Chapter_54 Motor Functions of the Spinal Cord.pdf

UNIT XI Chapter 54: Motor Functions of the Spinal Cord; The Cord Reflexes Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Spinal Cord Is More Than Just a Conduit for Nerve Fib...

UNIT XI Chapter 54: Motor Functions of the Spinal Cord; The Cord Reflexes Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Spinal Cord Is More Than Just a Conduit for Nerve Fibers neuronal circuits for walking and various reflexes are contained within the spinal cord. higher brain centers activate and command these circuits. – walking. – maintaining equilibrium. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Motor Organization of the Spinal Cord sensory fibers enter the cord and are transmitted to higher centers, or they synapse locally to elicit motor reflexes. motor neurons are located in the anterior portion of the cord. – motor neurons are 50 - 100 % bigger than other neurons. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Anterior Motor Neurons alpha motor neurons – give rise to large type A alpha fibers (~14 microns). – stimulation can excite 3 - 100extrafusalmuscle fibers collectively called a motor unit. gamma motor neurons – give rise to smaller type A gamma fibers (~5 microns). – stimulation excites intrafusal fibers, a special type of sensory receptor. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Anterior Motor Neurons Figure 54-2 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Interneurons and Propriospinal Fibers Interneurons – 30 times as many as anterior motor neurons. – small and very excitable. – comprise the neural circuitry for the motor reflexes. Propriospinal fibers – travel up and down the cord for 1 - 2 segments. – provide pathways for multisegmental reflexes. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Sensory Receptors of the Muscle Muscle Spindle – sense muscle length and change in length. Golgi Tendon Organ – sense tendon tension and change in tension. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Muscle Spindle Figure 54-3 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Muscle Spindle Ends have contractile elements The center has no contractile elements and responds to stretch Figure 54-3 Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill Fig36-3 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Muscle Spindle Three types of fibers Secondary or Group II afferents innervate static bag and chain fibers 1a afferents innervate all three fiber types Gamma motor neurons innervate the ends of the Kandel, Schwartz and Jessell, 4th edition intrafusal fibers 2000, McGraw Hill, fig 36-3 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Static Response of the Muscle Spindle when the center of spindle is stretched slowly - the number of impulses generated by the primary and secondary endings increases in proportion to the degree of stretch. this is the ‘static response’. function of the static nuclear bag and nuclear chain fibers. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Dynamic Response of the Muscle Spindle when the center of the spindle is stretched rapidly - the number of impulses generated by the primary endings increases in proportion to the rate of change of the length. this is the ‘dynamic response’. function of the dynamic nuclear bag fiber. Copyright © 2011 by Saunders, an imprint of Elsevier Inc. Gamma Motor Nerves to the Spindle divided into gamma s and gamma d innervate the contractile ends of the respective intrafusal fibers. contraction of the ends will stretch the middle of the fiber. Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 36-3 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Stretch of the spindle causes activity of the receptors Contraction of the muscle without contracting the ends of the spindle makes the stretch response unavailable Simultaneous contraction of the muscle and the ends of the spindle keep the stretch response intact Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 36-9 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Physiologic Function of the Muscle Spindle comparator of length between the intrafusal and extrafusal muscle fiber. opposes a change in length of the muscle. when the muscle is stretched the spindle returns it to its original length. leads to the stretch reflex. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Stretch Reflex 3. this reflex opposes further stretch of the muscle 2. afferent impulses to the cord excite the alpha motor neuron which results in contraction of the muscle. 1. sudden stretch of the muscle excites the muscle spindle Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig36-2b1 Copyright © 2011 by Saunders, an imprint of Elsevier Inc. How Can We Change Muscle Length ?? Coactivation of alpha and gamma motor neurons Causes contraction of the muscle and the ends of the spindle. This results in a shortened spindle with an intact stretch response (otherwise the spindle would hang loose within the muscle) Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 36-12a Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Coactivation of alpha and gamma motor neurons Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 36-9 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Function of the Gamma System - spindle is normally tonically active as a result of input from higher brain centers. - controls the intensity of the stretch reflex. - performs a damping function by adjusting sensitivity. Figure 54-2 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Control of the Gamma Motor System (Fusimotor System) gamma signal excited by the bulboreticularfacilatory area of the brain stem. secondarily by areas that send impulses to this area – cerebellum, basal ganglia, cortex. little is known about the precise control of this system. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Clinical Application of the Stretch Reflex knee jerk reflex – striking the patellar tendon with a hammer stretches the quadriceps muscle. – this initiates a stretch reflex which shortens the muscle and causes the knee to move forward. can be done with almost any muscle. index of the facilitation of the gamma efferents. cortical lesions usually increase muscle stretch reflexes. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Golgi Tendon Reflex mediated by the golgi tendon organ receptor located in the tendon. this receptor responds to tension. when the tension becomes too great the reflex inhibits the motor fibers attached to the tendon. function is to equalize force among muscle fibers. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Transmission of Stretch Information to Higher Centers muscle spindle and golgi tendon signals are transmitted to higher centers. this informs the brain of the tension and stretch of the muscle. information is transmitted at 120 m/sec. important for feedback control of motor activity. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  The Withdrawal Reflexes a painful stimulus causes the limb to automatically withdraw from the stimulus. neural pathways for reflex – nociceptor activation transmitted to the spinal cord. – synapses with pool of interneurons that diverge the to the muscles for withdrawal, inhibit antagonist muscles, and activate reverberating circuits to prolong muscle contraction. – duration of the after discharge depends on strength of the stimulus. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Crossed Extensor Reflex painful stimulus elicits a flexor reflex in affected limb and an extensor reflex in the opposite limb. extensor reflex begins 0.2 - 0.5 seconds after the painful stimulus. serves to push body away from the stimulus, also to shift weight to the opposite limb. Copyright © 2011 by Saunders, an imprint of Elsevier Inc. Neuronal circuits for withdrawal and crossed extensor reflex Figure 54-9 Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Other Reflexes for Posture and Locomotion pressure on the bottom of the feet cause extensor reflex. – more complex than flexor-crossed extensor reflex. basic walking reflexes reside in the spinal cord. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.  Reflexes that Cause Muscle Spasm pain signals can cause reflex activation and spasm of local muscles. inflammation of peritoneum can cause abdominal muscle spasm. muscle cramps caused by painful stimulus in muscle – can be due to cold, ischemia, of overactivity. – reflex contraction increases painful stimulus and causes more muscle contraction. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.