krit.club logo

Neural Control and Coordination - Reflex action

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Reflex action is an involuntary, nearly instantaneous response to a stimulus that occurs without conscious effort, mediated by the peripheral nervous system (PNSPNS) and spinal cord.

•

The pathway of a reflex action is known as the Reflex Arc, which consists of at least one afferent (sensory) neuron and one efferent (motor) neuron arranged in a series.

•

The Reflex Arc involves five main components: ReceptorReceptor (sensory organ), AfferentAfferent neuronneuron (sensory), CNSCNS (integration center in the spinal cord), EfferentEfferent neuronneuron (motor), and EffectorEffector (muscle or gland).

•

Afferent neurons receive signals from the receptor and transmit the impulse via the dorsal nerve root into the CNSCNS at the level of the spinal cord.

•

Efferent neurons then carry signals from the CNSCNS via the ventral root to the effector organ to produce a response.

•

The knee-jerk reflex is a classic example of a monosynaptic reflex where the sensory neuron synapses directly with the motor neuron.

•

During the transmission of a reflex impulse, the membrane potential changes from a resting state of approximately −70 mV-70\ mV to a peak depolarization of +30 mV+30\ mV as Na+Na^+ ions rush into the axon.

📐Formulae

Reflex Arc Sequence: Receptor→Afferent Neuron→Spinal Cord (Interneuron)→Efferent Neuron→Effector\text{Reflex Arc Sequence: Receptor} \rightarrow \text{Afferent Neuron} \rightarrow \text{Spinal Cord (Interneuron)} \rightarrow \text{Efferent Neuron} \rightarrow \text{Effector}

Vrest≈−70 mVV_{rest} \approx -70\ mV

Vthreshold≈−55 mVV_{threshold} \approx -55\ mV

Total Response Time=Tconduction+Tsynaptic delay+Tneuromuscular delay\text{Total Response Time} = T_{conduction} + T_{synaptic\ delay} + T_{neuromuscular\ delay}

💡Examples

Problem 1:

Explain the pathway of the 'Knee-jerk Reflex' when the patellar tendon is struck.

Solution:

Stimulus (Hammer)→Receptor (Muscle Spindle)→Afferent Pathway→Spinal Cord→Efferent Pathway→Effector (Quadriceps Muscle)→Response (Leg Jerks)Stimulus\ (Hammer) \rightarrow Receptor\ (Muscle\ Spindle) \rightarrow Afferent\ Pathway \rightarrow Spinal\ Cord \rightarrow Efferent\ Pathway \rightarrow Effector\ (Quadriceps\ Muscle) \rightarrow Response\ (Leg\ Jerks)

Explanation:

Striking the patellar tendon stretches the quadriceps muscle. This stretch is sensed by muscle spindles (receptors). The afferent neuron carries the signal to the spinal cord. In the spinal cord, it synapses directly with a motor neuron (monosynaptic). The efferent motor neuron then triggers the quadriceps to contract, causing the leg to kick forward.

Problem 2:

Calculate the state of a neuron if the internal charge moves from −70 mV-70\ mV to +20 mV+20\ mV during a reflex response.

Solution:

ΔV=Vfinal−Vinitial=20 mV−(−70 mV)=+90 mV\Delta V = V_{final} - V_{initial} = 20\ mV - (-70\ mV) = +90\ mV

Explanation:

This change indicates a phase of 'Depolarization'. The influx of Na+Na^+ ions through voltage-gated channels causes the membrane potential to become more positive, reaching the threshold required to propagate the action potential through the reflex arc.

Reflex action - Revision Notes & Key Diagrams | CBSE Class 11 Biology