Concept of the reflex and reflex arc
A reflex is the body's response to a stimulus mediated by the nervous system. The morphological substrate of a reflex is the reflex arc — a sequential chain of neurons along which a nerve impulse travels from a receptor to an effector organ. Neurons are interconnected by axons via synapses, ensuring unidirectional conduction of excitation.
Depending on the number of neurons in the chain, two types of reflex arcs are distinguished: simple and complex. The classification is based on the number of synaptic switchings between the sensory and motor components.
Classification of reflex arcs
- A simple (monosynaptic) reflex arc consists of two neurons: sensory (afferent) and motor (efferent). There is only one synapse between them. A classic example is the knee-jerk reflex arc, which occurs when the patellar tendon of the quadriceps femoris muscle is struck with a neurological hammer.
- A complex (polysynaptic) reflex arc includes three or more neurons: one or more interneurons are located between the afferent and efferent components. An example is the flexor withdrawal reflex when touching a hot object.
Neuronal composition of the reflex arc
Three functional types of neurons are distinguished in the somatic reflex arc:
- Afferent (sensory) neuron — a pseudounipolar neuron whose cell body is located in the spinal ganglion. Its peripheral process ends in a receptor, while its central process extends into the posterior horns of the spinal cord. A distinctive feature of pseudounipolar cells is that a single process arises and then T-branches into two.
- Interneuron — located in the gray matter of the spinal cord, primarily in its posterior and intermediate regions. It connects the afferent and efferent components in polysynaptic arcs.
- Efferent (motor) neuron — a motoneuron of the anterior horns of the spinal cord. Its axon exits via the anterior root and travels as part of the spinal nerve to the skeletal muscle, which acts as the effector organ.
Simple reflex arc using the knee-jerk reflex as an example
Stimulation originates in the receptors of the quadriceps femoris tendon when it is stretched. The impulse travels along the sensory fibers of the peripheral nerve to the spinal nerve, then enters the spinal ganglion via the posterior root, where the cell body of the pseudounipolar neuron is located.
The central process of the afferent neuron passes through the posterior horns to the anterior horns of the spinal cord, where it forms a single synapse with the cell body of the motoneuron. The axon of the motor neuron exits via the anterior root, enters the spinal nerve, and reaches the quadriceps femoris, causing it to contract and extend the knee joint.
Complex reflex arc using the withdrawal reflex as an example
When touching a hot object, skin receptors are stimulated. The afferent impulse travels via peripheral nerves into the spinal nerve, passes through the posterior root into the spinal ganglion to the pseudounipolar sensory neuron.
The central process heads to the posterior horns of the spinal cord, where it forms a synapse with the interneuron. The axon of the interneuron transmits excitation to the motoneuron of the anterior horns. Next, the impulse travels via the anterior root and spinal nerve to the muscles of the upper limb, causing the hand to withdraw. Thus, at least two synaptic switchings take place in a polysynaptic arc.
Anatomic landmarks on a transverse section of the spinal cord
- The butterfly-shaped gray matter contains neuronal cell bodies: the posterior horns are predominantly sensory and interneuronal, while the anterior horns are motor.
- The posterior root is formed by the axons of pseudounipolar neurons and features a swelling — the spinal ganglion.
- The anterior root is formed by the axons of the anterior horn motoneurons.
- The combination of the roots forms the spinal nerve, which exits through the intervertebral foramen.
Key points
- The reflex arc is the morphological basis of a reflex, a chain of neurons from receptor to effector.
- A simple arc includes two neurons and one synapse; a complex one includes three or more neurons involving interneurons.
- The afferent neuron of the somatic arc is pseudounipolar, localized in the spinal ganglion.
- The efferent neuron is a motoneuron of the anterior horns of the spinal cord.
- The knee-jerk reflex is an example of a monosynaptic arc, while the withdrawal reflex is polysynaptic.
- The posterior root carries sensory fibers, while the anterior root carries motor fibers.
- Interneurons are located in the gray matter of the spinal cord and provide interneuronal connections.