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23 Cards in this Set

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  • Back
What is a reflex, what 3 things do they involve
-A reflex is an Involuntary, Stereotyped (happens same way eveytime) response to sensory stimuli
-All reflexes must involve a Receptor Structure and associated Afferent (sensory) neuron and an Efferent (motor) neuron. In Spinal cord, they involve dorsal root ganglion neurons (sensory), ventral horn motor neurons and often cells that reside in the spinal dorsal horn gray matter (Interneurons).
Neurons that reside ENTIRELY in the CNS.
Spinal cord interneurons frequently "connect" sensory and motor neurons forming Reflex Arcs
What are three principal spinal (or segmental) reflexes involving the limbs
1) Muscle Stretch Reflex (deep tendon reflex(DTR), myotatic reflex)
2) Golgi Tendon Organ Reflex (inverse stretch reflex)
3) Flexor Withdrawal Reflex

These three types of reflex originate from different sensory receptors and have different intraspinal circuitry.
Muscle Stretch Reflex (myotatic reflex, DTR)
-The peripheral process of the sensory neuron is associated with a sensory ending or receptor in the muscle called the Muscle Spindle
-The central process of the sensory neurons synapse directly with the Alpha-Motor Neurons in the spinal cord
-The reflex arc created is terned a Monosynaptic Reflex since only two neurons (and One Synapse) are involved (no interneuron/very fast b/c of less components involved).
Functional example of muscle stretch reflex
Knee-jerk reflex
-Patellar ligament is tapped which stretched quadriceps muscle
-Muscle spindles are stretched, excite sensory endings of sensory neuron (dorsal root ganglion cells), this excites alpha-motor neurons in ventral horn, causes contraction of quadriceps.
-Since this type of reflex involves tapping a tendon, they are often called Deep Tendon Reflexes (DTRs).
Inhibitory Interneurons
During DTRs, the sensory fibers also innervate inhibitory interneurons that project to motorneurons that supply antagonistic muscles. The antagonistic muscle to the quads (flexor muscle) is inhibited
How are DTRs useful
They are used to gain clinical info about the integrity of both the peripheral nerve and the spinal cord segments involved, and the descending input.
Injuries associated with bad DTR
Reflexes may be increased or absent depending on the type of peripheral nerve injury or spinal cord injury:
1) With peripheral nerve injury or a spinal cord segment injury, a DTR is frequently diminished (Hyporeflexia) or absent (Areflexia) = Lower Motor Neuron Lesion
2) An important fact for clinicians is that the reflex becomes hyperactive (exxagerated) or clonus occurs when descending pathways are disrupted (Hyperreflexia) = Upper Motor Neuron Lesion. The reason for this is that descending systems inhibit segmental reflexes via inhibitory interneurons. When the descending inhibition is removed, the reflexes are disinhibited (Release Phenomenon).
Stretch reflex used to gain clinical info about integrity of peripheral nerve, spinal cord segment, descending input to spinal cord
Triceps Reflex - Muscles involved, spinal cord segments involved, peripheral nerve involved
-Triceps Brachii
-C6, C7
-Radial nerve
Biiceps Reflex - Muscles involved, spinal cord segments involved, peripheral nerve involved
-Biceps Brachii
-C5, C6
-Musculocutaneous nerve
Knee-jerk (patellar) Reflex - Muscles involved, spinal cord segments involved, peripheral nerve involved
-Quadriceps Femoris
-L3, L4
-Femoral nerve
Ankle-jerk (Achilles) Reflex - Muscles involved, spinal cord segments involved, peripheral nerve involved
-Gastrocnemius, soleus
-S1, S2
The Gamma Loop - what does it control - background info
-Gamma motorneurons control intrafusal muscle fibers within the muscle spindle
-Descending systems can control the level of activity of gamma motorneurons, so if you cut the descending inhibitory control, cell firing increases, thus muscle tone increases (spasticity) -> phenomenal resistance when passively extending and flexing limb (upper motor neuron lesion)
-Afferents from the muscle spindle control the degree of tonic activation of the alpha-motor neurons.
The Gamma Loop Pathway (5)
Supraspinal activation -> gamma-motor neuron activation -> intrafusal fiber contraction -> increase sensory fiber activity -> activation of alpa-motor neuron
-In this way, regulation of the gamma system by descending pathways determines muscle tone.
How is muscle tone assessed clinically? Abnormalities?
By PASSIVELY Extending and Flexing the Limbs.
-Abnormal activity of the gamma system is manifested by increase in resistance to movement (stiffness) b/c of increase in muscle tone (Hypertonia) - clinically, this is termed Spasticity.
Upper Motor Neuron Lesion symptoms
-Often a patient with a spinal cord injury or stroke will have a Combination of Paralysis, Hyperactive Reflexes and Increased Muscle Tone. When the descending control of gamma motor neurons is cut, there is an increase in gamma and alpha motor neuron cell firing -> spasticity/hypertonia
-They will be described as having Spastic Paralysis with Hyperreflexia - components of Upper motor neuron lesion
-Hyperactivity of the stretch reflex also results in an abnormal response to rapid stretch - a prolonged oscillation of contraction and relaxation of the limb termed Clonus.
Flexor Withdrawal Reflex - things involved
Involves cutaneous (skin) receptors and movement (withdrawal) of a whole limb that experiences a painful stimulus. Interneurons are involved so its a bit slower than monosynaptic, but only by a few milliseconds.
Flexor Withdrawal Reflex - examples
Withdrawal of a limb from a painful stimulus such as removing the hand from a hot stove by flexing the arm of lifting the foot after stepping on a sharp object on the floor.
Flexor Withdrawal Reflex - Pathway
Afferents from the pain receptors Activate Excitatory Interneurons which synapse with flexor muscles to withdraw the limb (at the same time the afferents synapse with Inhibitory Interneurons which synapse with antagonistic Extensor Muscles).
Flexor Withdrawal Reflex - Crossed Extension Reflex
Musculature of the opposite side of the body may also be activated by what is called the Crossed Extension Reflex.
By way of interneurons, there is excitation of contralateral extensor muscles and inhibition of flexor muscles.
Golgi Tendon Organ Reflex (inverse stretch reflex) - where are golgi tendon organs localized, what info does the receptor provide
-Golgi tendon organs are localized at the junction between the muscle fiber and the tendon
-This receptor is a stretch receptor that provides info on the amount of Tenson in the muscle.
Golgi Tendon Organ Reflex - Pathway
The afferent fiber of the Golgi tendon organ innervates in hibitory interneurons that project to motorneurons supplying the same muscle from which the afferent originates.
-Thus activation of the afferent results in inhibition of motor neurons supplying the muscle that experiences the stretch, leading to RELAXATION of the muscle to PREVENT INJURY to that muscle.
-While the Golgi tendon reflex is present in normal individuals, it is difficult to demostrate unless it is hyperactive as a result of disease or injury , eg. the clasp-knife reflex)
Anatomy of Muscle Spindles and their sensitivity
They are sensory organs embedded within the main skeletal muscle mass itself. It is made up of a connective tissue sheath surrounding 10-12 very small muscle fibers called Intrafusal Muscles or intrafusal fibers. The main skeletal fibers are called Extrafusal fibers.
The sensitivity of the stretch receptor within the sheath depends on the tension (tone) of the intrafusion muscle fibers, which is regulated by input from the gamma Motorneurons.