Spinal Cord, Nerves, and Muscles
The spinal cord is the connection between the brain and all muscles of the body, with the exception of the cranial nerves to the head and neck. The cord is organized into "segments" at each level between the neck and the sacrum. Each segment controls a distinct group of muscles at that level. Motor commands to the spinal cord travel downward, in multiple tracts.
The descending tracts contact cells in the intermediate and anterior portions of the spinal cord. The anterior "horn" contains the motor neurons that transmit signals directly to muscles. The intermediate layers contain relay neurons that directly trigger motor neurons or modulate their activity.
Motor neurons send signals to muscle fibers, making the fibers contract. Each motor neuron may contact up to several hundred muscle fibers. The neuron plus its muscle fibers is called the "motor unit." There are two types of motor neurons: alpha and gamma. The gamma neurons are further divided into gamma-static and gamma-dynamic subtypes. The alpha motor neurons are responsible for moving the muscles that do the work of moving the body. The gamma motor neurons adjust the sensitivity of special intrinsic "spindle" fibers that can sense the length or velocity of a muscle's movement.
The Stretch Reflex
Spindle fibers send sensory signals back to the spinal cord along fast sensory nerves. These sensory nerves enter the back of the spinal cord in the dorsal horn. Some fibers ascend immediately to the brainstem, cerebellum, or thalamus via the dorsal columns or spinocerebellar tracts. Other fibers split off and contact cells in the intermediate spinal layers or project directly onto the motor neurons in the anterior horn of the spinal cord. The loop from the spindle stretch receptors, through sensory nerves, to the motor neurons and thereby back to the muscle fibers is the basis for the stretch reflex. This reflex occurs when the knee or other muscle tendon is tapped with a rubber hammer. The hammer stretches the tendon, which then stretches the muscle, and triggers a sensory signal in the spindle fibers and sensory nerves. This signal then travels back to the spinal cord where it activates the motor neurons that cause the muscle to contract, making the knee jerk outwards. During normal activity, it is thought that the stretch reflex helps to maintain muscle tone and resist external forces on the limbs.
The stretch reflex is modulated by signals from the descending motor tracts and signals from the intermediate spinal layers. Normal movement requires inhibition of the stretch reflex so that voluntary movement of a joint does not trigger an involuntary contraction opposing the movement.
Damage to the spinal cord, brainstem, or motor cortex results in abnormal or absent signals in the descending tracts. It is thought that there may be decreased inhibition of the stretch reflex, and increased sensitivity of the alpha motor neurons. Therefore, the sensitivity of the motor neurons and stretch reflex increase over time. With increased sensitivity, even small attempted movements of the limb may lead to large opposing muscle contractions. This is thought to be the cause of spasticity. When the stretch reflex is strongly increased, it may lead to clonus, which is the occurrence of multiple jerks following a single tap to the tendon or a sudden stretch of a joint, particularly the ankle. Since the descending tracts may be injured, spasticity is usually associated with weakness, meaning that even though the muscles are strongly involuntarily contracted, the ability to make voluntary contractions is reduced, and there may be difficulty making finely differentiated movements.
Medications affecting inhibitory GABA receptors including baclofen or diazepam (Valium®) are thought to increase inhibition within the spinal cord and thereby decrease the strength of the stretch reflex. Botulinum toxin may also decrease the stretch reflex by weakening the connection between alpha motor neurons and muscles as well as by weakening the connection between gamma motor neurons and intrinsic fibers (that send the stretch signals back to the spinal cord). Dorsal rhizotomy is a surgical procedure that attempts to cut the sensory fibers where they enter the spinal cord in order to reduce or eliminate the stretch reflex.
Properties of muscles
Humans are constantly moving within an environment that may be changing and unpredictable and there is interaction with objects and tools of different weights and physical properties. In order to accomplish this, humans make use of the ability of muscles to adjust their stiffness (springiness) and viscosity (resistance to fast movement) in order to match the needs of the current task. Control of stiffness and viscosity is performed through several mechanisms:
When a doctor, therapist, or parent attempts to move an affected limb, the resultant muscle stiffness occurs as a result of a combination of the effects mentioned above. In addition, "passive" muscle properties reflect the level of tissue stiffness in the muscles, tendons, and joints. When affected children attempt to move their own limb, they do so by increasing the stiffness of one set of muscles (known as the agonists), while decreasing the stiffness of the opposing set of muscles (known as the "antagonists"). The difference in pull of the two sets of muscles leads to movement. If children wish to hold their limb in a fixed position, they may increase the stiffness of both the agonists and antagonists; thereby, the entire joint becomes difficult to move and maintains the desired position.
Increased stiffness is frequently seen in children with movement disorders and is referred to as "hypertonia." There are many different causes of hypertonia. At the muscle level, increased resistance to movement is caused by a combination of at least three elements:
Diagnosis of those elements are contributing to the stiffness is important as different medications are helpful only in some of these situations. For example, botulinum toxin, which blocks electrical conduction from nerve to muscle, does not have any immediate effect on passive muscle properties; however, it is possible that there may be long-term effects. The toxin may be able to reduce stiffness due to the initially contracting fibers or fibers that become active during the movement.
It is important to distinguish the stiffness felt by an examiner attempting to move the child's limb from the problems that a child may actually face. In particular, the child's complaints may be more closely related to weakness or poorly differentiated motor control, with the stiffness being noticed only by the doctor or therapist.
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Section Author: Terence Sanger, MD PhD
Scientific Reviewers: Leon Dure, MD, Associate Professor of Pediatrics and Neurology, The University of Alabama at Birmingham; Marjorie A Garvey, MD, Pediatrics and Developmental Neuropsychiatry Branch, NIMH, Human Motor Control Section, NINDS; Jonathan W. Mink, MD PhD, Associate Professor of Neurology, Neurobiology & Anatomy, and Pediatrics Chief, Child Neurology, University of Rochester Medical Center, Rochester, New York
Editor: Joy B. Leffler, NASW, AMIA