Lesson 6.4 REFLEXES AND PROPRIOCEPTION

Transcription

1 Lesson 6.4 REFLEXES AND PROPRIOCEPTION (a) The Reflex Arc ~ ~ ~ TOPICS COVERED IN THIS LESSON (b) Proprioception and Proprioceptors 2015 Thompson Educational Publishing, Inc. 1

2 What Are Reflexes? Reflexes are an important part of all physical movement. They are an automatic, rapid, and unconscious response to a particular stimulus. If the command centre, or control, for the reflex is located in the brain, it is called a cerebral reflex. If the control is located in the spinal cord, it is called a spinal reflex Thompson Educational Publishing, Inc. 2

3 Classification of Reflexes Reflex contraction of the skeletal muscle is not dependent on conscious inter vention by higher centres of the brain but are a way in which the body responds to an unexpected stimulus. Autonomic reflexes are mediated by the autonomic division of the nervous system and usually involve the activation of smooth muscle, cardiac muscle, and glands. These reflexes regulate such bodily functions as digestion, elimination, blood pressure, salivation, and sweating. Somatic reflexes involve stimulation of skeletal muscles by the somatic division of the nervous system, and include such reflexes as the stretch reflex and the withdrawal reflex Thompson Educational Publishing, Inc. 3

4 What Is the Reflex Arc? Neurons in our bodies transmit information to each other through a series of neural connections that form a pathway, or circuit. A reflex arc is a simple neural pathway along which an initial sensory stimulus and a corresponding message travel. The stimulus from sensory neurons is sent to the central nervous system (CNS), but there is little or no interpretation of the signal. Few, if any, interneurons are involved. The signal is transmitted to motor neurons, which elicit a response, e.g., a knee jerk Thompson Educational Publishing, Inc. 4

5 The Five Parts to a Reflex Arc The receptor, which receives the initial stimulus (e.g., a pinprick to the skin or a loud noise) The sensory (or afferent) nerve, which carries the impulse to the spinal column or brain The intermediate nerve fibre (the adjustor or interneuron), which interprets the signal and issues an appropriate response The motor (or efferent) nerve, which then carries the response message from the spinal cord to the muscle or organ, and the effector organ itself (e.g., a skeletal muscle), which carries out the response (such as removing the hand or leg away from danger) 2015 Thompson Educational Publishing, Inc. 5

6 The Reflex Arc: The Body s Response to the Threat of Pain 2015 Thompson Educational Publishing, Inc. 6

7 Proprioception and Proprioceptors Proprioception is a person s ability to sense the position, orientation, and movement of the body. Proprioceptors are sensory receptors found in muscles, tendons, joints, and the inner ear that can detect the motion or position of the entire body or a limb by responding to stimuli from within the organism. Muscle spindles and tendon organs are two proprioceptors that continuously monitor muscle actions and are essential components of the neuromuscular system. They tell the nervous system about the state of muscle contraction and allow the ner vous system to respond accordingly Thompson Educational Publishing, Inc. 7

8 Muscle Spindles and the Stretch Reflex Muscle spindles are sensory receptors (i.e., proprioceptors) within a muscle fibre that help to maintain muscle tension and that are sensitive to changes in muscle length (rather than tension). Muscle spindles are involved in the reflex contraction of muscles (the so-called stretch reflex). The usual example of the stretch reflex action is the knee-jerk reflex Thompson Educational Publishing, Inc. 8

9 A Muscle Spindle (Stretch Detector) 2015 Thompson Educational Publishing, Inc. 9

10 The Knee-Jerk Reflex (The Patella Reflex) 2015 Thompson Educational Publishing, Inc. 10

11 The Crossed-Extensor Reflex: A More Complex Reflex Action [CATCH: FIGURE 6.17 ON PAGE 187 THE CROSSED-EXTENSOR REFLEX] 2015 Thompson Educational Publishing, Inc. 60

12 Golgi Tendon Organs and the Tension Reflex Golgi tendon organs are found at the end of muscle fibres that merge into the tendon itself. GTOs detect changes in muscle tension. They help protect the muscle from excessive tension that might damage the muscle, joint, or both. A Golgi tendon organ projects to the motor neurons located within the spinal cord. When the change in tension is detected, an impulse is sent along afferent neurons to the central nervous system (CNS), where they synapse with motor neurons of the same muscle. The efferent neurons instantly transmit an impulse, causing the muscle to relax, thereby preventing injur y Thompson Educational Publishing, Inc. 12

13 A Golgi Tendon Organ (Tension Detector) 2015 Thompson Educational Publishing, Inc. 13