes through it to enter the eye.

Front 27

What is the iris, and what is its function?

Back 27

It is the inside of the cornea.

It controls the amount of light reaches the layer of sensory cells in the retina at the back of the eye.

Front 28

What is the pupil, and what is its function?

Back 28

It is the central opening of the iris.

When the iris constricts, as in bright light, the pupil is small. When the iris relaxes, as in dimmer light, the pupil is large.

Front 29

What is the lens, and what is its function?

Back 29

It is to the inside of the iris.

It is made of (highly structured) crystalline proteins.

It is involved in focusing of images on the retina. In particular, changes in its shape, which are controlled ciliary muscles, enable the eye to shift focus from distant objects to near objects, and vice versa.

Front 30

What are rod cells, and what is their basic structure and function?

Back 30

It is one type of sensory cell (photoreceptor). They contain the receptor protein, rhodopsin.

It is a modified neuron with a highly specialized structure.

Each rod cell has an outer segment, an inner segment, and a synaptic terminal.

The function of the discs is to capture photons of light energy passing through the rod cell.
The inner segment contains the nucleus and abundant mitochondria.
The synaptic terminal is where the rod cell communicates with other neurons via neurotransmitters.

Rod cells are very sensitive to light, and thus are well-suited for vision under low light conditions.

Front 31

What are cone cells, and what is their basic structure and function?

Back 31

It is one type of sensory cell (photoreceptor). They contain the receptor protein, rhodopsin.

It is also a specialized protein.

The human retina has about 100 million rod cells, but only about 5 million cone cells (20:1 ratio).

Cone cells are less sensitive to light, and thus function well under high light conditions. They also are responsible for color vision in humans.

The human retina contains three types of cone cells, which differ in wavelengths of visible light that they absorb.

Front 32

What is rhodopsin?

Back 32

It is a key sensory protein in sensory cells. It consists of both an opsin protein and a light-absorbing component, 11-cis-retinal.

The 11-cis-retinal is cradled in the center of the opsin protein, and is bound to it.

Front 33

How is rhodopsin activated?

Back 33

When 11-cis-retinal absorbs a photon of light energy, it changes into a different isomer (or form) of retinal, known as all-trans-retinal.

The change to all-trans-retinal puts a strain on the bonds between the opsin and retinal components, and results in a change in the conformation of opsin.

The latter change in conformation signals the detection of light energy (i.e., the rhodopsin is considered to be activated, or excited).

Front 34

What sequence of events follows the activation of rhodopsin and leads to the closing of sodium channels in the plasma membrane of the photoreceptor?

Back 34

When rhodopsin is activated (or excited) by the absorption of light energy, it activates an adjacent G protein, known as transducin, in the disc plasma membrane of the rod or cone cell.

Activated transducin in turn activates the enzyme phosphodiesterase, which converts cyclic GMP to 5'-GMP.

The latter conversion leads to the closing of sodium channels in the outer plasma membrane.

Thus, absorption of light energy by rhodopsin has the effect of hyperpolarizing (outside of rod cell will be hyperpolarized), rather than depolarizing, the outer plasma membrane (i.e., the membrane potential becomes more negative).

Hyperpolarization of the outer plasma membrane, results, in turn, in a decrease in the rate of neurotransmitter release by the rod cell or cone cell.

Front 35

What are the functions of bipolar cells and ganglion cells in the retina?

Back 35

Rod cells and cone cells do not generate or propagate action potentials.

Ganglion cells, by contrast, are capable of generating and propagating action potentials. The axons of ganglion cells form part of the optic nerve that carries information to the brain.

Connecting the ganglion cells to the rod and cone cells are bipolar cells.

Front 36

How is light energy converted into electrical signals in the retina?

Back 36

Absorption of light energy by rhodopsin results in a decrease in the rate of neurotransmitter release by the rod cells or cone cells at their synapses with the bipolar cells.

This decrease in the rate of neurotransmitter release causes a change in the membrane potential of the bipolar cells.

This change in membrane potential in turn alters the rate of neurotransmitter release by the bipolar cells at their synapses with the ganglion cells.

This in turn determines the rate at which the ganglion cells generate and propagate action potentials.

Front 37

What key factors (as discussed in lecture) can lead to impaired vision?

Back 37

1. Clouding of the Lens
- Causes the light that passes through the lens to be scattered
- Results from a breakdown int eh structure of the crystalline proteins that form the lens
- Accompanies aging

2. Damage to (or loss) of Photoreceptors
- The central portion of the retina is no longer able to absorb light energy effectively and convert it into action potentials.

3. Damage to the Optic Nerve
- Injury
- Action potentials generated by ganglion cells in the retina are no longer able to reach the occipital lobe of the brain.
- Ganglion cells still fire, but the information is not carried to the brain.