Reflexes are automatic responses, such as movements, initiated by stimulation of sensory receptors, which involve synaptic contacts between nerve cells.
They are the basic level of regulation of muscles or glands, and the simplest reflexes are found in the spinal cord.
Spinal Reflexes are automatic movements that involvea stimuus to a specific type of sensory receptor, at leas one synapse on another neurone, and an efferent, motor, pathway that activates an effector organ.
The simplest reflex is the monosynaptic reflex, consisting of a muscle spindle afferent synapsing on a motoneurone that causes the muscle to contract. This is the reflex circuit that operates in the knee jerk; tapping the patellar tendon stretches the muscle spindle, whose message reaches the spinal cord and excites motoneurones innervating the quadriceps muscle.
Reciprocal Innervation is the pathway that inhibits the activity of the hamstring muscles at the same time as the quadriceps contraction, and involves the addition of an inhibitory interneurone that hyperpolarises flexor motoneurones.
The diagram shows the pathway followed by impulses generated in muscle spindles when the extensor muscle of the knee is stretched by a tap on the patellar tendon. The afferent impulses cause the release of an excitatory transmitter on motoneurones innervating the extensor muscle, causing this muscle to contract. This is the pathway of the stretch reflex.
Connections between Neurones: Networks and Reflexes
Neurones make synaptic contacts with each other within all levels of the central nervous system. The simplest neuronal connection in mammals occurs in the ventral horn of the spinal cord, where large myelinated afferent axons that carry information from sensory nerve endings in a muscle (the muscle spindles) make direct contact with, and excite, motoneurones that innervate the same muscle. It consists of two neurones with one synapse between them, and is therefore called the monosynaptic reflex
The monosynaptic reflex is the basis of the stretch reflex (knee jerk) in which stretch of an extensor muscle causes a reflex contraction of that muscle.
During the knee jerk reflex (stretch reflex, myotatic reflex) a tap on the patellar tendon causes muscle spindle receptors in the attached muscle to be stretched. Impulses are passed into the spinal cord, and alpha motoneurones innervating the muscle that has been stretched are excited.
As a consequence of this connection between muscle spindles and alpha motoneurones, a tap on the patellar tendon causes the knee jerk. One essential component of this network is the synapse - a close connection between an axon terminal and a neuronal cell body.
Image source: docstoc.com
The pathway for reciprocal innervation.
Inhibitory interneurones and reciprocal innervation
Muscle spindle afferents branch into several collaterals when they enter the cord. Some of these are involved in a disynaptic reflex.
These branches of the spindle afferents synapse on interneurones that make contact with flexor motoneurones that innervate the antagonistic, muscle groups that oppose the contraction of the extensor muscles.
These interneurones release an inhibitory transmitter that hyperpolarises the flexor motoneurones.
There is therefore a simple form of coordination that ensures that the contraction of a muscle is not opposed by simultaneous contraction of its antagonists. Instead, it is more efficient to relax the opposing muscles, and this is done by using inhibitory interneurones.