reflexes

Objectives of the Lecture

• By the end of this lecture, the student should be able to:

  • Describe and sketch the general plan of a reflex pathway.

  • Give examples of ‘protective’ and ‘regulatory or control’ reflexes.

  • List and describe the defining characteristics of a reflex response.

  • Draw a labelled diagram illustrating:

  • The reflex pathway for the monosynaptic muscle stretch reflex (the tendon jerk).

  • The reflex pathway for the reciprocal stretch reflex.

  • The withdrawal reflex.

  • Outline the functional roles of the stretch reflex and withdrawal reflex.

Motor System

• The motor system is responsible for the:

  • INITIATION, COORDINATION, & CONTROL of skeletal muscle activity.

Components of the Motor System:

• Higher Motor Level

  • Association areas in the cortex (Idea).

• Middle Motor Level

  • Motor cortex, Basal Ganglia, and Cerebellum (Program).

• Lower Motor Level

  • Spinal cord and brainstem nuclei (Performance).

Types of Skeletal Muscle Contraction

1. Voluntary Contraction:

   • Purposeful contraction.

   • Types:

     • Fine (skilled): Performed by distal limb muscles.

     • Gross (slow): Performed by axial & proximal limb muscles.

2. Reflex Contraction:

   • Automatically occurs, e.g., Muscle tone.

   • Aim:

     • To maintain upright posture.

     • To provide postural background for voluntary movements.

Requirements for Voluntary and Reflex Contraction

Voluntary Contraction Requires:

1. Intact Upper Motor Neuron (from higher centers to the spinal cord, OR cranial nerve nuclei).

2. Intact Lower Motor Neuron (from anterior horn of spinal cord to muscle, OR cranial nerve nuclei to muscle).

3. Intact Muscle.

Reflex Contraction Requires:

1. Intact Muscle.

2. Intact Lower Motor Neuron (from anterior horn of spinal cord to muscle, OR from cranial nerve nuclei to muscle).

3. Intact Pathway of a Reflex Arc.

Introduction to Reflexes

• A reflex is a simple response to sensory input.

  • Reflexes engage:

  • Sensory receptor ➔ Afferent limb ➔ Integrating center ➔ Efferent limb ➔ Effector organ.

  • Integration is relatively limited ➔ Reflexes are quite stereotyped (spontaneous).

  • Skeletal muscle reflexes involve pathways through the spinal cord or brainstem (different centers).

Structure of Reflex Arc

1. Stimulus: Initiates the reflex.

2. Receptor: Detects changes.

3. CNS (Central Nervous System): Processes information.

4. Effector: Executes the response.

5. Response: Result of the reflex.

Types of Reflexes

Protective Reflexes:

• Eye blink.

• Gag reflex.

• Limb withdrawal reflex (dating back to Descartes in 1664).

Regulatory Reflexes:

• Vestibulo-ocular reflex: Maintains gaze (steady look).

• Muscle-stretch reflex: Helps maintain limb position.

Spinal Cord Anatomy

• Motor neurons (motoneurons) are located in the ventral horn and are organized according to muscle innervation:

  • Motor neurons controlling distal muscles (e.g., fingers) are positioned laterally.

  • Motor neurons controlling proximal muscles (e.g., shoulder) are positioned medially.

Cytoarchitecture of Spinal Cord Gray Matter

• General mammalian classification includes:

  • Nuclear cell columns (human classification).

  • Notable Nuclei include:

  • Posterior marginalis.

  • Substantia gelatinosa (lamina II).

  • Nucleus proprius of posterior horn.

  • Clarke's column (T1-L3).

  • Intermediomedial cell columns, associated autonomic motor neurons (T1-L2 and S1-S4).

Characteristics of Alpha and Gamma Motor Neurons

1. Alpha motor neurons (AHCs):

   • Supply ordinary contractile skeletal muscle fibers (extrafusal).

   • Control proximal and distal muscles mainly located in cervical and lumbosacral segments of the spinal cord.

   • In the thoracic segments below T1, they only associate with axial muscles.

2. Gamma motor neurons:

   • Supply modified contractile skeletal muscle fibers (intrafusal fibers).

   • Located at the periphery of the muscle spindle.

Intrafusal Muscle Fibers and Sensory Neurons

• Functions of Sensory Neurons:

  • Afferent fibers from muscle spindles:

  • Ia (Aa) fibers from annulospiral endings (proprioception).

  • II (AS) fibers from flower spray endings (proprioception).

  • Various types of fibers include IV (unmyelinated) fibers (pain).

Muscle Spindle

• Specialized encapsulated mechanoreceptor that monitors muscle length.

  • Structure:

  • Capsule attached to the tendon of the muscle.

  • Consists of 3-10 intrafusal fibers parallel to extrafusal fibers.

  • Types of Intrafusal Fibers:

  • Nuclear bag fibers (1-3 fibers per spindle).

  • Nuclear chain fibers (3-9 fibers per spindle).

  • Each intrafusal fiber has a central receptor stretchable part (non-contractile) and a peripheral contractile part.

Mechanism of Stretch Reflex

• Definition: Reflex contraction of a muscle in response to its stretch.

• Importance: It is a negative feedback reflex that returns the muscle to its normal length when stretched.

• Components:

  • Stimulus: Stretch of the muscle.

  • Receptor: Muscle spindle (intrafusal fibers).

  • Afferent: Types Ia and II fibers.

  • Center: Spinal cord, AHCs (only monosynaptic reflex in the body).

  • Efferent: Aα (lower motor neuron = α-motor neuron).

  • Effector: Contractile extrafusal fibers.

  • Response: Contraction of the stretched muscle.

Proprioception and Muscle Control

• Changes in muscle length and tension are detected by:

  • Muscle spindle: Detects stretch.

  • Golgi tendon organ: Detects tension.

• Proprioception information is vital for accurate control of movement.

• Information from muscle proprioceptors enters patterns of neural connections within the spinal cord that organize simple reflex movements, as well as transmitting to the brain for higher-level movement control.

Stretch Reflex Properties

1. It is a spinal deep reflex involving deep receptors in the muscle.

2. It is a monosynaptic reflex.

3. It has a short reflex time (only afferent & efferent, no interneurons).

4. It shows reciprocal innervation (contraction of a muscle accompanied by relaxation of its antagonist, thus referred to as the reciprocal stretch reflex).

5. Its activity is controlled by higher centers changing the activity of gamma (γ) neurons.

6. Inhibited by:

   • Active muscle contraction.

   • Stimulation of the stretch reflex of the antagonist muscle.

   • Stimulation of Golgi tendon organ reflex (inverse stretch reflex).

Functions of Stretch Reflex

1. Basis of muscle tone:

   • Continuous partial submaximal contraction of skeletal muscle during rest (static stretch reflex).

   • Maintains body temperature and organ positioning, and aids venous & lymphatic return.

2. Assists voluntary movements:

   • Alpha-gamma coactivation (load reflex).

   • Allows for muscle contraction with minimal nervous energy and the ability to maintain a constant position despite varying weights.

   • Compensates for muscle weakness or fatigue.

Golgi Tendon Reflex

• Definition: Reflex relaxation of a muscle in response to severe stretch or contraction.

• Importance: Prevents tendon rupture and avulsion during extreme tension; equalizes contraction force among muscle fibers.

• Components:

  • Stimulus: High tension due to severe stretch or contraction.

  • Receptor: Golgi tendon organ.

  • Afferent: Type Ib fibers.

  • Center: Spinal cord, AHCs with inhibitory interneuron.

  • Efferent: Inhibition of Aα (lower motor neuron).

  • Effector: Relaxation of contractile extrafusal fibers.

Reflex Examples

Flexion Withdrawal Reflex:

• A painful stimulus to the lower limb produces:

  • Flexion withdrawal of the stimulated limb.

  • Extension of the contralateral limb (provides body support).

References

• Suggested Readings:

  • The Physiology of motor systems Chapter 9.1 - 9.6 in Human Physiology by Pocock and Richards.

  • Guyton & Hall: Textbook Of Medical Physiology.