How the reflex is triggered: stimulus and afferent limb
Any reflex begins with a stimulus. For the depressor reflex, the specific stimulus is a sudden increase in systemic blood pressure. This pressure surge leads to mechanical stretching of the vascular wall.
Sensory nerve endings are located in key reflexogenic zones — the aortic arch and the carotid sinus. These are baroreceptors. When the vessel is stretched, they are instantly stimulated, and a nerve impulse is generated. The higher the blood pressure rises, the greater the frequency of impulsation along the afferent (sensory) fibers.
The primary pathway transmitting this signal "upward" is the depressor nerve (N. depressor), which is a branch of the vagus nerve. Through it, the stream of impulses is rapidly directed to the brain structures for subsequent processing.
Central regulation: function of the vasomotor center
The afferent signal reaches the medulla oblongata, where the vasomotor center (VMC) is located. This center has a complex structure and is functionally divided into zones that operate on a reciprocal (mutually exclusive) basis.
The impulses arriving from the baroreceptors cause activation of the depressor zone of the center. As soon as the depressor region is excited, it immediately exerts an inhibitory effect on the pressor zone. Thus, mechanisms that lower blood pressure are simultaneously turned on, and structures that could raise it are suppressed. This strict balance allows the nervous system to precisely adjust the state of circulation.
Efferent pathway and target organ response
After information processing in the medulla oblongata, a response command is formed and transmitted to the effector organs via the autonomic nervous system. This process proceeds along two parallel pathways:
- Activation of the parasympathetic system. Excitation of the vagus nerve nuclei (N. vagus) occurs, with impulses directed from them to the heart.
- Inhibition of the sympathetic system. Signals maintaining vascular tone and cardiac activity are blocked at the level of the spinal cord and sympathetic ganglia.
As a result, the function of the main target organs changes:
- Heart: under the influence of the vagus nerve, the heart rate and force of contraction decrease (bradycardia develops). Overall cardiac output drops.
- Blood vessels: due to the removal of sympathetic influence, the tone of the smooth musculature of the vascular wall decreases. Vasodilation (expansion of the vessel lumen) occurs.
The logical outcome of decreased cardiac output and dropped vascular resistance is the normalization of systemic blood pressure.
Briefly: answers to topical questions
Through which nerve does afferent impulsation from the carotid sinus travel?
Afferent impulsation from the carotid sinus is carried out via Hering's depressor nerve.
Main conduction pathways:
- Hering's nerve (sinus nerve) — originates directly from the receptors of the carotid sinus.
- Glossopharyngeal nerve (n. glossopharyngeus) — incorporates the fibers of Hering's nerve, via which impulses reach the vasomotor center of the medulla oblongata.
What neurotransmitters are released in the effector synapses of target organs during the depressor reflex?
The following neurotransmitters are indicated in the effector synapses of target organs for components of the autonomic nervous system:
- Acetylcholine — released by the endings of postganglionic parasympathetic fibers. During the depressor reflex, the parasympathetic system is excited; the effect on the heart is cardiac inhibition, decreased heart rate/bradycardia.
- Norepinephrine — neurotransmitter of postganglionic sympathetic fiber endings. During the depressor reflex, the sympathetic nervous system is inhibited; the net effects include decreased vascular tone and lowered blood pressure.
It is easy to remember the reaction of the autonomic nervous system using the "brake and gas" principle: at high blood pressure, the body hits the "brake" (excites the parasympathetic system and depressor center) and releases the "gas" (inhibits the sympathetic system and pressor center).
Class and exam questions
Where are the receptors located that trigger the depressor reflex?
Baroreceptors that respond to increased pressure are localized primarily in the walls of the aortic arch and the carotid sinus region.
Through which nerve does the afferent signal travel to the brain?
Impulses from the vessels are transmitted to the medulla oblongata via the depressor nerve (N. depressor), which is a branch of the vagus nerve.
What does the efferent signal lead to in this reflex?
The efferent signal causes a decrease in heart rate and contractile force (bradycardia), as well as relaxation of the smooth muscle of blood vessels, leading to their dilation.
What else to study on this topic
- Structure and functions of the vasomotor center of the medulla oblongata
- Mechanism of pressure perception by aortic baroreceptors
- The role of vagus nerve nuclei in heart rate regulation
- Interaction of the sympathetic and parasympathetic divisions of the ANS
- Hemodynamic consequences of vasodilation
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Depressor reflex" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in "Vascular Bed Hemodynamics"
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.