Systems Biology
Reflexes are rapid because they are controlled by neurons without the need for direct instructions from the brain. There are two types of reflex arcs discussed: monosynaptic reflex arcs and polysynaptic reflex arcs. A monosynaptic reflex arc involves only two neurons which include an incoming afferent or sensory neuron and an outgoing efferent or motor neuron. The knee-jerk reflex serves as a common example of a monosynaptic reflex arc.
On the other hand, polysynaptic reflex arcs include at least one interneuron in addition to the sensory and motor components. Interneurons connect sensory and motor neurons to complete a reflex arc while also activating other pathways. A prime example of a polysynaptic reflex arc is the withdrawal reflex, which happens when pulling away from something hot or painful. Although reflexes occur without direct brain involvement, the brain can moderate reflex responses by synapsing with either interneurons or motor neurons.
Lesson Outline
<ul> <li>Introduction to Reflexes</li> <ul> <li>Definition of a reflex</li> <li>Controlled by neurons and don't require direct brain instruction</li> <li>At least two neurons involved: one afferent (sensory) neuron and one efferent (motor) neuron</li> </ul> <li>Skeletal Muscle Reflex Arcs</li> <ul> <li>Basic anatomy: signal passes to the spinal cord, but not to the brain</li> <li>Two types: Monosynaptic and Polysynaptic reflex arcs</li> </ul> <li>Monosynaptic Reflex Arcs</li> <ul> <li>Contains only two neurons: sensory and motor</li> <li>Example: Knee-jerk reflex</li> </ul> <li>Polysynaptic Reflex Arcs</li> <ul> <li>Includes interneurons as middlemen between sensory and motor neurons</li> <li>Example: Withdrawal reflex</li> </ul> <li>Brain's Role in Reflexes</li> <ul> <li>Can moderate reflex responses by synapsing with interneurons or motor neurons</li> </ul> </ul>
Don't stop here!
Get access to 71 more Systems Biology lessons & 8 more full MCAT courses with one subscription!
FAQs
Reflexes are involuntary, rapid, predictable, and automatic responses to certain stimuli that help protect the body from potentially harmful situations. They play a crucial role in maintaining balance, protecting the body from injury, and regulating internal processes such as digestion and blood pressure.
A monosynaptic reflex arc involves a single synapse between a sensory neuron and a motor neuron, such as in the knee-jerk reflex. In contrast, a polysynaptic reflex arc involves multiple synapses that include interneurons between the sensory and motor neurons, as seen in the withdrawal reflex. Polysynaptic reflex arcs generally have slower response times but increased complexity and integration of stimuli.
Sensory neurons, or afferent neurons, transmit information from sensory receptors to the central nervous system (CNS). During a reflex, sensory neurons detect a specific stimulus, such as pain or pressure, and relay the information through reflex arc pathways. Motor neurons, or efferent neurons, carry the response signal from the CNS to the target muscles or glands, causing them to contract or secrete substances. This coordinated effort between sensory and motor neurons enables rapid and efficient reflex responses.
The knee-jerk reflex, or patellar reflex, is an example of a skeletal muscle reflex where the leg extends in response to a tap on the patellar tendon below the kneecap. It is considered a monosynaptic reflex arc because there is only one synapse between the sensory neuron, which detects the tendon stretch, and the motor neuron, which triggers the contraction of the quadriceps muscle, causing the knee to extend. There are no interneurons involved in this response, making it a simple and rapid reflex.
The withdrawal reflex is a protective reflex in response to a painful stimulus, causing the affected body part to move away from the source of the stimulus. The reflex arc in this case is polysynaptic, as it involves the integration of multiple synapses and interneurons within the spinal cord. Sensory neurons detect the painful stimulus and send signals to the spinal cord, where interneurons relay the information to both motor neurons that control flexor muscles (to contract the affected limb muscles) and extensor muscles (to relax the muscles that would otherwise oppose the flexion). This more complex pathway allows for better coordination and control during the withdrawal response.
MCAT® is a registered trademark of the Association of American Medical Colleges. The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). NAPLEX® is a registered trademark of the National Association of Boards of Pharmacy. PANCE© is a registered trademark of the National Commission on Certification of Physician Assistants. NCLEX® is a registered trademark and service mark of the National Council of State Boards of Nursing, Inc. None of the trademark holders are endorsed by nor affiliated with Sketchy or this website.
