Lab #7: Nerve Pathways and Somatosensory Physiology
Background
The nervous system plays a central role in
homeostasis.
The central nervous system
functions as the primary controller and
integrator for most of the physiological
regulatory mechanisms of the human body. The
peripheral nervous system, in turn, links the
central nervous system both with sensors (cells
that detect environmental changes) and with
effectors (cells that act on environmental
changes in a way that ultimately counteracts
those changes). In order for most physiological
regulatory processes to work, therefore,
information must be conducted through a series
of neurons leading from the sensors to the
central nervous system, through various regions
of the central nervous system, and ultimately
from the central nervous system to the effector
tissue.
change (Figure 7.1). This is largely because the
involvement of multiple interneurons between
the sensory neuron(s) and the motor neuron(s)
allows integration of information from other
sensory inputs, a higher degree of modulation of
the signal (impeding some aspects of the
signaling path while amplifying others) and the
involvement of multiple structures in the body in
producing a coordinated response. For example
if you were to read this text aloud, sensory
information would travel through three different
Peripheral NS
Central NS
Peripheral NS
Effector
Sensor
Nerve Pathway Structure and Spinal Reflexes
Regulatory mechanisms that involve the nervous
system require the transmittance of information
through a series of neurons forming a nerve
pathway. Most nerve pathways include neurons
that perform three basic roles: a) those that
conduct sensory information from the periphery
into the central nervous system and perhaps
through the central nervous system to a
particular location, b) those that process,
integrate, and interpret sensory information, and
c) those that carry response information to the
effector tissues, first through the central nervous
system and then out through the peripheral
nervous system to the appropriate muscles or
glands.
The specific number of neurons that
information needs to travel through to go from
sensor to effector can be quite variable.
Differences in the lengths of nerve pathways can
lead to wide variation in the complexity and
precision of response to some environmental
change as well as the speed of such response.
Longer nerve pathways often lead to much more
complex and precise responses to environmental
Peripheral NS
Central NS
Peripheral NS
Other
sensory
inputs
Sensor
Other
response
outputs
Effector
Fig 7.1. Outlines of a simple nerve pathway (top)
and a complex nerve pathway (bottom). Blue arrows
in the central nervous system represent an excitatory
effect of the neuron on the next neuron in the
pathway, whereas the red dashed arrows indicate the
neuron will have an inhibitory effect on the next
neuron. Note that the more complex pathway allows
a greater degree of integration from multiple sensory
inputs, and that based upon these sensory inputs the
response will be modulate in terms of the number of
motor neurons activated, the frequency of action
potential generation in these motor neurons, etc.
However, since more chemical synapses separate the
sensor from the effector, the response time is slower
in a complex pathway than in a simple one.
neurons from your eye to the occipital lobe of
the cerebral cortex.
Information from the
occipital lobe would then be relayed to a number
of different areas of the brain, including the
angular gyrus of the parietal lobe, regions of the
temporal lobe such as Wernicke’s area (for
interpretation of the symbols and formulation of
words), areas of frontal lobe such as the motor
cortex and Broca’s area (for controlling the
muscular activity needed for vocalization), and a
host of other areas that control the extrinsic eye
muscles, etc. Once integration of the sensory
information is made, selective stimulation and
inhibition of the somatic motor neurons
controlling the muscle activity of the tongue,
lipids, larynx, etc. are engaged to allow the
proper pronunciation of the words.
Shorter motor pathways, however, tend to
have much simpler responses that are localized
on one particular effector organ or a small group
of organs, and exhibit little integration of
multiple sensory inputs in the development of a
response, and thus little modulation of that
response.
Although complex nerve pathways enable
greater control and modulation of responses to
environmental change, there is a trade off with
the speed of response. The slowest point of
information conduction through a nerve pathway
occurs at chemical synapses, where the
presynaptic cell must couple the action potential
to neurotransmitter release, the released
neurotransmitter must diffuse across the synaptic
cleft and bind to receptors on the postsynaptic
cell, the binding of the messenger must be
coupled to a change in the permeability of the
membrane to specific ions, and the membrane
must depolarize up to threshold before an action
potential can be generated in the postsynaptic
cell (Fig 7.2). The more chemical synapses a
signal must travel through as it passes through
the central nervous system, the slower response
time will be. Therefore, the most rapid types of
responses tend to be relatively simple behaviors
derived from relatively short nerve pathways.
The simplest, and fastest, types of nerve
pathways are called reflex arcs. A reflex is a
simple, stereotyped involuntary behavior that
occurs in response to a specific stimulation.
Reflexes are unconscious actions where
evaluation, integration, and control involve the
Presynaptic Cell
• Influx of Ca2+
• Calmodulin Activation
• Protein Kinase C activation
• Synapsin Activation
• Exocytosis
Synaptic Cleft
• Diffusion of neurotransmitter
Postsynaptic Cell
• Binding of neurotransmitter
• Opening of ion channels
(directly or through second
messenger systems)
• Depolarization to threshold
Fig. 7.2. Diagram of a chemical synapse between
two neurons and the events that must take place in
order for the action potential of one neuron to induce
an action potential in a second neuron. Transferring
information from one neuron to another in a
chemical synapse is the slow point of information
transmittance through a nerve pathway
spinal cord and brain stem rather than the
cerebral cortex. As a result, the coordination of
the responses does not directly involve the
cerebral cortex, and thus the response may be
made before a person becomes conscious of the
stimulation or even without the person ever
becoming conscious of the stimulus.
The simplest and fastest of the reflex arcs
are for spinal reflexes. In these particular nerve
pathways, connections between sensory neurons
and motor neurons are made within the spinal
cord itself. Information may be relayed up to
the brain for conscious perception, but the brain
itself is not directly involved in the elicitation of
a response.
The simplest nerve pathway possible is
called a monosynaptic pathway, since there is
only one chemical synapse connecting the
sensory portion of the pathway to the motor
portion of the pathway. In such pathways, a
sensory neuron synapses directly with a motor
neuron, with no interneuron connecting the two.
An example of such a pathway can be found in
the patellar or “knee-jerk” reflex (Fig 7.3).
Tapping a subject on the patellar ligament pulls
Fig 7.3. The reflex arc of the patellar or “knee jerk”
reflex.
1) Tapping on the patellar ligament
stimulates stretch receptors (spindle fibers)
imbedded in the quadriceps femoris. 2) This
generates action potentials in a sensory neuron,
conducting this signal to the spinal cord. 3) The
sensory neuron synapses directly with an α motor
neuron. The α motor neuron conducts an action
potential through the same spinal nerve back to the
quadriceps femoris. 5) The quadriceps contract,
extending the lower leg.
Illustration from
www.merck.com/mmhe/ sec06/ch077/ch077c.html.
See also Fig 12.27 in your textbook.
the patella downward, in turn stretching the
quadriceps femoris through the connecting
tendons. Imbedded within the muscles are
modified skeletal muscle fibers called spindle
fibers (sometimes called intrafusal fibers) which
have the dendritic endings of sensory neurons
attached to them. Stretching the spindle fibers
induces action potentials to form in the sensory
neurons, which are conducted to the spinal cord.
In the grey matter of the spinal cord, the sensory
neuron synapses directly with the dendrites and
cell bodies of specific motor neurons (α motor
neurons) that lead to the contractile (extrafusal)
skeletal muscles fibers in the same muscle that
had been stretched. Stimulation of the muscle
fibers by the motor neurons induces contraction
of the quadriceps femoris, which extends the leg.
Other reflex pathways may involve one or
more interneurons connecting the sensory
portion of the pathway to the motor portion.
The inclusion of interneurons enables both
inhibitory and excitatory elements to be included
in reflexes. An example of an inhibitory reflex
arc is that involving the Golgi tendon organs—
structures imbedded in the tendons of skeletal
muscles that consist of the dendritic endings of
sensory neurons wrapped around the collagen
bands of the tendon (Fig 7.4). Golgi tendon
organs are stimulated when tension on the
tendons is increased (usually due to excessive
contraction of the muscle). Action potentials are
propagated down the length of sensory neurons
into the spinal cord, where they synapse with
interneurons. The interneurons are stimulated to
undergo action potentials, and in turn release
inhibitory neurotransmitter to the α-motor
neurons that control the contraction of the
affected muscle. The inhibitory post-synaptic
potentials triggered in the motor neurons slow
signaling rates to the muscle, thus decreasing
contractile strength and, in turn, the tension
being exerted on the tendon. Since there are, in
this case, two chemical synapses separating the
sensory portion of the pathway with the motor
portion (one between sensory neuron and
interneuron, and one between interneuron and
motor neuron), we refer to this reflex arc design
as a disynaptic reflex arc.
One should be aware that nerve pathways
can branch, and this occurs even within the
relatively simple nerve pathways involved in
spinal reflex arcs. For example, a sensory
neuron undergoing an action potential may
+
-
Fig 7.4. Illustration of the disynaptic reflex associated
with Golgi tendon organs. Stimulation of the sensory
neuron (black) induces it to stimulate the interneuron
(yellow) in the spinal cord. The interneuron, in turn,
inhibits the motor neuron (blue) innervating the muscle
undergoing contraction
Anatagonistic Muscle
Stimulated Muscle
+
+
-
Fig 7.5. An example of reciprocal innervation in a
muscle-stretch reflex. The sensor neuron (black) not
only stimulates a motor neuron leading to the muscle
that had been stretched, inducing contraction in it, but
also stimulates an interneuron that inhibits a motor
neuron leading to the stretched muscle’s antagonist.
synapse with one or more neurons involved in
the reflex response, but also synapse with
interneurons in ascending tracts of the spinal
cord that relay information to the brain so that
the brain can process the sensory information.
For example, if you place your hand on a
burning hot stove, not only is a reflex arc
activated to pull your hand away, but sensory
information is also relayed up to the sensory
cortex so that you become consciously aware
that you have burned yourself. Branching of
pathways is also important in coordinating the
actions of multiple organs so they do not
interfere with one another during the reflexive
response. For example, many skeletal muscles
function in antagonistic pairs, where one muscle
creates a motion when it contracts that is the
opposite of the motion created when its
counterpart contracts. In order to generate
reflexive motion in a particular direction (for
example, pulling your hand away from a hot
stove), not only must a specific muscle be
stimulated to undergo contraction but its
antagonist must be inhibited from contracting at
the same time. Thus many reflexive pathways
involve reciprocal innervation, where signals
from a sensory neuron actually stimulate two
different reflexive pathways, one that leads to
the excitation of motor neurons and the
contraction of a muscle, and another that inhibits
action potential generation in the motor neurons
leading to the antagonistic muscle (Fig 7.5).
Fig. 7.6. Examples of cutaneous receptors. Image
from http://www3.open.uoguelph.ca/de/ideaExchange
/zoo1500/cwork/unit3/strat_skin.html
Somatosensory Physiology
The central nervous system must receive
information regarding environmental changes
occurring outside the body as well as inside in
order to regulate the internal environment. A
variety of sensors are distributed throughout the
body which monitor environmental conditions
and relay information regarding these conditions
to the central nervous system. Some sensors are
specialized organs that are located within a
particular region of the body (e.g., eyes,
vestibular apparatuses, etc.), whereas others are
relatively simple structures that are distributed
throughout the body. These latter sensors are
referred to as somatic sensors, and are
responsible for the sensations of touch, pressure,
limb movement, body position, temperature, and
pain.
Somatic sensors typically have a simple
design, consisting of either the free dendritic
endings of sensory neurons or a small structure
(e.g., a single cell or bundle of connective tissue)
around which are wrapped the dendritic ends of
sensory neurons.
These sensors can be
classified into two categories based on location.
Cutaneous receptors are those somatic sensors
that are distributed in the skin near the surface of
the body, such as those responsible for the sense
of touch, pressure, and temperature (Fig 7.6).
Proprioceptors are sensors imbedded within
muscle and associated connective tissue and
which monitor tension exerted on those
structures, thus enabling perception of limb
movements, contractile tension and body
position. The spindle fibers and Golgi tendon
Heat
Cold
Touch
Figure 7.7. A diagram of a cross section of the skin
illustrating the punctate distribution of cutaneous
receptors at the surface. Each sensor has an exclusive
receptive field that, when stimulated with the proper
stimulus, induces action potentials in a sensory neuron.
organs that monitor muscle tension are examples
of proprioceptors.
Cutaneous receptors are designed to monitor
environmental conditions at the surface of the
body. Each receptor detects environmental
changes in a specific location on the surface of
the skin, referred to as a receptive field. For
those sensors located particularly close to the
surface of the skin (e.g., those responsible for
sensations of light touch and those that detect
the temperature of objects in contact with the
skin), typically only one receptor is found in a
specific region of the skin. Therefore, these
cutaneous receptors tend to have a punctate
distribution, where a specific region of the skin
would need to be stimulated in order to detect
heat, or cold, or touch (Fig. 7.7)
The size and density of the receptive fields
found within a region of skin can have a
profound influence on sensory acuity, the ability
to discriminate fine details of an object such as
shape and texture. When a stimulus is applied
anywhere within a receptive field, one specific
sensory neuron is stimulated, which sends
signals to the central nervous system. However,
the central nervous system is unable to discern
where, precisely the stimulus was applied within
a receptive field, since the same neuron is
stimulated regardless of what specific point in
the receptive field is stimulated. To detect the
multiple points of stimulation needed to discern
the shape and texture of objects, then, the
stimulus must be applied to multiple receptive
fields.
Smaller receptive fields, therefore,
enable more precision in determining where the
surface of the skin is being stimulated and where
it is not. Thus, regions of the skin that have
small receptive fields tend to have greater
degrees of sensory acuity than do areas with
larger receptive fields (Fig 7.8). Receptive field
size, in turn, is related to the density of sensory
receptors in a region of the skin. For example,
some regions of the skin (e.g., the back of the
lower leg) have relatively low densities of touch
receptors.
Thus each touch receptor is
responsible for a relatively large area of the
surface of the skin, and hence the receptive field
for each touch receptor is large. However, other
regions of the skin (e.g., the lips and finger tips)
have very high densities of sensory receptors,
and thus the receptive field size for each
receptor is comparatively small.
Sensors can vary widely in the way they
respond to prolonged stimulation. Many sensors
will undergo sensory adaptation, adjusting their
rates of action potential generation in response
to chronic stimulation (Fig 7.9). Sensors that
readily undergo sensory adaptation are called
Large Fields, Low Density
Small Fields, High Density
Fig 7.8. A diagram illustrating the effect of receptor
field size on acuity for the sensor of touch. The grey
four-pointed arrow is an object placed in contact with
the skin, and receptive fields filled in black are the
fields stimulated by contact with the object. Notice that
the pattern of receptive field stimulation for the small
fields better reflects the actual shape of the object than
does that for the large receptive fields.
Membrane
potential
Phasic receptor
Membrane
potential
Tonic receptor
Fig 7.8. Reponses to sustained stimulation in phasic
and tonic receptors.
phasic receptors. Other sensors, however, show
little sensory adaptation with continuous
stimulation, and continue to generate action
potentials at a constant rate as long as the
stimulus is applied. These sensors are called
tonic receptors.
Somatosensory receptors, like all sensory
receptors, function as transducers. They respond
to changes in the environment by generating
action potentials in sensory neurons. Ultimately,
it is the signals delivered into specific regions of
the brain or spinal cord from specific sensory
neurons that enable the central nervous system
to perceive environmental changes, and not the
application of the stimulus itself. Sensors are
designed to be most sensitive to specific stimuli
(the so-called “adequate stimulus” for that
sensor), but it is possible for other types of
stimuli to cause a sensor to evoke an action
potential in a sensory neuron if the stimulus is
strong enough. As a result, it is possible to
perceive a stimulus as being a different stimulus.
For example, receptors that detect cold
temperatures can also be stimulated by the
chemical methanol. It is also possible for a
stimulus to be perceived at a location other than
where it is actually being applied, a phenomenon
called referred pain. For example, stimulating
the endings of a severed nerve in an amputee
could lead to the sensation that the limb nonexistent limb is being stimulated (so-called
“phantom limb pain”). Moreover, the pain
receptors of many visceral organs synapse with
the same interneurons in the spinal cord as do
pain receptors from regions at the surface of the
body. Thus when damage occurs to the visceral
organ, the pain is perceived as originating from a
particular region near the surface of the body
(Fig. 7.9). Angina pectoris—the pain in the left
chest, shoulder, and arm that is associated with
heart disease, is one such example.
Fig 7.9. Sites where pain originating from visceral
organs are perceived cutaneously.
Yellow =
diaphragm/pericardium/heart; red = heart; violet =
digestive tract; orange = liver/gall bladder; green =
kidney/ureters; black = pelvic organs. Photograph
from http://www.med.umich.edu/lrc/coursepages/M1
/anatomy/html/surface/abdomen/referred.html.
(Dude! Put some pants on!)
Experimental Procedures
Experiment I: Spinal reflexes.
Using a rubber mallet, test one of your lab group
members for their stretch reflexes (see Fig 7.10).
a)
1) Patellar reflex: The subject should be seated and
relaxed, with their lower legs dangling and
their feet off the floor. The examiner should
locate the patellar ligament, and with a
relatively loose grip on the hammer, the
examiner applies a brisk tap to the ligament.
If the subject does not respond well, have the
subject lock fingers and pull his/her arms apart
to amplify the response.
2) Gastrocnemius (Achilles) reflex: The subject
should be seated and relaxed, with their lower
legs dangling and their feet off the floor. The
examiner should hold the bottom of the
subject’s foot with one hand, and with a
relatively loose grip on the rubber mallet,
apply a brisk tap to the Achilles tendon. If the
subject does not respond well, have the subject
lock fingers and pull his/her arms apart to
amplify the response.
b)
c)
Fig 7.10. Depictions of the techniques used for triggering
stretch reflexes: a) patellar reflex, b) gastrocnemius
(Achilles) reflex, c) biceps brachii reflex. Note that for
most of these reflexes the examiner is holding the portion
of the limb that will be moved as a result of the muscle
contracting.
Illustrations
are
from
http://medicine.tamu.edu/ neuro/reflex.htm
3) Biceps brachii reflex: The subject should be
seated and relaxed. The examiner should
palpate the anterior distal end of the humerus
to locate the biceps tendon, the place his/her
thumb over the tendon. Have the subject cross
his/her legs and squeeze them together (this
should amplify the response). With a relatively loose grip on the hammer, the examiner applies a
brisk tap to his/her thumbnail. Flick the hammer repetitively to test several times.
Experiment II: Mapping cutaneous receptors.
Draw a 2 cm × 2cm square on the back or the hand or the
ventral forearm of the subject with a pen (Fig 7.11). Have
the subject close their eyes. Place a metal probe into a
beaker of ice water to chill it, then remove it from the water
and dry the probe with a paper towel. Lightly touch the
probe to different points in the square, and have the subject
tell you when they feel a cold sensation. Mark the location
of cold sensation with a dark dot (●) with a pen. Repeat the
procedure with a probe that had been heated in hot water
Fig 7.11. The mapping of cutaneous receptors
in a small region of skin. Illustration is from
http://www.mcrel.org/ whelmers/whelm25.asp
then blotted dry and mark points where the subject feels heat with an open circle (○). Repeat once again
with a single bristle from a paint brush for, and have the subject tell you when they feel the bristle in
contact with their skin, and mark the location with a small “x” (×). Record the distribution of the sensors
in the subject’s skin on your datasheet.
Experiment III: Two-point touch discrimination.
This procedure is used to examine acuity for the sensor
of touch in different area of the skin. Two points are
applied to the skin simultaneously (Fig 7.12). If they fall
in two separate receptive fields, then two points of
contact will be felt. However, if both touch the skin in
the same receptive field, then only one point of contact
will be felt. By carefully adjusting the distance between
the two points so that they are as far apart as possible
when only one point is felt, the approximate diameter of
the receptive field can be estimated.
Use the following procedure to measure receptive
field size for the following regions of the skin: the tip of
the index finger, the palm of the hand, the medial lower
arm, and the nape of the neck. Obtain a pair of calipers,
and move the tines of the calipers 2-2.5 cm apart. Have
a member of your group serve as a subject, and have
them close their eyes. Select a location on their body,
and lightly touch both tines of the calipers to their skin
simultaneously. Ask the subject if they feel one point or
two. If they feel two points, move the tines slightly
closer together and repeat. Repeat this procedure until
the subject can only feel one point. Measure the distance
between the two tines to the nearest millimeter. This
distance is roughly equal to the diameter of the receptive
fields for touch in that area of the skin.
Fig 7.12. Two-point touch discrimination. When
the skin is touched at two points that fall in different
receptive fields, then two different sensory neurons
will be stimulated , and two separate points of
contact will be perceived. However, if the skin is
touched at two points in the same receptive field,
then only one sensory neuron will be stimulated, and
only one point of contact will be perceived
Experiment IV: Thermosensory adaptation.
In this experiment, different groups of cutaneous thermoreceptors (which tend to be phasic) will undergo
sensory adaptation. The result of differential adaptation of thermoreceptors, in turn, will illustrate how
the same environmental conditions can be perceived in different ways based on the sensitivity of different
sensors.
Place one hand into each of two water baths (one hot, one cold) simultaneously for 1 minute.
Afterwards, place both hands simultaneously into a bath containing lukewarm water. Describe the
sensation you feel in each hand.
Experiment V: Referred pain.
Select one person from your group. Obtain a large axe.
Position the subject’s arm on the counter surface and restrain
the subject. With a single, sweeping downward blow of the
axe, attempt to sever the lower
Identify the person in your group with the poorest
cardiovascular health, and have them eat a couple of Monster
Thickburgers©, drink a pot of strong coffee, chain smoke a
carton of cigarettes, and take a couple doses each of Vioxx
and Fen-Phen. Then have them run wind sprints around the
Tap firmly on the ulnar nerve where it crosses the median
epicondyle of the elbow with your finger or with the rubber
mallet (Fig 7.13). Describe where the tingling sensation is
perceived.
Figure 7.13. Illustration depicting stimulation of
the ulnar nerve at the median epicondyle of the
elbow.
Image from http://www.nsbri.org/
HumanPhysSpace/focus7/ep_metabolism.html