Anatomy and Physiology
Reflexes are involuntary, predictable responses to a stimulus, which serve to protect the body, maintain balance, and carry out automatic functions. They are mediated by the neural pathways known as reflex arcs, which involve sensory receptors, afferent neurons, integration centers, efferent neurons, and effector organs.
– Sensory Receptors: Detect the stimulus.
– Afferent (Sensory) Neurons: Carry the impulse to the central nervous system (CNS).
– Integration Center: Processes the information, often involving interneurons, and is typically located within the spinal cord or brainstem.
– Efferent (Motor) Neurons: Transmit the response command from the CNS.
– Effector Organs: Carry out the response, such as muscles contracting or glands secreting.
Classification of Reflexes:
Reflexes can be classified in several ways:
– Innate vs. Acquired: Innate reflexes are those we are born with, such as the suckling reflex. Acquired reflexes are learned through experience, like driving a car.
– Somatic vs. Autonomic: Somatic reflexes involve the skeletal muscles, such as the knee-jerk reflex, while autonomic reflexes regulate body functions, like heart rate and digestion.
– Spinal vs. Cranial: Spinal reflexes are integrated within the spinal cord, while cranial reflexes are mediated by the brainstem.
– Monosynaptic vs. Polysynaptic: Monosynaptic reflexes have a single synapse between the afferent and efferent neuron, like the stretch reflex. Polysynaptic reflexes have two or more synapses involving interneurons.
Reflex Arcs Specifics:
The simplest reflex arc is the monosynaptic reflex arc, like the patellar reflex. When the patellar tendon is tapped, the muscle spindles in the quadriceps are stretched, sending a signal via afferent neurons to the spinal cord, which immediately sends a signal back through efferent neurons to the quadriceps, causing it to contract.
Polysynaptic reflex arcs involve more complex pathways with interneurons facilitating more elaborate responses, such as the withdrawal reflex. This reflex involves pulling away from a painful stimulus and requires coordination with other muscles for balance.
Testing Reflexes:
Reflex testing is a critical component of the neurological exam, as it helps to assess the integrity of the nervous system. Common reflexes tested include:
- Corneal Reflex: Evaluating the trigeminal (V) and facial (VII) cranial nerves. A normal response is a bilateral blink.
- Jaw Jerk Reflex: Testing the integrity of the trigeminal nerve (V). A normal response is a slight closure of the jaw.
- Styloradial Reflex: Assessing the function of the C5-C6 spinal nerves and the brachioradialis muscle. It typically results in slight pronation or supination and slight elbow flexion.
- Biceps and Triceps Reflexes: Assessing C5-C6 and C6-C7 spinal nerves, respectively.
- Abdominal Reflexes: Testing the T8-T12 spinal nerves. A normal response is the contraction of the abdominal muscles and deviation of the umbilicus toward the stimulus.
- Patellar Reflex: Testing the L2-L4 spinal nerves and the femoral nerve.
- Achilles Reflex: Evaluating S1-S2 spinal nerves and the tibial nerve.
- Plantar Reflex: Evaluating the corticospinal tract. A normal response is the plantar flexion of the toes.
Normal and Pathological Reflex Responses:
Normal reflex responses are consistent, reproducible, and symmetrical. Abnormal responses can indicate neurological disorders. For example:
– Hyperreflexia: Exaggerated response, often seen in conditions like stroke or spinal cord injury above the level of reflex integration.
– Hyporeflexia/Areflexia: Diminished or absent reflex, which can occur with peripheral neuropathy or spinal cord injury at the level of reflex integration.
– Babinski Sign: An abnormal plantar reflex in adults, where the big toe extends upwards, indicating CNS damage.
Understanding reflexes and their responses is vital for diagnosing and managing neurological conditions. Medical students should become proficient in testing and interpreting these reflexes to aid in their clinical assessments.
Verified by Dr. Petya Stefanova
