Sensory systems summary
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Lecture 11 Notes: Sensory Systems
There are sensory receptors for many different modalities. There are receptors that sense,
for example, chemical stimuli (chemoreceptors), mechanical movement
(mechanoreceptors), light (photoreceptors), temperature (thermal receptors), noxious
substances (nocioceptors), etc…
In general, a receptor protein on a sensory (receptor) cell detects the stimulus. This
usually causes the opening of ion channels either alone or in conjunction with activation
of second messenger systems. Ultimately there is a change in membrane potential that
leads a signal being sent to an integrating centre in the brain. The sensory receptor itself
may send the signal to the brain (i.e., sensory neurons) or the sensory cell may release
neurotransmitters onto another neuron (interneuron) that conveys the signal to the brain
(i.e., epithelial sensory receptor cells). One important integrating centre is the nucleus of
the solitary tract (nucleus tractus solitarius; NTS) located in the brainstem at the junction
between the medulla oblongata and the spinal cord.
Tonic and Phasic Receptors
Tonic receptors are those that fire action potentials for the duration of the time that the
receptor cell is stimulated. However, if the stimulus lasts a very long time, the receptors
may become desensitized or down-regulated resulting in a reduction (or cessation) of
action potential activity.
Phasic receptors are those which fire APs either at the beginning of a stimulus, at the end
of a stimulus or at both the beginning and end of a stimulus.
Pulmonary Stretch Receptors
Pulmonary (lung) stretch receptors (PSR) are located in the walls of the lungs. They
respond to the stretching of the lungs during inspiration. There are three types of
pulmonary stretch receptors.
Rapidly-adapting pulmonary stretch receptors fire APs immediately when the lungs begin
to inflate and then cease firing.
Slowly-adapting PSR begin to fire action potentials at around the time when the lung
inflation reaches its maximum. They fire until the lung volume returns to normal.
Mixed rapidly-adapting and slowly-adapting PSR exhibit a rapid and a slow component.
There is activity immediately upon lung inflation (rapid firing of APs) followed by a
second phase of APs that are slower than the initial phase. These mixed PSR regulate the
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Hering-Breuer Inspiratory Reflex (also called the Hering-Breuer Inspiratory off-switch).
This reflex is as follows. As the lungs inflate, the mixed PSR begin to fire. They signal to
the lungs that the lungs are inflated and it is now time to stop inspiring.
During a breath hold there is constant PSR stimulation. The constant PSR activity
reduces the drive to breathe. However, there reaches a point, during a breath hold, when
blood O2 is lowered and blood CO2 is raised to a point that these chemical stimuli override
the PSR and cause breathing to commence again. During the breath hold blood O2 has
gone down as O2 is used by the tissues and blood CO2 has gone up as tissues dump
metabolically produced CO2 into the blood.
In this case it is the CO2 levels that are critical rather than blood O2 levels. Chemical
regulation of breathing in air-breathing vertebrates is driven by changes in CO2 not O2.
When the blood CO2 level reaches a critical threshold during a breath hold, the breath
hold is terminated and breathing commences.
At any time, if blood CO2 falls below a certain threshold there will be no breathing. It
doesn’t matter how low blood O2 falls if CO2 levels are too low to trigger breathing. We
will not breathe to obtain O2 if blood CO2 is below a certain level. Air-breathing is
controlled by the need to get rid of CO2 not by the need to obtain O2.
Central pH/CO2 Chemoreceptors
The text above talks about blood CO2 levels. However, levels of CO2 in the blood are
actually sensed in the cerebral spinal fluid (CSF) surrounding the brain as CO2 diffuses
from the blood into the CSF. CO2 is sensed by central (brain) chemoreceptors that are
located in many regions of the brain but are heavily concentrated on the ventral surface of
the medulla oblongata.
These receptors are called pH/CO2 sensitive chemoreceptors due to the relationship
between CO2 and pH. As CO2 levels rise, pH decreases due to the CO2 hydration reaction
in which CO2 reacts with H2O to form HCO3- and an H+ ion. It is generally considered to
be dogma that CO2 can cross the blood brain barrier but H+ ions cannot.
A putative mechanism for CO2 sensing is illustrated in the Power Point slides. It is as
follows. CO2 diffuses from the blood, into the CSF and then into the chemoreceptor cell.
There it is hydrated, under carbonic anhydrase (CA) catalysis to form a proton and a
bicarbonate ion. The proton is then removed from the cell is exchange for a sodium ion.
The sodium ion is then removed from the cell in exchange for calcium. Therefore,
intracellular calcium levels rise and cause the chemoreceptor cell to release
neurotransmitters onto other neurons that project to the respiratory control centres. This
will cause an increase in breathing.
Thermal Regulation and Thermal Chemoreceptors
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