Reflex Arcs and Reflexes

Reflex Arcs and Reflexes

The routes followed by nerve impulses as they travel through the nervous system are called nerve pathways. As a rule, activity on a nerve pathway begins with sensory receptors associated with sensory neurons, involves several inter-neurons, and may include some motor neurons.

Parts of Reflex Arcs

Parts	Description	Function
Receptor	The receptor end of a dendrite or a specialized receptor cell in a sensory organ	Sensitive to a specific type of internal or external change
Sensory neuron	Dendrite, cell body, and axon of a sensory Neuron	Transmits nerve impulse from the receptor into the brain or spinal cord
Interneuron	Dendrite, cell body, and axon of a neuron within the brain or spinal cord	Conducts nerve impulse from the sensory neuron to a motor neuron
Motor neuron	Dendrite, cell body, and axon of a motor Neuron	Transmits nerve impulse from the brain or spinal cord out to an effector
Effector	A muscle or gland outside the nervous system	Responds to stimulation by the motor neuron and produces the reflex or behavioral action

Reflex Behavior

The simplest nerve pathways lead directly from sensory to motor neurons and include only two or three cells.

Such pathways, known as reflex arcs, form the behavioral units of the nervous system. That is, they constitute the structural and functional bases for the simplest acts—the reflexes.

Reflexes are automatic, unconscious responses to changes occurring inside or outside the body.

They help to control many of the body's involuntary processes such as heart rate, breathing rate, blood pressure, and digestive activities.

Reflexes are also involved in swallowing, sneezing, coughing, and vomiting.

The knee-jerk reflex (patellar reflex) is an example of a simple reflex that employs only two neurons.

This reflex is initiated by striking the patellar ligament just below the patella. As a result, the quadriceps femoris muscle, which is attached to the patella by a tendon, is pulled slightly, and stretch receptors located within the muscle are stimulated.

These receptors, in turn, trigger impulses that pass along the fibers of a sensory neuron into the spinal cord. Within the spinal cord, the sensory axon forms a synapse with a dendrite of a motor neuron.

The impulse then continues along the axon of the motor neuron and travels back to the quadriceps femoris.

The muscle responds by contracting, and the reflex is completed as the lower leg extends.

This reflex is helpful in maintaining an upright posture. For example, when a person is standing still, if the knee begins to bend as a result of gravity pulling downward, the quadriceps femoris is stretched, the reflex is triggered, and the leg straightens again.

Another type of reflex, called a withdrawal reflex, occurs when a person unexpectedly touches a finger to something hot or sharp.

As this happens, some of the skin receptors are activated, and sensory impulses travel to the spinal cord.

There the impulses pass on to interneurons and arc directed to motor neurons. The motor neurons transmit the signals to flexor muscles in the arm, and they contract in response.

At the same time, the antagonistic extensor muscles are inhibited, and the hand is rapidly and unconsciously withdrawn from the harmful source of stimulation.

While the flexor muscles of the stimulated side (ipsilateral side) are caused to contract, the flexor muscles of the other arm (contralateral side) are being inhibited.

Furthermore, the extensor muscles on this other side are caused to contract. This phenomenon, which is called a crossed extensor reflex, is due to interneuron pathways within the spinal cord that allow sensory impulses arriving on one side of the cord to pass across to the other side and produce an opposite effect.

Concurrent with the withdrawal reflex, other interneurons in the spinal cord carry sensory impulses to the brain, and the person becomes aware of the painful experience.

A withdrawal reflex is, of course, protective because it serves to prevent excessive tissue damage when a body part touches something that is potentially harmful.