Two different reflexes exist in skeletal muscles: the Golgi tendon reflex and the stretch reflex. It is very important for a Physical Therapist to understand the structure and function of these two reflexes, because he will use their effect on muscles in many different cases.
In the conceptual model above, I have drawn an agonist muscle (the big orange cylinder), an antagonist muscle (the smaller orange cylinder) and a spinal segment (the grey region). In the agonist muscle I have also drawn two sensory organs: the Golgi tendon organ and the muscle spindle.
The Golgi tendon organ senses changes in muscle tension. It is a proprioceptive sensory receptor organ that is located at the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle. It provides the sensory component of the Golgi tendon reflex. In a Golgi tendon reflex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. Though muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited through the Inhibitory Postsynaptic Potential. However, antagonistic muscles are activated.
An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential. The opposite of an inhibitory postsynaptic potential is an excitatory postsynaptic potential (EPSP), which is a synaptic potential that makes a postsynaptic neuron more likely to generate an action potential. They can take place at all chemical synapses which use the secretion of neurotransmitters to create cell to cell signalling. Inhibitory presynaptic neurons release neurotransmitters which then bind to the postsynaptic receptors; this induces a postsynaptic conductance change as ion channels open or close. An electrical current is generated which changes the postsynaptic membrane potential to create a more negative postsynaptic potential.
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurons via the stretch reflex to resist muscle stretch. The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.
When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length.
