Receptors allow us to detect what is occurring
both outside and inside the body. Without
receptors, we would be unable to respond to changes in the environment, and thus
homeostasis could not be maintained. Some receptors, called exteroreceptors,
respond to stimuli that occur outside the body. Exteroreceptors located in your ears, eyes, and
skin allow you to sense information about the
external environment. For example, photoreceptors within your
retinas allow you to view this picture. There are also receptors within the body,
called enteroreceptors that allow us to detect
internal stimuli. For example, receptors called baroreceptors
within the aortic arch allow the body to detect
changes in blood pressure. Other sets of receptors would allow us to
detect the blood oxygen levels, pH, joint
position, etc. Another method of classifying receptors is
according to their modality, or the type of
sensation that they produce via the nervous system. Receptors can produce a number of
different sensations that may be either
consciously or unconsciously perceived by the CNS. For example, as you
watch this video there are a number of different
modalities that are active. You are seeing the images on the screen, and
hearing my voice narrating the slides. You may be warm or cold based on the
temperature in the room. Here’s a test: Without looking down, can you
tell me what position your feet are in? How do you know this? There are receptors
called proprioceptors which are constantly
monitoring the position and movement of your muscles, tendons, and joints.
Unconsciously, your body may also be
detecting a number of other things. Are your feet getting adequate oxygen supply in
the current sitting position? Your body would
be detecting the partial pressure of oxygen in your bloodstream. Have you eaten recently?
The body would be monitoring your blood
glucose levels. Are you stressed? Your body would be detecting changes in blood
pressure. The amount of information constantly being
produced by the body is staggering. Receptors may also be classified into 6 main
types based on the type of stimulus they
detect: Chemoreceptors-detect chemicals in the body
such as oxygen in the bloodstream, odorants
in the nose, or chemicals in the food you eat Mechanoreceptors-respond to the
deformation, stretching, or bending of cells. For example, the baroreceptors that detect
pressure in the aortic arch are mechanical
receptors that respond to stretching of the arterial wall Nociceptors-pain receptors that respond to
noxious, potentially damaging stimuli.
Nociceptors would be stimulated when physical or chemical damage to the body
occurs. Osmoreceptors-detect the osmotic pressures
within the body. Osmoreceptors in the
hypothalamus are responsible for detecting body fluid osmotic pressure, and assist in the
proper control of hydration in the body. Photoreceptors-are the rods and cones in the
retina of the eye, which respond to light. Thermal Receptors-detect alterations in
temperature and help assist in the regulation
of core body temperature. Now that we know WHAT the receptors are
responding to, let’s discuss HOW the
receptors actually carry out this process. Microscopically, receptors may contain free or
encapsulated dendritic nerve endings, or they
may be separate cells that synapse with sensory neurons.
We learned in the nervous system that
neurons communicate through action potentials, which are all or none signals. The
magnitude of an action potential is always the
same. Neurons communicate intensity through
changing the frequency with which action
potentials are generated. A strong signal = a rapid succession of action
potentials. The dendrites or separate receptor cells,
however, do not produce action potentials. They instead produce what are called
generator potentials. These are graded
changes in the membrane potential of the receptor. For example, let’s consider one of
the mechanoreceptors in the skin responsible
for the sensation of touch. If the skin is touched very lightly, this would
produce a small change in the membrane
potential of the mechanoreceptor. This might occur through a change in the
membrane permeability to a specific type of
ion. If this generator potential is of sufficient
magnitude to reach threshold, an action
potential will fire in the neuron’s axon. If the skin is touched with a greater pressure, a
large change in the membrane potential would
occur. This would result in a higher frequency of
action potentials in the neuronal axon. Once the receptors detect a signal, they must
be able to communicate via action potentials
with downstream control centers and effectors in order to maintain homeostasis.
Thus, the nervous system has three main
functions. A sensory function that detects the changes
that are happening both inside and outside the
body. This function is largely carried out by the
receptors of the body. The sensory receptors convey messages to
the system via sensory or afferent nerve fibers. The messages are transmitted by the use of
action potentials through the neurons. Next, an integrative function analyzes the
incoming information, processes it by
comparing it to previous experiences or to set points, and sends output to effectors. This
process is usually carried out by the central
nervous system in the brain or spinal cord. Motor or efferent fibers carry the signal via
action potentials to effector organs or tissues
that can respond to the original detected signal. The motor function responds to the stimuli with
an action to correct or respond to the original
signal The functions of the nervous system often
occur as a reflex arc. A reflex is an automatic
sequence of events that occurs in response to a stimulus. A great analogy for a reflex arc can
be found in a heating and cooling unit. A sensor within the thermostat is able to detect
the ambient temperature in the room. This
would be analogous to thermal receptors that could be found in the skin. The thermostat then compares the detected
temperature with the set point established by
the user. In the human body, this task would be
accomplished by the hypothalamus, which is
responsible for setting an appropriate range for temperatures. The thermostat then sends this signal to the
HVAC unit, which in this case would turn the air
conditioning on. The air conditioning is thus an effector for the
system. In a human, if the core body
temperature was too high, the nervous system would respond by increasing circulation
through the superficial cutaneous circulation,
and through the formation of perspiration. Another example of a reflex arc found within the
human body is the baroreceptor reflex. As we
saw before, baroreceptors are mechanoreceptors that monitor blood
pressure. These receptors are embedded in
the walls of major blood vessels, primarily in the aortic arch and carotid sinus.
Baroreceptors are embedded in the wall of the
blood vessel, and can be stretched as the blood vessel is exposed to increasing blood
pressures and/or blood volume. Afferent fibers
from these receptors travel to the cardiovascular center. When the
baroreceptors are stretched, they increase
their frequency of action potentials sent to the cardiovascular center in the medulla.
Within the medulla and pons, the
cardiovascular center integrates information from the afferent fibers. There are three main
centers within this region: The cardiac pressor
center, the cardiac depressor center, and the vasoconstrictor center.
Efferent fibers then travel via the autonomic
nervous system, leading to the heart and blood vessels. The cardiovascular system initiates
reflexes by increasing the parasympathetic
drive, and decreasing the sympathetic drive. vessels. The cardiovascular system initiates
reflexes by increasing the parasympathetic
drive, and decreasing the sympathetic drive. The net effect of these actions is to decrease
cardiac output, vasodilate the resistant blood
vessels, and increase urine production. Consequently, the blood pressure will begin to
drop, and the baroreceptors will decrease their
frequency of action potentials. Reflex arcs may be autonomic, such as the
baroreceptor reflex we have just seen, which
involve a division of the nervous system called the autonomic nervous system. Other reflex
arcs, such as the knee jerk reflex a doctor may
conduct during a physical, involve the somatic nervous system.
The Nervous System has many different
divisions, each of which is responsible for carrying out different tasks in the body.
The two main branches of the nervous system
are the central nervous system and the peripheral nervous system. The central nervous system is comprised of
the brain and spinal cord, while the peripheral
nervous system consists of all nervous tissue outside of the brain and spinal cord.
The efferent pathways of the peripheral
nervous system have 3 main divisions. The somatic nervous system innervates the
skeletal muscle within the body and is under
conscious control. This system would control any physical activity
or exercise. The enteric nervous system controls the
motility and secretions of the GI tract.
This control is important for the proper digestion and absorption of nutrients in the
body. The autonomic nervous system is responsible
for the unconscious activities of the body. The
efferent pathways of the autonomic nervous system travel to involuntary effectors such as
smooth muscle, cardiac muscle, and glands. There are two different divisions of the
autonomic nervous system, the sympathetic
nervous system and the parasympathetic nervous system. If you would like to learn
more about the autonomic nervous system, please view my upcoming video entitled “The
Autonomic Nervous System.”