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Physiology
Dr. Basim Mohammed Alwan Lecture (3)
SENSORY NERVES
Sensory nerves
(afferent nerves)
are the nerves which convey
the sensory information from different parts of the body to the
central nervous system. They make the first order of neurons in
the nervous pathways of all sensations.
All the spinal sensory nerves enter the spinal cord through the
dorsal roots (sensory roots) of the spinal nerves. The cell bodies
of these nerves are in the dorsal root ganglia of the spinal
nerves. As these nerves carry impulses towards the cell bodies
they are considered as long dendrites of the ganglion cells.
BELL-MAGENDI LAW
: It states that the dorsal roots of the
spinal nerves are sensory and the ventral roots are motor".
SENSORY UNIT:
Is the
primary afferent nerve (sensory) and all its branches.
RECEPTIVE FIELD:
Is the area which is supplied by a certain unit?
DERMATOME
A dermatome is the area of skin which is supplied by a spinal
dorsal root i.e. it is the cutaneous receptive field of a spinal dorsal
root (fig. 3-1). The dermatomes of different roots overlap with
each other.
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Figure 3 - 1: Arrangements of Dermatomes
THE DORSAL HORN OF THE SPINAL GRAY MATTER
The
dorsal horn of the spinal gray matter is the site of relay of
most of the afferent sensory fibers. A cross section in the spinal
cord shows several anatomical laminae of the gray matter. Each
type of sensory fibers relays in certain specific laminae as shown
in fig. 3-2.
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Fig.3-2 the dorsal and ventral horn of SC
THE COURSE OF THE AFFERENT SPINAL NERVE AFTER
ENTERING THE SPINAL CORD
As the afferent spinal nerve fiber enters the spinal cord, it takes
one of the following courses:
1. Ascends or descends for few segments at the tip of the dorsal
horn forming the Lissaur's tract before entering into the dorsal
horn.
2. Enter the dorsal horn to relay on neurons in different
laminae of the dorsal horn. These neurons are either
interneuron's (intermediate neurons) or second order neurons of
their sensory pathway.
3. Proceeds in the spinal gray matter to relay on motor neurons
in the ventral horn (the reflex arc of the stretch reflex).
4. Ascends without relay in the dorsal column of the spinal
white matter forming the dorsal column tracts (gracile's and
cuneate's tracts) terminate in the dorsal column nuclei (gracile's
and cuneate's nuclei) in the medulla oblongata.
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SENSORY RECEPTORS
A sensory receptor is a specialized nerve ending which is
sensitive to a specific type of stimulus and produces a
specific type of sensation.
FUNCTIONS OF RECEPTORS
1. Detectors of the type of the stimulus. Each receptor is
specialized to detect a certain type of stimulus, e.g. touch, heat,
pain.
2. Sensitizers, which lower the threshold of stimulation of nerve
endings.
3. Transducers, which convert the energy of the stimulus into
an electric response, i.e. a membrane potential which generates an
action potential in the afferent nerve.
4. Gauges, which measure the intensity of the stimulus.
Accordingly, it can be concluded that without receptors, the
CNS becomes almost useless.
PROPERTIES OF THE SENSORY RECEPTORS
(A) SPECIFICITY
Each receptor is highly sensitive to a certain type of stimulus
which is called the "adequate stimulus" for this receptor. When
the adequate stimulus is used, the receptor is stimulated by the
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least amount of energy. If the receptor is stimulated by a
stimulus other than its adequate stimulus, it needs high energy and
still gives its specific type of sensation; e.g. light receptors in the
eye could be stimulated by a strong mechanical blow on the eye,
these results in seeing flashes and stars.
(B) EXCITABILITY
(THE GENERATOR OR THE RECEPTOR POTENTIAL)
When a receptor is stimulated, it increases the permeability of
the membrane of the nerve ending to Na
+
so Na
+
influx and
membrane depolarization is produced. The depolarization of the
membrane of the sensory nerve ending on stimulation of the
receptor is called the "generator potential" or the "receptor
potential" (fig. 3-3). The generator potential is characterized
by:
Figure 3 - 3: The generator potentials produced by 4 increasing intensities of
stimuli (1, 2, 3, 4). When the generator potential reaches the firing level, a full
action potential is produced in the afferent nerve.
The generator potential is characterized by:
1. It does not obey the all or none rule. Its magnitude
increases proportionately with the intensity of the stimulus.
2. It is not followed by a refractory period.
3. It has a long duration (more than 5 ms). So, it can be
temporally summated.
4. When it reaches a threshold value, it activates the first node of
Ranvier of the afferent nerve to generate an action potential in
the afferent nerve.
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Figure 3 - 3: The relationship between the magnitude of the generator
potential and the frequency of discharge of impulses in its afferent nerve.
5. It is not blocked by local anesthetics. Local anesthetics
prevent the development of the action potential at the first node
of Ranvier but do not prevent the development of the generator
potential.
When the generator potential exceeds the threshold value, the
frequency of discharge of impulses in the sensory nerve
becomes directly proportionate with the amplitude of the
generator potential (fig. 3-3).
(C) DISCHARGE OF IMPULSE
The response of a receptor to a change in the intensity of the
stimulus is by changing the "frequency" of impulses generated
in the afferent nerve. That is why the response of receptors: to a
change in stimulus intensity is called a "frequency modulated
response" or "FM response".
Weber-Fechner law states that "The frequency of discharge of
impulses from a receptor is directly proportionate with the log
intensity of stimulus.
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Figure 3 - 4: The compression function of receptors
.
For example, a hundred-fold increase in the intensity of the
stimulus leads to only twofold increase in the frequency of
discharge of impulses from the receptor. The modulation of large
changes in the intensity of the stimulus to small changes in the
frequency of impulses is referred to as "the compression
function" of the receptor (fig.3-5). This function enables the
receptor to respond to, and discriminate, a wide range of
stimulus intensity, e.g. sound receptors can inform the CNS of
sound intensity range of one to ten billion-folds.
(D) ADAPTATION OF RECEPTORS
Adaptation is the decline in response to a constant maintained
stimulus.
If a constant maintained stimulus is applied to a receptor, the
frequency of discharge of impulses in its afferent nerve declines
with time. The rate of decline depends on the type of receptor.
The sensory receptors are classified according to their rate of
adaptation, into three types:
1. Rapidly adapting receptors (phasic receptors), e.g. touch and hair
touch receptors.
2. Moderately adapting receptors e.g. thermoreceptors in the
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temperature rang ebetween 20-40°C.
3. Slowly adapting receptors (tonic receptors), e.g. muscle
spindles and Golgi tendon organs.
The rate of adaptation of each type of receptors fits its function.
E.g. touch receptors adapt rapidly, so after putting clothes on; it
would be irritating to feel the touch of clothes all the time. So,
touch receptors adapt rapidly and stop discharging. On the other
hand, muscle spindles send continuous proprioceptive signals to
the CNS which maintain body posture and equilibrium which
are needed all the time, so they adapt very slowly.
Figure 3 - 5: rate of adaptation of different types of receptors.
MECHANISM OF ADAPTATION OF RECEPTORS
The following factors contribute to the process of adaptation of
receptors:
1. Gradual inactivation (closure) of some Na
+
channels in the
membrane of the nerve ending so membrane depolarization
becomes difficult.
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2. Dissipation of some of the stimulus energy to the surrounding
tissues i.e. redistribution of the energy of the stimulus.
3. Decreased sensitivity of the node of Ranvier.
CLASSIFICATION OF RECEPTORS ACCORDING TO THEIR
ADEQUATE STIMULUS
According to the type of their adequate stimulus, receptors are
classified into 6 main categories as shown in fig. 3-8.
Figure 3-8: The types of receptors according to their adequate stimuli.
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CODING OF SENSORY INFORMATION
All stimuli are transduced by the sensory receptors into nerve
impulses in the afferent nerves. These impulses reach the brain
acting as code signals specific for each sensation. The brain then
deciphers these code signals and identifies the modality (type),
the locality and the intensity of the stimulus.
1. CODING OF THE MODALITY OF THE STIMULUS
Each sensory pathway from the receptor up to the final sensory
neuron in the brain is specialized to serve a specific sensory
modality. Stimulation of any point along the course of any
sensory pathway produces its specific sensation. The reservation
of a specific sensory pathway to serve a specific type of
stimulus is referred to as "the labeled line principle"; i.e., the
whole line from the receptor to the final sensory neuron in the
brain is "labeled" for conducting impulses of a specific
sensation.
So, the coding of a stimulus modality is by sending the
impulses through its specific sensory pathway (i.e. its labeled
line).
LABELED LINE PRINCIPLE
(MULLER'S LAW)
This law states that "stimulation of any point along the course
of a sensory pathway produces the specific sensation served by
this pathway regardless of the type of the stimulus used",
2. CODING OF THE LOCALITY OF THE STIMULUS
Each locality sends sensory impulses to the brain via a specific
sensory pathway. Stimulation of any point along the course of
this pathway produces a sensation felt at this specific locality.
So, the locality of the stimulus is also coded by the specific
sensory pathway which serves this locality. This is called the
"law of projection". It explains the phantom limb phenomenon
which occurs in amputees.
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THE PHANTOM LIMB PHENOMENON
This is the false sensation from a limb when the limb does not
really exist. It occurs in amputees who complain of pain, touch,
itching or pressure sensation felt in the absent limb. This false
sensation is due to irritation of the cut ends of the afferent nerves of
the amputated limb which send impulses up to the brain. The brain
projects the sensation on to the absent limb as if it is exist.
3. CODING OF THE INTENSITY OF THE STIMULUS
The intensity of the stimulus is coded by two factors:
a.
The frequency of discharge from the sensory receptor. The
frequency is proportionate with the log intensity of the stimulus
(Weber Feschner law)
b.
The number of activated receptors. A stronger stimulus
activates more receptors. The activation of more receptors by
stronger stimuli is called "recruitment of receptors".