reflex

REFLEXES

• Definition: A reflex is an automatic, involuntary, reproducible effector response to a sensory stimulus.
• Reflex Arc:
  • Forms the functional unit of the nervous system.
  • Consists of:
  • Receptor
  • Afferent neuron
  • Centre
  • Efferent neuron
  • Effector
• Bell-Magendie Law:
  • Principle in the spinal cord that states dorsal roots are sensory and ventral roots are motor.

CLASSIFICATION OF REFLEXES

BASED ON THE NUMBER OF SYNAPSES

1. Monosynaptic Reflexes:
   • Only one synapse between afferent and efferent neuron.
   • Example: All stretch reflexes.
2. Bisynaptic Reflex:
   • Inverse stretch reflex; reciprocal innervation to antagonistic muscle.
3. Polysynaptic Reflexes:
   • More than two synapses in the centre.
   • Example: Withdrawal reflexes.

CLINICAL CLASSIFICATION OF REFLEXES

1. Superficial Reflexes:
   • Elicited from the surface of the body.
   • Examples: Abdominal reflex, plantar reflex, etc.
2. Deep Tendon Reflexes (DTR):
   • Examples: Knee jerk, biceps jerk, etc.
3. Visceral Reflexes:
   • Originating from internal organs.
   • Examples: Deglutition reflex, defecation reflex, micturition reflex.

EXAMPLE OF DTR – KNEE JERK

• Tapping on the patellar tendon (quadriceps muscle) with a knee hammer stretches the muscle.
• Response: Extension of knee.
• Centre: L2, L3, and L4.

ANKLE JERK

• Involves S1 and S2 spinal nerves.

BICEPS JERK

• Involves C5 and C6 spinal nerves.

MUSCLE SPINDLE

Definition

• Muscle Spindle: A macroscopic, spindle-shaped structure approximately 4 mm long, widely distributed in muscles.
• Functions as receptors that respond to changes in muscle length and velocity of lengthening.
• Composed of 8-12 muscle fibers enclosed in a connective tissue capsule known as intrafusal fibers (IFF).

Extrafusal Fibers (EFF)

• Regular contractile units of muscle; IFF are in parallel with the rest of the muscle fibers, attached either to the ends of the muscle or the sides of EFF.

INTRAFUSAL FIBERS

1. Nuclear Bag Fibers (NB):
   • Longer; dilated at the center to form a bag-like structure that contains many nuclei.
   • Usually, two nuclear bag fibers are present in a spindle.
2. Nuclear Chain (NC) Fibers:
   • Show a single line of nuclei lying in a chain; usually, four or more NC fibers are seen in a spindle.
   • Their ends are connected to the sides of NB fibers.

INNERVATION OF MUSCLE SPINDLE

Sensory Innervation

• Each muscle spindle has two types of sensory endings:
  1. Primary or Annulospiral Ending:
  • Terminals of rapidly conducting group Ia afferent fibers.
  • They wrap around the center of NC and NB fibers.
  • One branch of Group Ia fiber supplies nuclear bag fiber while another supplies nuclear chain fiber.
  1. Secondary or Flower Spray Ending:
  • Terminations of group II (A beta) fibers.
  • Respond mainly to sustained stretch.

MOTOR SUPPLY

• Motor supply to the muscle spindle comes from the anterior horn cells of the spinal cord by gamma efferent fibers (also called fusimotor fibers).
• Types of Gamma Motor Neurons:
  • Gamma 1 Fibers: Supply ends of dynamic nuclear bag fibers.
  • Gamma 2 Fibers: Supply static ends of nuclear chain fibers.
  • Firing Patterns of Gamma Motor Neurons:
  • Stimulation of dynamic gamma motor neurons increases Ia afferent response during the dynamic phase of muscle stretch.
  • Stimulation of static gamma motor neurons increases type II afferent response during the static phase of muscle stretch.

FUNCTIONS OF MUSCLE SPINDLE

• Receptors sensitive to stretch; when a muscle is stretched, sensory endings distort, generating action potentials in afferent fibers, stimulating alpha motor neurons leading to muscle contraction.
• Mechanism of Muscle Contraction in Response to Muscle Stretch: