Control of Body Movement

Integrative Physiology I: Control 13 of Body Movement Neural Refl exes Neural Refl ex Pathways Can Be Classifi ed in Diff erent Ways Autonomic Refl exes Skeletal Muscle Refl exes Muscle Spindles Respond to Muscle Stretch Golgi Tendon Organs Respond to Muscle Tension Stretch Refl exes and Reciprocal Inhibition Control Movement Around a Joint Flexion Refl exes Pull Limbs away from Painful Stimuli The Integrated Control of Body Movement Movement Can Be Classifi ed as Refl ex, Voluntary, or Rhythmic The CNS Integrates Movement Symptoms of Parkinson’s Disease Refl ect Basal Ganglia Function Control of Movement in Visceral Muscles Extracting signals directly from the brain to directly control robotic devices has been a science fi ction theme that seems destined to become fact. — Dr. Eberhard E. Fetz, Science News 156: 142, 8/28/99 Background Basics Refl ex pathways Central nervous system Summation of action potentials Isometric contraction Each dot of Sensory pathways and a microarray receptors represents one Graded potentials gene. Genes that are active Tonic control show up in Tendons bright colors. From Chapter 13 of Human Physiology: An Integrated Approach, Sixth Edition. Dee Unglaub Silverthorn. Copyright © 2013 by Pearson Education, Inc. All rights reserved. 465 Integrative Physiology I: Control of Body Movement hink of a baseball pitcher standing on the mound. As he RUNNING PROBLEM looks at the fi rst batter, he receives sensory information from multiple sources: the sound of the crowd, the sight T Tetanus of the batter and the catcher, the smell of grass, the feel of the ball in his hand, and the alignment of his body as he begins his “She hasn’t been able to talk to us. We’re afraid she may have windup. Sensory receptors code this information and send it to had a stroke.” That is how her neighbors described 77-year- old Cecile Evans when they brought her to the emergency the central nervous system (CNS), where it is integrated. room. But when a neurological examination revealed no Th e pitcher acts consciously on some of the information: problems other than Mrs. Evans’s inability to open her mouth he decides to throw a fastball. But he processes other informa- and stiff ness in her neck, emergency room physician Dr. Doris tion at the subconscious level and acts on it without conscious Ling began to consider other diagnoses. She noticed some thought. As he thinks about starting his motion, for instance, he scratches healing on Mrs. Evans’s arms and legs and asked the shift s his weight to off set the impending movement of his arm. neighbors if they knew what had caused them. “Oh, yes. She Th e integration of sensory information into an involuntary re- told us a few days ago that her dog jumped up and knocked sponse is the hallmark of a refl ex. her against the barbed wire fence.” At that point, Dr. Ling realized she was probably dealing with her fi rst case of tetanus. Neural Refl exes All neural refl exes begin with a stimulus that activates a sensory receptor. Th e sensor sends information in the form of action potentials through sensory aff erent neurons to the CNS. Th e CNS through experience. Th e example of Pavlov’s dogs salivating is the integrating center that evaluates all incoming information upon hearing a bell is the classic example of a learned re- and selects an appropriate response. It then initiates action po- fl ex , also referred to as a conditioned refl ex . tentials in eff erent neurons to direct the response of muscles and 4 By the number of neurons in the refl ex pathway . Th e sim- glands—the targets. plest refl ex is a monosynaptic refl ex , named for the single A key feature of many reflex pathways is negative feedback . synapse between the two neurons in the pathway: a sen- Feedback signals from muscle and joint receptors keep the CNS sory aff erent neuron (oft en just called a sensory aff erent ) continuously informed of changing body position. Some refl exes and an eff erent somatic motor neuron ( Fig. 13.1 a). Th ese have a feedforward component that allows the body to anticipate a stimulus and begin the response. Bracing yourself in anticipation of Table a collision is an example of a feedforward response. Classifi cation of Neural Refl exes 13.1 Neural Refl ex Pathways Can Be Classifi ed in Diff erent Ways Neural Refl exes Can Be Classifi ed by: Refl ex pathways in the nervous system consist of chains or net- 1 . Efferent division that controls the effector works of neurons that link sensory receptors to muscles or glands. a. Somatic motor neurons control skeletal muscles. Neural refl exes can be classifi ed in several ways ( Tbl. 13.1 ): b. Autonomic neurons control smooth and cardiac muscle, glands, and adipose tissue. 1 By the eff erent division of the nervous system that controls the response. Reflexes that involve somatic motor neu- 2 . Integrating region within the central nervous system a. Spinal reflexes do not require input from the brain. rons and skeletal muscles are known as somatic refl exes . b. Cranial reflexes are integrated within the brain. Reflexes whose responses are controlled by autonomic neurons are called autonomic refl exes . 3 . Time at which the reflex develops 2 By the CNS location where the refl ex is integrated . Spinal a. Innate (inborn) reflexes are genetically determined. reflexes are integrated in the spinal cord. These reflexes b. Learned (conditioned) reflexes are acquired through may be modulated by higher input from the brain, but experience. they can occur without that input. Refl exes integrated in 4 . The number of neurons in the reflex pathway the brain are called cranial refl exes . a. Monosynaptic reflexes have only two neurons: one 3 By whether the refl ex is innate or learned . Many refl exes are afferent (sensory) and one efferent. Only somatic innate ; in other words, we are born with them, and they are motor reflexes can be monosynaptic. genetically determined. One example is the knee jerk, or pa- b. Polysynaptic reflexes include one or more interneurons between the afferent and efferent neurons. All tellar tendon refl ex: when the patellar tendon at the lower autonomic reflexes are polysynaptic because they have edge of the kneecap is stretched with a tap from a reflex three neurons: one afferent and two efferent. hammer, the lower leg kicks out. Other refl exes are acquired 466 Fig. 13.1 ESSENTIALS Neural Reflexes SKELETAL MUSCLE REFLEXES (a) A monosynaptic reflex has a single synapse between the afferent and Sensory Receptor efferent neurons. Stimulus neuron Spinal cord integrating center Skeletal muscle Somatic motor neuron Target Efferent One Response cell neuron synapse (b) Polysynaptic reflexes have two or more Synapse 1 synapses. This somatic Sensory Spinal Receptor motor reflex has both Stimulus neuron cord synapses in the CNS. integrating center Interneuron Synapse 2 Target Efferent Response cell neuron AUTONOMIC REFLEXES (c) All autonomic reflexes are polysynaptic, with at least one synapse in the CNS Sensory and another in the Stimulus Receptor neuron autonomic ganglion. CNS integrating center Preganglionic autonomic Response neuron Postganglionic autonomic neuron Target Autonomic cell ganglion 467 Integrative Physiology I: Control of Body Movement Other autonomic refl exes are integrated in the brain, pri- RUNNING PROBLEM marily in the hypothalamus, thalamus, and brain stem. Th ese Tetanus {tetanus, a muscle spasm}, also known as lockjaw, is regions contain centers that coordinate body functions needed a devastating disease caused by the bacterium Clostridium to maintain homeostasis, such as heart rate, blood pressure, tetani . These bacteria are commonly found in soil and enter breathing, eating, water balance, and maintenance of body tem- the human body through a cut or wound. As the bacteria perature. Th e brain stem also contains the integrating centers reproduce in the tissues, they release a protein neurotoxin. for autonomic refl exes such as salivating, vomiting, sneezing, This toxin, called tetanospasmin, is taken up by somatic motor coughing, swallowing, and gagging. neurons at the axon terminals. Tetanospasmin then travels An interesting type of autonomic refl ex is the conversion of along the axons until it reaches the nerve cell body in the emotional stimuli into visceral responses. Th e limbic system— spinal cord. the site of primitive drives such as sex, fear, rage, aggression, and Q1: a. Tetanospasmin is a protein. By what process is it taken hunger—has been called the “visceral brain” because of its role up into neurons? in these emotionally driven refl exes. We speak of “gut feelings” and “butterfl ies in the stomach”—all transformations of emo- b. By what process does it travel up the axon to the tion into somatic sensation and visceral function. Other emo- nerve cell body? tion-linked autonomic reflexes include urination, defecation, blushing, blanching, and piloerection, in which tiny muscles in the hair follicles pull the shaft of the hair erect (“I was so scared two neurons synapse in the spinal cord, allowing a signal my hair stood on end!”). initiated at the receptor to go directly from the sensory Autonomic refl exes are all polysynaptic, with at least one neuron to the motor neuron. (Th e synapse between the so- synapse in the CNS between the sensory neuron and the pre- matic motor neuron and its muscle target is ignored.) ganglionic autonomic neuron, and an additional synapse in the ganglion between the preganglionic and postganglionic neurons Most refl exes have three or more neurons in the pathway ( Fig. 13.1 c). (and at least two synapses), leading to their designation as poly- Many autonomic refl exes are characterized by tonic activ- synaptic refl exes ( Fig.

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