Module 1: The Somato-Motor System: Tendon Tap reflex Module Objectives: 1. Describe the anatomic pathway of a tendon tap reflex. 2. Explain how a tendon tap reflex assessment assists in diagnosis of a neural system injury or disorder. Have you ever wondered why clinicians often check a patient's reflex? What exactly is the point? Even before learning about the anatomy and clinical applications of the tendon tap reflex, you first need to know that a number of different names are often used besides "tendon tap" reflex. Knowing these other names will help to greatly reduce confusion or frustration in the future if you are attempting to learn more about this topic. Names for the tendon tap reflex vary based on whether you are reading scientific or clinical publications. Some of the most commonly used clinical names include: 1. A knee jerk reflex (or what ever joint at which the tendon is being tapped) 2. In the specific case of the knee it is also sometimes called the patellar reflex or patellar tendon reflex 3. A Deep Tendon Reflex or DTR Some of the most commonly used scientific or research names include: 1. A Muscle Stretch Reflex or MSR 2. A myotactic reflex 3. A monosynaptic reflex In order to avoid confusion, it is extremely helpful to remember that all of these terms refer to the same thing, a tendon tap reflex. Heretofore, for this module, tendon tap reflex will be the preferred nomenclature! 1
How does a tendon tap reflex work? The response demonstrated by a patient after their tendon is tapped with a rubber hammer provides the clinician with helpful information concerning nervous system integrity. Throughout this module the knee jerk reflex will be used as the reflex example. It is one of the most commonly and easily assessed tendon tap reflexes. So what actually happens when you tap someone on the patellar tendon with a reflex hammer? The tendon tap causes a quick stretch to occur in the fibers of the muscle which the tendon is attached to. This quick stretch stimulates nerve receptors located within the muscle to fire off a signal. This signal travels along the muscle's nerve fibers. This neural pathway is a relatively simple one. A tendon tap reflex arc has five components (animated to the left): 1. A sensory receptor is stimulated (tendon tap) 4. An efferent signal is sent out to the muscle 5. A response occurs (muscle contraction) The afferent pathway of the tendon tap reflex (the initiation of the reflex) More detail is needed in order for you to fully understand this reflex and why it is clinically important. As stated previously, the tendon tap reflex arc has five primary components: 1. A sensory receptor is stimulated (tendon tap) 4. An efferent signal is sent out to the muscle 5. A response occurs (contraction) Specifics: The sensory (afferent) portion of a tendon tap reflex is comprised of two of the five components of the reflex arc. a. The quick muscle stretch produced via the tendon tap stimulates type 1A receptors (annulospinal endings) (http://www.umds.ac.uk/physiology/mcal/spin1.html) located within the muscle belly fibers (intrafusal fibers) (http://www.umds.ac.uk/physiology/mcal/spin1.html). b. The stimulated 1A receptors send a signal to the spinal cord. c. The afferent pathway ends in the spinal cord where a synapse occurs. 2
Lastly, an injury or disease process could damage a part of the pathway, thus altering the reflex response of a patient. Further detail on this will be provided soon! The efferent pathway of a tendon tap reflex Next, the efferent component of the pathway is explained in greater detail. As you know, the tendon tap reflex arc (or pathway) has five primary components: 1. A sensory receptor is stimulated (tendon tap) 4. An efferent signal is sent out to the muscle 5. A response occurs (contraction) Specifics: The motor (efferent) component of a tendon tap reflex. a. A synapse between the sensory and motor pathways of the reflex arc occurs in the gray matter of the spinal cord. b. This is the only synapse occurring on this pathway (monosynaptic). c. The efferent signal travels out to the muscle on an alpha motor neuron (http://www.placidity.net/neuro/page/3sect/0.shtml) and stimulates a muscle to contract. Diseases or injuries can damage this specific component of the pathway and affect the reflex response in a patient. The diagnostic value of the tendon tap reflex The tendon tap reflex is considered to have significant clinical diagnostic value. In fact, tendon tap reflex assessment has been identified as one of the most important physical exam diagnostic tools in neurology. In order to understand how a tendon tap reflex may assist in the process of diagnosing nervous system disease or injury, even greater understanding of the anatomy of a tendon tap reflex is required. Previously, the five basic components of a tendon tap reflex were described. In addition to these components, it is important to highlight the fact that signals from the brain can travel down the spinal cord and influence the tendon tap response (see animation). This is why individuals can sometimes voluntarily reduce or eliminate their response to a tendon tap. Thus, an additional component must be added to the model pathway of a tendon tap reflex: 3
1. A sensory receptor is stimulated (tendon tap) 4. An efferent signal from the brain, sent down the spinal cord can influence the reflex pathway 5. An efferent signal is sent out to the muscle 6. A response occurs (contraction) The diagnostic value of tendon tap reflex testing is based on the fact that injuries or diseases of the nervous system will alter the reflex response. In order to understand how reflex responses can become altered, one must consider the basic anatomical makeup of the nervous system. It may be divided into two main components: Peripheral Nervous System (PNS) Central Nervous System (CNS) In conjunction with knowledge of the patient's medical history, the type of muscle contraction (response) elicited by a tendon tap allows the clinician to determine whether a disease or injury is PNS or CNS related. Distinctly different diseases occur in these two nervous system areas. Tendon Tap response of a patient with a PNS disorder This is an example of a typical knee tendon tap reflex response of a patient with a Peripheral Nervous System http://www.ultranet.com/~jkimball/biologypages/p/pns.html) (PNS) disorder. If either the PNS (afferent or efferent pathway components) or the muscle the tendon is attached to are damaged due to disease or injury, a patient will typically demonstrate a diminished or absent muscle contraction response from the tendon tap (ie. hyporeflexia). Therefore, if a tendon tap reflex is reduced or absent on one side, a disease or injury affecting the PNS or muscle tissue is usually implicated Examples of PNS or muscle diseases or injuries (http://www.medinfo.ufl.edu/year2/neuro/review/pereiph.html) which will result in hyporeflexia include: 1. Compression of a nerve at the intervertebral foramen caused by a herniated disk. 2. Poliomyelitis - damage at the origin of the efferent pathway due to a viral infection. 3. Muscular Dystrophies - a group of inherited disorders which results in loss of muscle function. The anatomical make-up of the PNS (Peripheral Nervous System) consists of all the neurons which give rise to nerve fibers which travel outside the CNS (both afferent and efferent). 4
The tendon tap reflex pathway consists primarily of PNS components. Injury or disease of either the afferent or efferent component will result in a diminished or absent reflex response. This is an illustration of an absent tendon tap reflex response due to an injury to the efferent component of the pathway. A lumbar disc herniation compressing the efferent fibers of a spinal nerve is a common example of how this might occur. Additional useful scientific and clinical information: the route the efferent signal travels from the spinal cord to the muscle has been termed the final common pathway of the nervous system. It is the only pathway signals from the brain or spinal cord can travel to create a muscle response. Tendon Tap response of a patient with a CNS disorder When the Central Nervous System (http://www.ultranet.com/~jkimball/biologypages/c/cns.html ) (CNS) is affected by disease or injury, a much greater than expected muscle contraction typically occurs in response to a tendon tap (ie. hyper-reflexia). Therefore, if a greater than expected muscle contraction occurs with a tendon tap, the presence of a disease or injury affecting the CNS may be indicated. There are exceptions to this rule. They will be covered in a future course. Examples of CNS diseases (http://cpmcnet.columbia.edu/texts/guide/hmg26_0003.html) or injuries: 1. Paraplegia (paralysis of the lower limbs) - may occur due to spinal cord injury. 2. Multiple Sclerosis (MS) - a progressive disease which damages the myelin sheath of CNS neurons. 3. Cerebrovascular Accident (CVA) - a loss of blood supply to a portion of the brain resulting in permanent damage The anatomical make-up of the CNS (Central Nervous System) consists of all the neurons contained within the brain and the spinal cord. The tendon tap reflex assesses the integrity of the CNS because: While the tendon tap reflex pathway consists primarily of PNS components, it does enter into the CNS where a synapse occurs. This synapse within the spinal cord permits the CNS to influence the pathway. (see diagram) Typically, efferent signals from the brain dampen the efferent signal elicited by the tendon tap before it travels out to the muscle via the PNS. If either the area of the brain where the dampening signal is produced or the pathway in the spinal cord that the dampening signal travels are injured or damaged by disease, the influence of the CNS on the tendon tap response is eliminated. 5
This is the final content section of this module. When you click on the "continue" button you will be transferred to the post-test for the module. 6