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Moving Neurophysiology 8 Notes Reflexes are the simplest neural pathways connecting sensory stimuli to motor responses. They are rapid and repeatable, and usually involuntary. Reflexes can involve spinal nerves, cranial nerves, or autonomic pathways. Testing reflexes is useful in a clinical setting...

Moving Neurophysiology 8 Notes Reflexes are the simplest neural pathways connecting sensory stimuli to motor responses. They are rapid and repeatable, and usually involuntary. Reflexes can involve spinal nerves, cranial nerves, or autonomic pathways. Testing reflexes is useful in a clinical setting because loss of a reflex helps to localize nervous system damage to a specific pathway. The reflexes we are going to cover in this lecture include the myotatic, inverse myotatic, withdrawal, cutaneous trunci, and diving reflexes. The components of a reflex are the sensory receptor (may be a specialized sensory cell or part of the sensory neuron), the sensory neuron, one or more synapses in the central nervous system (spinal cord and/or brain), the motor neuron, and the effector organ (muscle or gland). Reflexes are often classified by the number of neurons in the pathway. Monosynaptic reflex pathways consist of just two neurons: the sensory neuron and the motor neuron. Polysynaptic reflex pathways involve one or more interneurons in the central nervous system. The myotatic reflex, also called the stretch reflex, can be elicited in all skeletal muscles. In the quadriceps, this reflex is called the knee jerk or patellar tendon reflex. The sensory organ for the myotatic reflex is the muscle spindle. The muscle spindle is a specialized bundle of skeletal muscle cells enclosed in a layer of fibrous connective tissue. These specialized muscle fibers, also called intrafusal fibers, are aligned parallel to the regular muscle fibers (called extrafusal fibers), and detect when the muscle is stretched by external forces. The intrafusal fibers are innervated by two types of sensory neurons that detect muscle length and velocity of stretch. When the entire muscle is stretched, the muscle spindle is stretched, resulting in an action potential in the sensory neuron. The axon of that sensory neuron travels through a spinal nerve to the spinal cord where it synapses on an alpha motor neuron. The cell body of the sensory neuron is located in the dorsal root ganglion near the spinal cord. When the reflex is activated through stretch of the muscle, the result is contraction of the muscle in order to provide postural stability. In the patellar tendon reflex, striking the tendon of the quadriceps muscle stretches the muscle, which also stretches the intrafusal muscle fibers, sending a sensory signal through the sensory neurons to the spinal cord. Those neurons synapse on and activate the alpha motor neurons innervating the quadriceps muscle. If the stimulus is strong enough, the result is contraction of the extrafusal muscle fibers of the quadriceps. The intrafusal fibers also contract, so that they can detect any additional stretch in the muscle. The motor neurons that innervate the intrafusal fibers are called gamma motor neurons and their cell bodies are also located in the ventral horn of the spinal cord. Like muscle spindles, Golgi tendon organs are sensory receptors that detect body position. These receptors are located in tendons that connect skeletal muscles to bones. Golgi tendon organs provide information about the degree of muscle contraction and prevent injury from excessive contraction. While muscle spindles are located parallel to the extrafusal muscle fibers, Golgi tendon organs are located in series with the muscle. The Golgi reflex is sometimes called an inverse myotatic reflex. Contraction of a muscle is sensed by the GTO, relayed to the spinal cord by a sensory neuron, and synapses on inhibitory interneurons which synapse on alpha motor neurons to decrease the action potentials going to the muscle, leading to muscle relaxation. Since the Golgi tendon organ reflex involves an interneuron, it is classified as a polysynaptic reflex. The withdrawal reflex is a response to painful stimuli in the limbs. Nociceptor, or pain receptors, are the sensory receptors. Activation of these receptors is relayed through the sensory neuron to the spinal cord, where interneurons mediate several different responses: contraction of flexor muscles and relaxation of extensor muscles on the same side as the painful stimulus (ipsilateral flexion); and relaxation of flexor muscles and contraction of flexor muscles on the opposite side of the body (contralateral extension). The result is that the animal retracts the limb away from the painful stimulus, but extends the contralateral limb to maintain balance. This is a polysynaptic reflex. The cutaneous trunci reflex responds to light touch (insects crawling on the skin) by contracting the thin layer of skeletal muscle (cutaneous trunci muscle) located just below the skin, to cause a twitch. Mechanical receptors in the skin sense the light touch, and a signal is sent to the spinal cord by the sensory nerves. Interneurons then transmit that signal to the alpha motor neurons in spinal segments C1-T8 (the motor neuron pool for the cutaneous trunci muscle), which cause contraction of the muscle and twitching of the skin. Clinically, this reflex can be used to localize lesions in the spinal cord. Spinal reflexes and skeletal muscle tone can be useful in distinguishing between lesions of the upper motor neurons and lower motor neurons. Upper motor neuron lesions typically cause normal or increased reflexes and muscle tone while lower motor neuron lesions are generally associated with decreased reflexes and muscle tone. The diving reflex is a complex, polysynaptic reflex in mammals that is especially important in aquatic mammals. The sensory receptors are the mechanical and thermoreceptors of the face. The trigeminal nerve relays those signals to the pons where they synapse on interneurons. The interneurons synapse in the brainstem on preganglionic motor neurons of the vagus nerve (parasympathetic system) which mediate decrease in the heart rate, inhibit respiration, and cause bronchoconstriction.