The pressure of arterial blood is significantly greater than the pressure of venous blood. A precapillary sphincter is a ring of smooth muscle located where a capillary branches off of an arteriole. When the sphincter contracts, it closes the vessel and prevents blood flow into the capillary. During the knee-jerk reflex, a small but rapid stretch of the quadriceps muscle produces action potentials in the endings of a sensory axon innervating that muscle. Describe the sequence of steps by which this electrical signal leads to contraction of the quadriceps.
Sensory neurons conduct action potentials to the spinal cord.
In the spinal cord, the sensory neuron has chemical synapses onto motor neurons. The action potentials of the sensory neuron to elicit action potentials in the motor neurons at these synapses. The motor neurons conduct their action potentials out to the quadriceps muscle. The motor neurons have chemical synapses onto the muscle cells of the quadriceps. The action potentials in the motor neurons elicit action potentials in the muscle cells at these synapses, resulting in contraction.