2. What is reflex Arc?
• A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory
neurons do not pass directly into the brain, but synapse in the spinal cord.
• A neural pathway that controls an action reflex. In higher animals, most sensory
neurons do not pass directly into the brain, but synapse in the spinal cord. This
characteristic allows reflex actions to occur relatively quickly by activating spinal
motor neurons without the delay of routing signals through the brain, although the
brain will receive sensory input while the reflex action occurs. There are two types
of reflex arcs: autonomic reflex arc and somatic reflex arc.
3. Parts of Reflex Arc
• 1. Receptor – detects the stimulus.
a) Description: the receptor end of a particular dendrite or a specialized receptor cell in
a sensory organ.
b) function: sensitive to a specific type of internal or external change.
• 2. sensory neuron – conveys the sensory info. to brain or spinal
• cord.
a. Description: Dendrite, cell body, and axon of a sensory neuron.
b. Function: transmit nerve impulses from the receptor into the brain or spinal cord.
4. • 3. Interneuron: relay neurons.
a. Description: dendrite, cell body, and axon of a neuron within the brain or spinal
cord.
b. function: serves as processing center, conducts nerve impulses from the sensory
neuron to a motor neuron.
• 4. Motor neuron: conduct motor output to the periphery.
a. Description: Dendrite, cell body, and axon of a motor neuron.
b. function: transmits nerve impulse from the brain or spinal cord out to an effecter.
5. • 5. Effecter:
a. Description: a muscle or gland.
b. function: Response to stimulation by the motor neuron and
• produces the reflex or behavioral action.
6. • alpha motor neuron: These are large, lower motor neurons of the
brainstem and spinal cord. They innervate the extrafusal muscle fibers of
skeletal muscle and are directly responsible for initiating their contraction.
Alpha motor neurons are distinct from gamma motor neurons that innervate
the intrafusal muscle fibers of muscle spindles.
7.
8. Spinal reflexes include the stretch reflex, the Golgi
tendon reflex, the crossed extensor reflex, and the
withdrawal reflex.
• Stretch Reflex : The stretch reflex is
a muscle contraction in response to
stretching within the muscle. This
reflex has the shortest latency of all
spinal reflexes. It is a monosynaptic
reflex that provides automatic
regulation of skeletal muscle length.
9. • Golgi Tendon Reflex : The Golgi
tendon reflex is a normal component
of the reflex arc of the peripheral
nervous system. The tendon reflex
operates as a feedback mechanism to
control muscle tension by causing
muscle relaxation before muscle force
becomes so great that tendons might
be torn.
10. • Crossed Extensor Reflex : The crossed
extensor reflex is a withdrawal reflex.
The reflex occurs when the flexors in the
withdrawing limb contract and the
extensors relax, while in the other limb,
the opposite occurs. An example of this
is when a person steps on a nail, the leg
that is stepping on the nail pulls away,
while the other leg takes the weight of
the whole body.
11. • Withdrawal Reflex : The
withdrawal reflex is a spinal
reflex intended to protect the
body from damaging stimuli. It
is polysynaptic, and causes the
stimulation of sensory,
association, and motor neurons.
