Lecture 7: Afferent Nervous System
Reading: Silverthorn Ch 10
Organization of the Nervous System
The nervous system can be organized anatomically, or functionally (with anatomical sprinklings!)
1.
2.
3.
4.
5.
6.
7.
8.
Central nervous system
Peripheral nervous system
Afferent nervous system
Efferent nervous system
Somatic nervous system
Autonomic nervous system
Sympathetic nervous system
Parasympathetic nervous system
Draw a visual that illustrates how the previous 8 terms are related to each other.
Categorizing NS by LOCATION of nerves:
1. CNS (brain, spinal cord)
A. Integrates incoming sensory info, evaluates, then initiates response
B. Only includes cells that BEGIN and END in the brain or spinal cord
2. PNS (afferent and efferent neurons)
A. Cranial nerves (originate in brain)
B. Spinal nerves (originate in spinal cord)
Categorizing NS by DIRECTION in which information is carried
1. Afferent (TOWARD CNS; sensory info; the word “affect” (a noun), actually means “feelings, or facial expression”)
2. Efferent (AWAY FROM CNS; action info; often called motor neurons because they create an “effect” (noun) by stimulating a
muscle to DO something)
A. Categorizing NS by the TYPE of effectors (actions) they regulate
i. Somatic Nervous System (SNS)-- Sends out an order for an “effect” thru somatic effectors (skeletal muscles)
ii. Autonomic Nervous System (ANS)-- Sends out an “effect” order to “involuntary” tissues
a. Sympathetic branch!!!!! Prepares body for “fight or flight” responses…
b. Parasympathetic branch…..“Rest and repair”, or “Feed and breed”
A closer look at PATHWAYS...
1.
2.
3.
4.
5.
Stimulus
Sensory receptor (receives stimulus)
Afferent neuron (transmits message to CNS)
Afferent and interneuron meet at a SYNAPSE
Interneuron (entirely within CNS) integrates info
A. Often a specific PART of the brain! Ex: (Fig 9-15)
B. Sometimes in a FASTER reflex (ie knee jerk) this is eliminated…one less step to go through, faster response
6. Inter and efferent meet at SYNAPSE
7. Efferent neuron (causes action)
A. Somatic response (skeletal muscle fires)
B. Autonomic response (other, involuntary response)
a. Sympathetic (!!!)
b. Parasympathetic (…)
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Sensory Receptors
1. Modified dendritic ends (or special cells associated with the dendrites) that are sensitive to certain environmental stimuli.
2. Stimulus triggers a change in membrane potential of the receptor, though an AP is not necessarily created.
3. Classified according to the type of stimulus that triggers an action potential…
A. Mechanoreceptors- respond to mechanical energy
i. Touch (skin)
ii. Deep tissue touch (muscle, pressure)
iii. Hearing
iv. Equilibrium
v. Arterial pressure (baroreceptors in carotid sinus and aorta)
B. Thermoreceptors- respond to temperature
C. Nociceptors- PAIN
D. Photoreceptors- respond to light
E. Chemoreceptors- respond to chemical energy
i. Taste
ii. Smell
iii. Arterial O2 and CO2 (in aortic and carotid bodies)
iv. Osmolarity (in hypothalamus)
v. Blood glucose (hypothalamus)
4. Super sensitive to their own stimuli…but can be made to fire if other stimuli is great enough (ex: photoreceptors in the eye, if hit
hard enough, can fire…seeing stars)
5. Intense stimulation of any receptor = pain.
6. Can ADAPT! (med 575)
A. They adapt partially or fully to continual sensory stimulus
B. At first, many impulses (like AP, but not all or none) are generated by the stimulus.
C. Over time, the impulses are generated more slowly, or not at all!
D. Mechanism of this varies…
Taste
mm Anatomy
A taste bud is a collection of about 50 cells (taste cells and sustentacular cells) with a taste pore in the
middle. These cells are embedded in the gustatory epithelium. Each taste bud is between 1/30 x 1/16 mm
in size and they are embedded in bumps called papilla. Taste cells are epithelial cells with microvilli (taste
hairs) on the surface. The microvilli are covered with RECEPTORS that respond to chemicals in the saliva.
1. Mechanism of action
A. Taste molecules dissolve in the saliva and come in contact with the receptors on the taste cell.
B. If salty and sour taste cells are stimulated, an AP is generated and serotonin is released on the postsynaptic neuron heading
to the brain.
C. If sweet, umami or bitter receptors are stimulated, a SECOND MESSENGER CASCADE is activated …end result is ATP
which stimulates postsynaptic neurons (and even neighboring cells)
D. Ultimately, the message is sent to glossopharyngeal and facial nerves)
2. Primary sensations of taste
A. Currently identified 13 taste receptors
B. All tastes (hundreds you can detect!) are supposedly combined forms of those 13 basic tastes
C. Some people have “taste-blindness” Like to PTC paper...15-30% of people have taste blindness to phenylthiocarbamide
D. Non-tasting is a recessive trait
3. Threshold
A. Different taste molecules are detected at different concentrations:
i. HCl: 9 x10-4
ii. NaCl: 1 x10-2
iii. Sucrose: 1 x10-2
iv. Quinine (bitter): 8 x10-6
B. What is the adaptive significance of this?
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Smell
mm Anatomy
Olfactory cilia line the olfactory epithelium. The cilia are covered with receptors that fire when smell
molecules bind. The message is sent into the olfactory nerve (CN I).
1. Mechanism of action
A. Cilia are covered with RECEPTORS
B. Odor molecules enter the nose and come in contact with the olfactory membrane.
C. They then diffuse through the mucus that covers the membrane.
D. Once they reach the olfactory cilia, they bind with a receptor, triggering a SECOND MESSENGER CASCADE with
culminates with the opening of a gated sodium channel!
i. Second messenger responses can be AMPLIFIED (one odor molecule is detected, but the response produces many
cAMP molecules which activate the , meaning small odors can be detected…smell is very sensitive.)
E. Action potential is generated, and message is sent to olfactory nerve
2. Primary sensations of smell
A. Possibly over 100 primary sensations of smell!
B. Some people have “odor-blindness” for certain smells
C. At least 50 different “odor-blindness” conditions have been identified.
3. Threshold
A. Very tiny concentrations of odor molecules can be detected.
B. Methylmercaptan can be smelled at a concentration of one 25 trillionth of a gram in 1 mL of air (used to scent natural gas)
Sight
mm Anatomy
The retina (lining back of eye) is composed of several layers of cells. From back to front (following the
path of signal transduction, NOT the path that LIGHT takes): Pigmented layer contains cells filled with
melanin that are DARK and so prevent light from ricocheting around the eye. Photoreceptors are cells
that fire action potentials when stimulated by light (100 million rods and 3 million cones). Horizontal cells
are cells that connect rods and cones laterally AND connect to bipolar cells, and INHIBIT lateral spread
of stimulus. Bipolar cells transmit info from rods, cones and horizontal cells; sometimes these messages
are excitatory and sometime they are inhibitory. Amacrine cells have a huge variety of cell functions that
essentially help interpret visual data before it leaves the eye. Finally, ganglion cells connect to the optic
nerve and different types are sensitive to different things (ex: one type is sensitive to sudden movement or
change, sending an immediate message!)
1. Mechanism of action (Interesting note: Very few of these processes in the eye generate ACTION POTENTIALS. Instead,
electric current is generated and transmitted through the cell…but it is not an all or none response. This enables GRADED
STIMULATION, and very fine distinctions- pg 635 med)
A. Light enters the eye and must pass through all layers of the RETINA to reach the photoreceptors.
i. Exception: FOVEA!
a. The fovea is the location on the retina where light hits when you look DIRECTLY at an object. It is the area where
your vision is the most acute.
ii. Each photoreceptor has similar general structure, but contains different visual pigments in the outer segment…
a. Rod visual pigment: Rhodopsin
b. Cone visual pigments: 3 different pigments, very similar to rhodopsin
B. Light energy triggers a chemical reaction in rhodopsin, cleaving it into opsin and retinal. This process is called
BLEACHING.
C. This also triggers a SECOND MESSENGER CASCADE producing large amounts of product that SHUT Na+ channels
i. Closed sodium channels…what happens to membrane potential? Hyperpolarization.
ii. Tiny numbers of photons can activate the rhodopsin and produce a measurable response, because of the second
messenger system (it amplifies the response).
iii. Cone pigments are 30-300x less sensitive than the rods (med pg 631), but this is sensitive enough to allow color
distinction even in very dim light! Check it out sometime…
iv. Each type of cone is sensitive to a unique set of wavelengths.
D. The hyperpolarized photoreceptor does NOT release glutamate onto bipolar cell (it decreases glutamate release at the
synapse. Interestingly, the unstimulated state of the photoreceptor involves a partially depolarized membrane and tonic
release of glutamate at the synapse). Therefore, counterintuitively, it is the absence of glutamate in the synapse that causes
the bipolar cell to fire and pass on the visual message.
E. The message ultimately reaches the optic nerve and is sent to the visual cortex.
2. Adaptation
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A. In bright light, lots of BLEACHING occurs, decreasing sensitivity to light= light adaptation.
B. In dark, rhodopsin is put back together = dark adaptation
C. Color afterimage:
i. Stare at a red square. Keep listening and don’t move your eyes.
ii. Right now, the “red” cones are firing. All the photoreceptors in the red cones are firing, and bleaching.
iii. The longer you stare, the more those cones fire…
iv. The more they fire, the more bleaching occurs!
v. *Stare for 1 min*
vi. Now, stare at a white piece of paper. Do you see the greenish/bluish square? Why is that there???
vii. White stimulus = all colors…to see white, all your cones fire!
viii.However, all those red cones seeing the square are bleached.
ix. Only the unbleached cones are firing…blue/green! (When the red cones recover, you will see white again).
Hearing
Sends messages to CN VII: Auditory (vestibulochochlear) nerve
mm Anatomy
Complex anatomy of the ear allows transfer of mechanical energy (sound waves in the air) to a nerve
impulse that is interpreted as SOUND by the brain. In the external ear, sound waves are “caught” by
the pinna, travel through the external auditory canal, and run into the tympanic membrane, which then
VIBRATES. The vibrations are transferred to the middle ear by three middle ear bones (malleus, incus, and
stapes) which are attached at one end to the tympanic membrane and at the other end to the oval window, a
thin membrane separating the middle and inner ear. These bones enable an increase in the FORCE applied
by the original sound wave to the oval window by a factor of 22. The mechanical force causes waves in the
fluid filled inner ear structures, including the cochlea and semicircular canals. Sound waves travel through
the fluid filled cochlea where non-neural receptor cells (hair cells- about 16000 of them) are stimulated and
transmit the information to the brain.
1. Mechanism of action: Hearing
A. Sound waves enter cochlear fluid from oval window and run into the flexible membranes of the cochlear duct.
B. Vibrations move the basilar membrane AND the tectorial membrane
C. These two membranes slide next to each other and stimulate the cilia ridge of the HAIR CELLS
D. When cilia are bending, K+ CHANNELS OPEN and the cell depolarizes.
E. Depolarized hair cells release NT on sensory neurons, causing AP in those neurons leading to the olfactory nerve
F. Lower frequency sounds travel farther up the cochlea…higher frequency sounds don’t travel so far.
2. Mechanism of action: Balance
A. Semicircular canals contain fluid that detects rotational acceleration as fluid inside sloshes
B. Utricles and sacules contain little rocks that trigger hair cells when you change your position…
3. Attenuation (accommodation)
A. Muscles can contract the ear drum and thus tighten the tympanic membrane, making it less sensitive…this happens when in
the presence of very loud noises…it takes a bit for the muscles to relax, which is why sounds can be fuzzy when you exit a
loud venue…
Touch, pressure and vibration…
1. All sensations are detected by the same type of receptor, but just at different locations
A. Touch- surface
B. Pressure- deep
C. Vibration- fast repetition of stimulus
2. At least 6 different kinds of receptors…respond to varying stimuli
3. These receptors are distributed unevenly throughout the skin, and their locations can be mapped.
4. Also, you can determine the sensitivity of an area by by using the two point threshold test.
A. Not all sensory receptors converge on a single neuron within the CNS. In fact, sometimes MANY sensory receptors
converge on ONE CNS neuron! This can lead to the sensation of a single stimulus when actually multiple stimuli are taking
place.
Thermoreceptors- respond to thermal energy (heat)
1. There are more cold receptors than warm receptors (text 346). The also adapt slowly so that they only respond to CHANGES…
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External Brain 7: Afferent Nervous System
Study Guide Questions
1. Draw a diagram that clearly relates the following terms: CNS, PNS, Somatic NS, Autonomic NS, afferent, efferent.
2. Be able to identify the different parts of the reflex arc given any scenario. (Ex: Larry sees a bear, Larry runs away). Your
answer does not need to DETAIL structures or processes that we have not yet studied...
3. Define “sensory receptor”.
4. Be able to clearly identify the different types of sensory stimuli that can stimulate sensory receptors.
5. Describe the structure of taste cells and explain exactly HOW a chemical stimulus is translated into an afferent message.
6. What is taste blindness? Relate this answer to PTC paper. How does this relate to “odor blindess” or “color blindness”?
7. Why are taste cells more sensitive to bitter flavors? (Your answer should address evolutionary “causes” for this phenomenon,
as well as the physiological mechanism by which it occurs.)
8. Does taste stimulus always result in an action potential? Explain.
9. Explain how the smell of baking chocolate chip cookies will be sent to your brain.
10.Describe the structure of the retina. Include the functions for the pigment layer, the photoreceptors, and “everything else”.
11. What is the fovea? Why is it so significant?
12.Be able to explain how light triggers an afferent message in a ROD.
13.What is the difference between the 3 types of cones? How are cones different from Rods?
14.What is bleaching?
15.Explain the physiology of the “afterimage”.
16. Be able to explain the role each of the following structures plays in “hearing”: Pinna, tympanic membrane, middle ear
bones, oval window, cochlea
17.Explain how sound waves are translated into an afferent nerve impulse.
18.What is the two point threshold test? What does this test tell you about sensory neuron location?
19.What is the blind spot?
20. Why don’t pain receptors “adapt”? What types of sensory receptors DO adapt? Why the difference?
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