Topics and Speakers John W. Krakauer, MD
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Synopsis
Spinal reflexes occur at the bottom of the hierarchical control system that integrates the cortex, the brainstem, and the spinal cord. As Dr John W. Krakauer explains, seemingly isolated reflexes, such as the tendon reflex, are much more complex than commonly imagined. Contrary to conventional understanding, spinal reflexes do not involve an obligatory or automatic response. In fact, as Dr Krakauer describes, they are highly modifiable by the task, content and intent of the stimulus (e.g., if the tendon is hit harder with a tendon hammer, the reflex will be much stronger).
Dr Krakauer begins by discussing two principal reflexes: the stretch and withdrawal reflexes (which are preprogrammed in the cortex, and are not under voluntary control), the muscle spindle, descending pathways in the spinal cord, and diseases of the spinal cord.
Dr Krakauer expands on one of the most familiar types of "involuntary" reflexes, the stretch reflex, which is also known as the knee-jerk reflex and is composed simply of a two-neuron loop, an afferent neuron and an efferent neuron. The afferent neuron detects a stretch in the muscle via the muscle spindle and makes the efferent neuron twitch the muscle.
He also explains the makeup of the muscle spindle in this reflex. The muscle spindle is a small group of muscle fibers that are separate from the rest of the muscle by a collagen sheath. In turn, this sheath is made of intrafusal fibers, which oversee muscle stretching. These intrafusal fibers are in turn sensitive to the Ia fiber, which fires when the muscle is stretching and quickly stops when the muscle is in the desired position. The Ib fibers, also known as Golgi tendon organs, are embedded in the tendon and act as a position sensor.
Descending pathways in the spinal cord include both voluntary and postural pathways. Voluntary pathways are responsible for the activity of hands and distal limbs. Postural pathways are involuntary and are primarily responsible for holding the body up and straight against gravity.
Dr Krakauer concludes the lecture by describing spinal diseases, such as upper motor neuron disease, which is characterized by hyperexcitability of the stretch reflex, resulting in stiffness, slowness, and clumsiness of movement. This disease is also marked by the Babinski response: when an object is scraped along the sole of the foot, the big toe moves upward while the rest of the toes fan down and outward. Dr Krakauer also briefly mentions other disorders such as flaccid toe and clonus.
