Reflex Action and Reflex Arc: Definition, Process, Examples

What Is a Reflex Action?

A reflex action is a quick response to stimuli and does not involve conscious thinking. It is essential for survival since it allows the body to respond almost instantly to possibly harmful situations, such as the withdrawal of a hand from a hot surface. Reflex actions are involuntary actions in which the brain is excluded. However, mediation occurs in the spinal cord.

What Is a Reflex Arc?

A reflex arc is the neural pathway that mediates a reflex action. Generally, it comprises a sensory receptor, a sensory neuron, an interneuron, a motor neuron, and an effector in the form of a muscle or a gland. The reflex arc enables quick response to the stimulus by directly sending the signals from the sensory neurons to the motor neurons through the spinal cord and bypassing the brain.

Types of Reflex Actions

Types of reflex actions are:

Simple (Monosynaptic) Reflexes

• Have only a direct pathway with one synapse.

Example:

• Patellar reflex

• Stretch reflex that helps maintain muscle tone

Complex (Polysynaptic) Reflexes

• Contains many synapses and interneurons.

Example:

• A withdrawal reflex when your hand makes contact with something hot.

• Crossed extensor reflex which helps in maintaining balance.

Mechanism Of Reflex Action

The mechanism of reflex action refers to a series of events occurring as a response to a stimulus resulting in an involuntary action.

Sensory Neurons & Receptors

• Detect stimuli and transmit the signal to the CNS.

• Convert external stimuli into electrical signals.

Receptors detect:

• Mechanoreceptors: They detect changes that are mechanical in nature, for example, touch and pressure.

• Thermoreceptors: They detect temperature changes.

• Nociceptors: They detect painful or harmful stimuli.

Role of Interneurons & Spinal Cord

• Interneurons act as connectors between sensory and motor neurons.

• Process information within the CNS to facilitate complex reflexes.

• Spinal Cord acts as the principal route for reflex actions.

• Processing and responding to stimuli without alerting the brain.

Motor Neurons & Effectors

• Transmit signals from the CNS to effectors- muscles or glands.

• Initiate the response to the stimulus.

• The functions of effectors involve muscles and glands.

• Carrying out the response, for example, muscle contraction or secretion by glands.

• Execute the reflex action to counteract the stimulus.

Reflex Arc – Structure & Pathway

A reflex arc is the simplest neural pathway controlling a reflex action, and a response to stimuli is made very quickly.

Components

• Receptor: detects the stimulus

• Sensory Neuron: transmit the signal to the CNS

• Interneuron: The signal is processed here, only in complex reflexes.

• Motor Neuron: Takes the impulse to the effector.

• Effector: A muscle or a gland performs the response.

Pathway

• The stimulus is detected by the receptor.

• The signal is transmitted to the spinal cord by the sensory neuron.

• The signal is then processed by an interneuron if there is one.

• The motor neuron carries the signal to the effector.

• Effector executes the response.

Types Of Reflex Arcs

Reflex arcs can be classified into monosynaptic and polysynaptic types based on the complexity and number of synapses involved.

Monosynaptic Reflex Arc

• Sensory neuron synapses directly with a motor neuron.

• Simple and rapid response.

• Example: Patellar Reflex

• When the patellar tendon is tapped, the leg extends suddenly.

Polysynaptic Reflex Arc

• More than one synapse with an interneuron.

• Capable of higher processing of stimuli.

• Example: Withdrawal Reflex

• Touching a hot object causes the hand to withdraw quickly.

Importance of Reflex Actions

Reflex actions are important in keeping the body safe from damage and maintaining homeostasis.

Protective Functions

• Immediate Response to Harmful Stimuli

• Reflex actions prevent injury by initiating a quick response.

• Examples: Blinking to protect eyes from foreign bodies.

• The cough reflex clears the airway.

• The gag reflex prevents choking.

Daily Life Functions

• It helps in maintaining posture and balance.

• Examples: Stretch reflexes that mediate muscle tone.

• Reflexes of walking and running.

• Grasp reflexes of adjustments when objects are held in the hand.

Disorders of Reflex Actions

Abnormal reflex action is a sign of neurological disorder and should be brought to the attention of medical personnel.

Hyperreflexia – Overactive reflexes

• Overactive reflex responses.

• Commonly linked to pathologies of the CNS.

Hyporeflexia – Weak reflex response

• A decreased reflex response.

• Can be a manifestation of nerve damage or peripheral neuropathy.

Areflexia – No reflex response

• No reflex responses at all.

• It can result from severe nerve damage or spinal cord injury.

Reflex Action & Reflex Arc NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Types of reflex actions and reflex arcs

• Importance of reflex arcs

Practice Questions for NEET

Q1. Process of response to a peripheral nervous stimulation that occurs involuntary termed as:

1. Symphathetic action

2. Reflex action

3. Parasympathetic action

4. Both a and c

Correct answer: 2) Reflex action

Explanation:

Reflex action is an involuntary and automatic response to a peripheral nervous stimulation, occurring without conscious effort or thought. This process involves a specific part of the central nervous system, ensuring rapid and immediate reactions to stimuli for protection and survival.

Hence, the correct answer is option 2) Reflex action.

Q2. Assertion: Involuntary striated muscles experience fatigue when subjected to repeated stimulation.

Reason: The occurrence of fatigue in these muscles is primarily attributed to the limited availability of ATP molecule

1. Both the assertion and reason are individually true, and the reason is the correct explanation of the assertion.

2. Both the assertion and reason are individually true, but the reason is not the correct explanation of the assertion.

3. The assertion is true, but the reason is false.

4. Both assertion and reason are false.

Correct answer: 4) Both assertion and reason are false.

Explanation:

Involuntary striated muscles, also known as smooth muscles, do not experience fatigue in the same way as skeletal muscles. Smooth muscles have different properties and mechanisms for contraction and energy production compared to skeletal muscles. Fatigue in skeletal muscles is mainly attributed to the depletion of ATP and the accumulation of metabolic byproducts, whereas smooth muscles are more resistant to fatigue due to their slower contraction and reliance on aerobic metabolism. Therefore, the reason provided is incorrect, and the assertion itself is false.

Hence, the correct answer is option 4) Both assertion and reason are false.

Q3. Removal of proximal convoluted tubule from the nephron will result in:

1. No change in quality and quantity of urine

2. No urine formation

3. More diluted urine

4. More concentrated urine

Correct answer: 3) More diluted urine

Explanation:

Removal of PCT from the nephron will result in urine that is further diluted. This is because, through tubular reabsorption, the PCT reabsorbs about 60-70% of the water, besides other important nutrients and ions including glucose, sodium, and bicarbonate, back into the blood vessels from the filtrate. With the PCT removed, most of these solutes would find their way to the urine. This means dilution of the urine and probable loss of useful solutes in the urine.

Hence, the correct answer is option 3) More diluted urine.

Also Read:

• MCQs on Reflex Action and Reflex Arc

• Neuron and Nerves

• Action Potential

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