SENSORY PHYSIOLOGY & SPINAL REFLEXES
Objectives
After completing this practical you should be able to:
1. Define the terms: sensory receptor, sensory unit, peripheral receptive field,
modality of sensation, sensory adaptation, generator potential, referred pain,
and phantom pain.
2.
Name at least four different categories of sensory receptors.
3. Name and describe the function and location within the skin of the five
primary types of cutaneous receptors.
4. Describe how to utilise a two-point touch threshold test to measure tactile
acuity.
5. Name and describe the conduction velocity and degree of myelination of
the four primary types of sensory afferent nerve fibres.
6. Explain the pattern of change in peripheral sensations which are observed
with ischaemia.
1. CUTANEOUS SENSATION:
Four independent modalities of cutaneous sensation have traditionally been
recognised - warmth, cold, touch, and pain (pressure is excluded because it is
mediated by receptors deep in the dermis, and the sensations of itch and tickle
are usually excluded because of their mysterious origin). Mapping of these
sensations on the surface of the skin has revealed that the receptors are not
generalised throughout the skin but are clustered at different points (have a
punctate distribution).
Since the punctate distribution is different for each of the four sensory
modalities, this suggested that each sensation was mediated by a different
sensory receptor, and this view was supported by the histological identification
of different cutaneous receptors. However, this identification has not been
possible for all modalities. Excision of areas of the skin corresponding to ‘cold’,
‘warm’ and ‘pain’ spots failed to reveal distinct receptors for those modalities;
later work showed that these sensations are mediated by bare nerve endings of
very small diameter, not visible in conventional histological sections.
A. Mapping the Temperature Receptors of the Skin.
Procedure
1. With a ball point pen, draw a square 2 x 2 cm on the ventral surface of the
subject's forearm.
2. With the subject's eyes closed, gently touch an ice-cold metal rod to
different points in the square. Mark the points of cold sensation with a dark
dot.
3. Heat the metal rod to about 45°C in a water bath, wipe the rod, and repeat
the mapping procedure, drawing open circles at the points where heat
sensation is felt (use a different colour or different symbol).
4. Reproduce this map in your laboratory report.
B. Adaptation of Temperature Receptors.
Many of our sense receptors respond strongly to acute changes in our
environment and then cease responding when these stimuli become constant.
This phenomenon is known as sensory adaptation. Our sense of smell, for
example, quickly adapts to the odours of the laboratory, and our touch receptors
soon cease to inform us of our clothing, until these stimuli change.
When one hand is placed in warm water and another in cold water, the
perceived strength of the stimulus gradually diminishes until both types of
temperature receptors have adapted to their new environment. If the two hands
are then placed in water at an intermediate temperature, the hand that was in the
cold water will feel warm, and the hand that was in the warm water will feel
cold. The "baseline," or "zero" of the receptors has obviously changed. The
sensations of temperature are therefore not absolute but relative to the baseline
previously established by sensory adaptation.
Procedure
1. Place one hand in warm water (about 40°C) and the other in cold water ~
12°C and leave them in the water for about 3 minutes.
2. Now place both hands in lukewarm water (about 22°C), and record your
sensations below
C. The Two-Point Threshold in Touch Perception.
The density of touch receptors in some parts of the body is greater than in other
parts; therefore, the areas of the sensory cortex (postcentral gyrus of the central
fissure) that correspond to different regions of the body are of different sizes.
Those areas of the body that have the largest density of touch receptors also
receive the greatest motor innervation; the areas of the motor cortex
(precentral gyrus) that serve these regions are correspondingly larger than other
areas. Therefore, a map of the sensory and motor areas of the brain reveals that
large areas are devoted to the touch perception and motor activity of the face
(particularly the tongue and lips) and hands, whereas relatively small areas are
devoted to the trunk, hips, and legs (Fig. 1).
The density of touch receptors is measured by the two-point threshold test (Fig.
2). The two points of a pair of adjustable calipers are simultaneously placed on
the subject's skin with equal pressure, and the subject is asked if two separate
contacts are felt. If so, the points of the divider are brought closer together, and
the test is repeated until only one point of contact is felt. The minimum distance
at which two points of contact can be felt is the two-point threshold.
Figure 1: Motor and sensory areas of the cerebral cortex. (a) Motor areas that
control skeletal muscles and (b) sensory areas that receive somatesthetic
sensations.
Figure 2: The two-point touch threshold test. If each point touches the receptive
fields of different sensory neurones, with one neurone in between, two separate
points of touch will be felt.
Procedure
1. Beginning with the calipers wide apart and the subject's eyes closed,
determine the two-point threshold on the back of the hand. (Randomly alternate
the two-point touch with one-point contacts, so that the subject will not try to
second guess the examiner.)
2. Repeat this procedure with the palm of the hand, fingertip, and back of the
neck.
3. Describe your findings below.
Clinical Significance:
Sensory information from the cutaneous receptors projects to the
postcentral gyrus of the cerebral cortex. Direct electrical stimulation of
this area therefore produces sensations similar to those felt when the
cutaneous receptors are stimulated. Much of this information has been
obtained by electrically stimulating the brain in patients that remain awake
whilst undergoing brain surgery; the surgeon must often map the areas of
the brain in order to locate the site of the lesion and avoid damage to
healthy tissue. Since the cutaneous receptors are more densely arranged in
the face, tongue, and hands than on the back and thighs, larger areas of the
brain are involved in analysing information from the former areas than
from the latter. Consequently, the areas of the brain map representing the
face, tongue, and hands are larger than those representing the back and
thighs. The map is also upside down, with the feet represented near the
superior surface and the head represented more inferiorly and laterally in
the cortex.
2. PHANTOM PAIN AND REFERRED PAIN:
Receptor organs are sensory transducers, changing environmental stimuli into
afferent nerve impulses. Since the action potentials in one nerve are the same as
another, the perception of the sensation is determined entirely by the area of the
brain stimulated, which is different for each sensory nerve. Although a given
sensory nerve is normally stimulated by a specific receptor, trauma to the nerve
proximal to the receptor may also evoke action potentials, and this will be
interpreted by the brain as the normal sensation (e.g., seeing flashes of light or
‘stars’ when punched in the eye).
A. Phantom pain:
Amputees frequently report feelings of pain in their missing limbs (this is
known as the phantom limb phenomenon). The nerve impulses arise at the cut
ends of the nerve fibres, but the pain is perceived as coming from the amputated
region of the body that normally produces action potentials along these nerves.
The following experiment demonstrates how we perceive sensations that arise
from impulses generated along the course of a nerve.
Procedure
1. Gently tap the ulnar nerve where it crosses the median epicondyle of the
elbow with a reflex hammer.
2. Describe the locations where sensation is felt.
B. Referred pain:
Referred pains are important clinically, particularly for deep visceral pain,
which is characteristically dull and poorly localised. Here, pain initiated by
impulses arising in internal organs is perceived as arising from an area of the
body surface, because visceral and somatic nociceptive afferents converge on
the same ascending neurone. For this reason, the deep visceral pain is referred
to a surface location served by nerves from the same segmental level of the
spinal cord (see Fig. 3). In ischaemic heart disease, for example, the pain is
referred to the left pectoral region and shoulder areas; this is called angina
pectoris. In many patients with stomach ulcers, the pain is referred to the
region between the scapulae of the back.
Figure 3: Referred pain. Convergence of visceral and somatic afferents onto
ascending pathways.
3. DEEP PAIN:
Arrest the circulation in one arm of the subject by rapidly inflating a cuff on the
upper arm to 200 mm Hg. The subject now momentarily but tightly grips a
tennis ball in each hand; the gripping movements, which should all be of equal
strength, are repeated once per second. After about 30 seconds pain will be felt
in the forearm, and perhaps in the hand, with the circulation arrested. It will
probably increase in intensity and become severe in about 60 seconds. The
exercise is then stopped, but the cuff is kept inflated. The pain continues
practically unchanged. On sudden release of the cuff the pain vanishes within a
second or two. Usually no pain is felt in the arm exercising with a free
circulation.
4. EFFECTS OF ISCHAEMIA ON PERIPHERAL SENSATIONS:
The subject holds his bared forearm above head level to drain the veins while a
sphygmomanometer cuff is rapidly inflated to 200 mm Hg. It must be
maintained at this pressure throughout the experiment. The subject then rests
his forearm comfortably on the bench and closes his eyes while the
experimenter repeatedly tests sensory function as follows, in the hand and
forearm.
Light touch sensation - touch lightly with a wisp of cotton wool; the subject
says 'yes' whenever he feels the touch. Vary the rhythm and site of testing
irregularly.
Superficial pain sensation – jab the arm and/or forearm lightly with either the
head or point of a pin; the subject reports 'head' or 'point'.
Temperature sensation - touch the skin with a boiling-tube containing water at
45°C or 10°C. The subject reports 'hot' or 'cold' as the case may be.
Position sensation - the experimenter waggles the subject's thumb or forefinger
for a second or two, finally holding it in either a flexed or extended position.
The subject reports the position.
Keep the arm cuff inflated for between 30 and 35 min. (there is an ample safety
margin even after this time period). The most interesting effects are
experienced only after 25-30 min of ischaemia, so do not discontinue it too
soon. If the superficial veins become uncomfortably distended the brachial
artery cannot have been completely occluded, and the experiment should be
discontinued.
Before release of the occluding pressure note wrist-drop and inability to
dorsiflex the wrist. On releasing the cuff, note the general subjective similarity
between 'pins and needles' (here an after-effect of the ischaemia, resulting from
impulses generated at the point where the cuff was pressing on the nerve trunk),
the sensation when the ulnar nerve is accidentally knocked where it passes
round the medial epicondyle, and the sensation of an electric shock. This type
of sensation is caused by the simultaneous stimulation of fibres which normally
mediate different sense modalities, so that no 'meaningful pattern' of sensory
inflow is formed.
5. CONDUCTION VELOCITY OF HUMAN MOTOR NERVE:
Figure 4: Representative screen cap of two compound action potentials traces
generated by stimulating the ulnar nerve at the wrist (left hand trace) and
elbow.
If the ulnar nerve is stimulated at various points along its course, the latency of
the response of the muscle which abducts the little finger (abductor digiti
minimi) can be recorded. A plot of latency against the length of the conduction
path will give a straight line plot, the slope of which is the conduction velocity.
It is useful to record the response evoked by stimulation at elbow and wrist. By
measuring the distance between points of stimulation and the time interval
between the responses an average conduction velocity can be found (Fig. 4).
Figure 5: Diagram illustrating the attachment points on the arm for the
stimulating, recording and earth electrodes when measuring conduction
velocity of the ulnar nerve.
Procedure:
1) Attach the electrodes wires according to the diagram in Fig. 5 above.
2) The bar stimulus electrode is placed over the ulnar nerve, increase the
stimulus voltage until a response occurs. You will probably have to explore
around with the electrode before locating the optimum position. You will feel
some sensation as a result of stimulation of sensory nerve fibres, but provided a
good contact is maintained by the stimulating electrode there should not be any
discomfort.
When the latency of the response has been measured by stimulation at elbow
and wrist, try the effect of changing the stimulation frequency. What is the
frequency at which tetanus occurs?
Clinical Significance:
Nerve conduction velocity measurement, using the method shown here, is a
commonly used diagnostic tool where demyelinating neuropathy is
suspected.
Breakdown of myelination along axons results in a slowing of action
potential conduction along it such that axons within a nerve begin to
conduct at slightly different velocities. Thus, the nerve subsequently loses
its normal synchrony of conduction in response to a single stimulus. For
example functions that depend upon the arrival of synchronous bursts of
neural activity, such as tendon reflexes and vibratory sensation are lost
soon after the onset of a chronic neuropathy.
6. REACTION TIMES:
Figure 6: Representative screen cap of trace for visual and auditory reaction
time experiments
This is the time elapsing between the reception of a sensory stimulus and the
acting upon it.
a) Visual Reaction Time
In this experiment, the computer timebase is triggered to sweep by the
experimenter; the time taken for the subject to respond visually is measured.
Procedure
1) On the computer screen, click Start in Sample panel.
2) The experimenter pushes the (hand-held) Start button to initiate a sweep; as
soon as the subject sees the sweep beginning, he pushes the (hand-held) Stop
button and so deflects the sweep. The time from the origin of the sweep to the
deflection is the subject's visual reaction time and can be measured from the
timebase at the bottom of the screen (Fig. 6).
b) Auditory Reaction Time
In this experiment, the computer timebase is again triggered to sweep by the
experimenter; the time taken for the subject to respond aurally is measured.
Procedure
The subject has his back to the computer screen and wears the earphones
supplied. Click Start on the computer screen. When the experimenter pushes
the (hand-held) Start button, the subject hears a click and the sweep starts; the
subject pushes the (hand-held) Stop button to deflect the sweep. This allows
measurement of the reaction time as before.
NOTE: The experimenter should not allow the subject to see the Start
button, and before the experimenter pushes the button he calls 'ready' to the
subject. At least six observations of both visual and auditory reaction times
should be made; the experimenter should vary randomly the interval between
calling 'ready' and pushing the start button.
In order to give these figures a meaning for everyday affairs, take the shortest
time for the visual reaction and calculate the distance in metres travelled in that
time by a motor car driven at 50 k.p.h. (13.9 m/sec). This will give the distance
traversed before you could begin to apply the brake after receiving a warning
('red') light signal when travelling at 50 k.p.h. Note that this is only part of the
distance required to bring the car to a standstill. A car travelling at 50 k.p.h. can
be stopped in about 10 metres from the moment of application of the brake,
assuming 100% braking efficiency with optimum coefficient of friction between
the tyres and the road. In a real situation the car driver cannot equal the low
reaction times of the laboratory because he is not prepared by a "ready" signal.
The reaction time is more likely to be about a second.
Now calculate what this means in terms of distance (metres)
Note also that older people (with slower reaction times) are often safer drivers
than younger people, who may depend unduly on their fast responses to
dangerous situations. This is explained by the better judgement ('road sense') of
the older people who anticipate a possible dangerous situation and avoid the
need for emergency measures ('defensive driving'). Most of our ordinary
reactions are not merely responses to simple sensory stimuli, but to a
complicated pattern of stimuli which must be integrated before an adequate
response is possible. An intelligence test may be said to measure the speed and
accuracy of such integrative responses.
In a real driving situation, attention may be distracted by other stimuli, thus
increasing reaction time. To measure the magnitude of this effect, repeat the
visual reaction time test above, but during a mobile phone conversation about a
topic that requires some concentration.
Conduction Velocity
(metres/sec)
Reaction Time
Visual
(ms)
Auditory
(ms)
Visual
In terms of
distance travelled
(metres)