Reflexes and reflex Arcs

Schematic for reflex Arc

Receptor, Sensory neuron, CNS, Motor neuron, Effector

Monosynaptic reflex

Contains 2 neurons, sensory and motor

Has only 1 synapse in spinal cord
Helps maintain an upright posture

Withdrawal Reflex

• Occurs when person touches or steps on something painful (stove, tack)

• Prevents or limits tissue damage, by removing limb from painful stimulus

• Polysynaptic: contains sensory neuron, interneuron, motor neuron

• Reciprocal innervation: flexors contract, extensors are inhibited

Crossed Extensor Reflex

• During withdrawal reflex, flexors on affected (ipsilateral) side contract, and extensors are inhibited

• At same time, extensors on opposite (contralateral) side contract, flexors are inhibited

• Also shifts body weight, so person remains upright

Ascending:
Fasciculus gracilis and fasciculus cuneatus

Sensation of touch, pressure and body movement

Ascending: Spinothalamic tracts

Sensations of pain, temperature, touch and pressure

Ascending: Spinocerebellar tracts

Coordination of muscle movement

Descending: Corticospinal tracts

Controls voluntary movement

Descending: Reticulospinal tracts

Controls sweat glands and muscle tone

Descending: Rubrospinal tract

Coordinates muscle actions

Ascending tracts

conduct sensory impulses to the brain

Descending tracts

conduct motor impulses from the brain via motor neurons reaching muscles and glands

Uses of Reflexes

• Reflexes are used to assess condition of nervous system.

• Routinely performed in neurological testing.

• In patients with nervous system injury, location and extent of damage can be judged by testing reflexes.

• Effectiveness of anesthetic during surgery can be assessed through reflex testing.

Examples of reflexes

• Knee-jerk reflex

• Plantar reflex

• Biceps-jerk reflex

• Triceps-jerk reflex

• Abdominal reflex

• Ankle-jerk reflex

• Cremasteric reflex

Reflex

• Automatic, subconscious response to a stimulus within or outside the body

• Maintain homeostasis by controlling involuntary processes, such as heart rate, blood pressure, swallowing, coughing

Reflex arc

• Neural pathway, consisting of a sensory receptor, 2 or more neurons, and an effector

• Simple reflex arc contains only sensory and motor neurons

• Most common reflex arc contains sensory neuron, interneurons, motor neurons