Reflexes: Nervous System | Medical Science Optional Notes for UPSC PDF Download

Reflex Action Overview

• In vertebrates, most sensory neurons do not directly enter the brain but instead synapse in the spinal cord.

• This arrangement facilitates faster reflex actions by activating spinal motor neurons without the delay of routing signals through the brain.

• During a reflex action, the brain receives sensory input, but the analysis of the signal occurs after the reflex is initiated.

• A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus.

• It is an automatic response that does not require conscious thought, serving as a protective mechanism to prevent irreparable harm.

• Examples of reflex actions include withdrawing a hand from a hot object or closing eyelids and constricting pupils in response to a bright light, preventing damage to the eyes.

Reflex Arc

• A reflex arc is a neural pathway responsible for controlling an action reflex.

• In this pathway, most sensory neurons do not directly enter the brain but instead synapse in the spinal cord.

• This design enables rapid reflex actions by activating spinal motor neurons without the delay of routing signals through the brain. However, the brain still receives sensory input during the reflex action.

There are two main types of reflex arcs:

1. Autonomic Reflex Arc: Influencing inner organs.
2. Somatic Reflex Arc: Affecting muscles.

Understanding these reflex arcs provides insight into the rapid and automatic nature of reflex actions, which play a crucial role in the body's response to stimuli.

Monosynaptic and Polysynaptic Reflexes

• In animal reflex arcs, if there is only one sensory neuron and one motor neuron involved, the reflex is classified as monosynaptic.

• The term "monosynaptic" denotes the presence of a single chemical synapse in this type of reflex arc.

• Monosynaptic reflexes involve a direct and rapid communication between the sensory and motor neurons, contributing to a quick response to a stimulus.

Example: Stretch Reflex (Patellar Reflex - DTRs)

1. When the patellar tendon is tapped just below the knee, it activates a specialized structure called a muscle spindle within the quadriceps.
2. An action potential is initiated in the muscle spindle and travels to the L3 and L4 nerve roots of the spinal cord through a sensory axon.
3. The sensory axon releases glutamate onto a motor nerve, leading to motor nerve activity.
4. The outcome is the contraction of the quadriceps muscle, causing extension of the lower leg at the knee, commonly known as the lower leg kicking forward.

Polysynaptic Reflex:

1. In contrast, polysynaptic reflex pathways involve one or more interneurons that connect afferent (sensory) and efferent (motor) signals.
2. There can be multiple synapses (two to hundreds) in a polysynaptic reflex arc.
3. Interneurons play a role in inhibiting the motor neurons of the antagonistic muscle.

Understanding these reflex mechanisms provides insights into the complexity and efficiency of the nervous system's responses to external stimuli.

Reflexes-Examples

Stretch Reflex

• When a skeletal muscle with an intact nerve supply is stretched, it undergoes a contraction, known as the stretch reflex or myotatic reflex.

• The initiating stimulus for this reflex is the stretch of the muscle, and the corresponding response is the contraction of the stretched muscle.

• The sensory organ responsible for sensing muscle stretch is a small encapsulated spindle-like or fusiform-shaped structure called the muscle spindle, situated within the fleshy part of the muscle.

• Impulses generated from the muscle spindle are conveyed to the Central Nervous System (CNS) via fast sensory fibers that directly reach the motor neurons supplying the same muscle.

• The neurotransmitter involved in the central synapse of this reflex is glutamate. This neurotransmitter facilitates the transmission of signals between nerve cells, contributing to the rapid and precise nature of the stretch reflex.

Muscle spindle

• Muscle spindles (groups la and II afferents) are arranged in parallel with extrafusal fibers.
• They detect both static and dynamic changes in muscle length.
• It consist of small, encapsulated intrafusal fibers connected in parallel with large (force generating) extrafusal fibers.

The finer the movement required, the greater the number of muscle spindles in a muscle.

Types of intrafusal fibers in muscle spindles 

• Nuclear bag fibers 
  • detect the rate of change in muscle length (fast, dynamic changes). 
  • are innervated by group la afferents. 
  • have nuclei collected in a central "bag" region. 
• Nuclear chain fibers 
  • detect static changes in muscle length, 
  • are innervated by group II afferents. 
  • are more numerous than nuclear bag fibers, 
  • have nuclei arranged in rows.

Inverse stretch reflex

• It occurs through Golgi tendon organs (GTOs) which are nerve endings located at the junction between tendon and muscle 
• They Respond to tension and force within the muscle

Golgi Tendon Organ

• The body of the Golgi tendon organ consists of collagen strands connected at one end to muscle fibers and merging at the other end into the tendon proper.

• Each Golgi tendon organ is innervated by a single afferent type Ib sensory nerve fiber (Aa fiber), which branches and terminates as spiral endings around the collagen strands.

• The Ib afferent axon is characterized by a large diameter and myelination.

• The neurotendinous spindle is enclosed in a capsule containing several enlarged tendon fasciculi (intrafusal fasciculi).

• One or more nerve fibers perforate the capsule's side, losing their medullary sheaths. The axis-cylinders subdivide and terminate between the tendon fibers in irregular disks or varicosities.

  

Understanding the structure and innervation of the Golgi tendon organ provides insights into its role in monitoring tension in tendons and regulating muscle contraction.

Withdrawal Reflex

• The withdrawal reflex is a typical polysynaptic reflex triggered by a noxious stimulus affecting the skin, subcutaneous tissues, or muscles.

• In response to this stimulus, the reflex induces contraction of flexor muscles and simultaneously inhibits extensor muscles. This leads to the flexion and withdrawal of the body part stimulated away from the source of irritation.

• In cases of a robust stimulus applied to a limb, the response may include not only flexion and withdrawal of that limb but also extension of the opposite limb.

• The withdrawal reflex serves a protective function by moving the stimulated limb away from the irritant, while the extension of the opposite limb provides support to the body.

• The coordinated pattern assumed by all four extremities positions the individual to escape from the offending stimulus, emphasizing the reflex's role in promoting self-preservation.

Reflexes which maintain balance and posture

Basal Ganglia:

• The basal ganglia are associated with various functions, including the control of voluntary motor movements, procedural learning, habit formation, eye movements, cognition, and emotion.

• Lesions in the basal ganglia can result in movement disorders and parkinsonism.

Cerebellum:

• Vestibulocerebellum: Control of balance and eye movement.
• Pontocerebellum: Planning and initiation of movement.
• Spinocerebellum: Synergy, involving the control of rate, force, range, and direction of both extensors and flexors, with the Rubrospinal tract stimulating flexors and inhibiting extensors.

Brain Stem:

• Pontine Reticulospinal Tract: Stimulation has a general predominant effect on extensors.
• Medullary Reticulospinal Tract: Stimulation has a general inhibitory effect on both extensors and flexors, with the predominant effect on extensors.
• Lateral Vestibulospinal Tract: Stimulation causes a powerful stimulation of extensors and inhibition of flexors.
• Tectospinal Tract: Involved in the control of neck muscles.

Understanding these neural pathways provides insight into the coordination and regulation of various motor functions within the central nervous system.

Nerve muscle physiology-Repeats 

Reflexes
Q1: Define reflex. Classify them with examples and their significance in daily life. (2012).
Q2: Describe neuronal pathways and reflexes which maintain balance and posture of human body (2007).
Q3: Describe the reflex arcs of stretch and inverse stretch reflexes.
Q4: What are the differences in the properties of monosynaptic and polysynaptic reflexes (2001)?
Q5: Compare the properties of monosynaptic and polysynaptic reflexes (1999).