Motor and Sensory System Coordination | Reflex Pathways
Motor and Sensory System Coordination | Reflex Pathways
Objective 10
Define “reflex”. For each of the following reflexes, name the components in the reflex arc: myotatic (stretch) reflex; flexor (withdrawal) reflex; pupillary reflex. Describe the abnormal plantar extensor reflex (Babinski reflex) and why it is observed.
A reflex arc consists of the following components:
- a sensory receptor which picks up information from the environment;
- a sensory neuron (which may or may not be the same as the receptor) to carry the information to the central nervous system);
- an integrating center (i.e., the CNS);
- a motor neuron to innervate the effector;
- an effector (generally, skeletal muscle, also called striated or voluntary muscle).
Although we often call things like braking for a small child or springing out of the blocks at the starting gun “reflexes,” technically, these are complex behaviors. Reflexes are hard-wired and require no conscious thought.
It is critically important for the muscles to detect passive stretch (for example, when a load suddenly increases in weight) in our muscles. This doesn’t seem important at first until we consider walking on uneven ground with (say) rocks shifting under our feet. In order to remain upright, we need to immediately recognize the muscle stretch that results and respond to it quickly.
The myotatic reflex (also called the stretch reflex or monosynaptic reflex) is the fastest reflex in the human body. The various names sketch out the elements of the reflex. It starts with a stretch receptor, anatomically called the intrafusal muscle fiber or muscle spindle receptor. (Everything seems to have two or three names.)
There is exactly one synapse between the sensory receptor and motor neuron effector — hence the name “monosynaptic”. The effector is a lower motor neuron (α motor neuron) with its cell body in the anterior horn of the spinal cord. The α motor neuron axon terminals form neuromuscular junctions where acetylcholine (ACh) is released onto skeletal muscle cells.
To review, then, there are two neurons and one synapse in the myotatic reflex.
The sensory arm (afferent limb) of the reflex is a muscle spindle receptor, with a modified dendrite in skeletal muscle, an axon which enters the spinal cord via the posterior root, and a cell body in the posterior root ganglion. This is a unipolar neuron.
There is a synapse between the muscle spindle receptor and the lower motor neuron (α motor neuron) in the anterior horn of the spinal cord.
The lower motor neuron sends an axon via the anterior root (efferent limb) to innervate the same muscle that originated the sensory signal.
Another way to break this down is to consider the functional steps in the reflex:
Some jerk hits your patellar tendon with a rubber hammer, stretching a muscle in quadriceps group and activating an intrafusal muscle fiber (stretch receptor; muscle spindle receptor).
The stretch receptor releases the excitatory neurotransmitter glutamate onto