Motor reflex

Motor Reflex
Diagram of a reflex arc
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A motor reflex is an involuntary and nearly instantaneous movement in response to a stimulus. Reflexes are mediated by neural pathways called reflex arcs that can act on an impulse before that impulse reaches the brain. This 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.

Physiology of Motor Reflexes

Motor reflexes are essential for maintaining homeostasis and protecting the body from harm. They are classified as either monosynaptic reflexes or polysynaptic reflexes.

Monosynaptic Reflexes

Monosynaptic reflexes involve a single synapse between the sensory neuron and the motor neuron. The classic example of a monosynaptic reflex is the patellar reflex, also known as the knee-jerk reflex. In this reflex, tapping the patellar tendon stretches the quadriceps muscle, which is detected by muscle spindles. The sensory neurons from the muscle spindles synapse directly onto motor neurons in the spinal cord, causing the quadriceps to contract and produce the knee-jerk response.

Polysynaptic Reflexes

Polysynaptic reflexes involve one or more interneurons between the sensory and motor neurons. These reflexes are more complex and can involve multiple muscle groups. An example of a polysynaptic reflex is the withdrawal reflex, which occurs when a person touches a hot object. The sensory neurons detect the heat and synapse onto interneurons in the spinal cord, which then activate motor neurons to withdraw the hand from the hot object.

Types of Motor Reflexes

Motor reflexes can be categorized based on their function and the type of stimulus that triggers them.

Stretch Reflex

The stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex that helps maintain muscle tone and posture. The patellar reflex is an example of a stretch reflex.

Withdrawal Reflex

The withdrawal reflex, or flexor reflex, is a polysynaptic reflex that causes a limb to withdraw from a painful stimulus. It involves the activation of flexor muscles and inhibition of extensor muscles to quickly move the limb away from harm.

Crossed Extensor Reflex

The crossed extensor reflex is a polysynaptic reflex that occurs in conjunction with the withdrawal reflex. When a withdrawal reflex is initiated in one limb, the crossed extensor reflex causes the opposite limb to extend. This helps maintain balance and posture when the body is subjected to a sudden shift in weight.

Clinical Significance

Motor reflexes are used clinically to assess the integrity of the nervous system. Abnormal reflexes can indicate neurological disorders or damage to specific areas of the nervous system. For example, hyperreflexia (exaggerated reflexes) can be a sign of upper motor neuron lesions, while hyporeflexia (diminished reflexes) can indicate lower motor neuron damage.

Also see

• Reflex arc
• Patellar reflex
• Withdrawal reflex
• Nervous system
• Neurology