Download Sensory Nerves and Receptors

Sensory Nerves and
Receptors
I: To define certain important terms which are related to the sensory
functions of CNS
II: To discuss the properties of the sensory receptors
III: To describe the coding of the sensory information by the CNS
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 2. Sensitizer 3. Transducers 4. Gauges
Accordingly, it can be concluded that without receptors, the CNS
becomes almost useless.
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".
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.
Fig.2-1 The dorsal and ventral horn of SC
SENSORY UNIT:
Is the primary afferent nerve
(sensory) and all its branches.
RECEPTIVE FIELD:
Is the area which is supplied by a
certain sensory unit?
DERMATOME:
A dermatome is the area of skin
which is supplied by a spinal dorsal
root (Fig. 2-2).
The dermatomes of different roots
overlap with each other.
Figure 2 - 2
Arrangements of Dermatomes
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
(B) EXCITABILITY
(THE GENERATOR OR THE RECEPTOR POTENTIAL)
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. 2-3).
Figure 2 - 3: The generator potentials
produced by 4 increasing intensities of
stimuli.
The generator potential is characterized by all the following
characteristics:
1. It does not obey the all or none rule.
2. It is not followed by a refractory period.
3. It has a long duration (more than 5 ms).
4. When it reaches a threshold value, it activates the
afferent nerve to generate an action potential.
5. It is not blocked by local anesthetics
(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.
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
(D) ADAPTATION OF RECEPTORS
Adaptation is defined as the decline in response to a constant
maintained stimulus.
The sensory receptors are classified into three types:
1. Rapidly adapting receptors (phasic receptors), as touch receptors.
2. Moderately adapting receptors as thermoreceptors (20-40°C).
3. Slowly adapting receptors (tonic receptors), as muscle spindles.
The rate of adaptation of each type of receptors fits its function.
MECHANISM OF ADAPTATION OF RECEPTORS
1. Gradual inactivation (closure) of some Na+ channels.
2. Dissipation of the stimulus energy to the surrounding tissues.
3. Decreased sensitivity of the node of Ranvier.
Figure 2 - 4: Rate of adaptation of different
types of receptors.
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
The reservation of a specific sensory pathway to serve a specific type of
stimulus is referred to as "the labeled line principle";
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
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.
THE PHANTOM LIMB PHENOMENON
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 that
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.
(Weber Feschner law)
b. The number of activated receptors. The activation of more receptors
by stronger stimuli is called "recruitment of receptors".