TABLE 2.2 • Muscle contraction and movement matrix Type of contraction (muscle action) Chapter Definitive and Isometric Isotonic Movement without 2 descriptive factors Concentric contraction Eccentric Agonist muscle Dictated solely by gravity length No appreciable Shortening Lengthening and/or external forces change Dictated solely by gravity Antagonist muscle •II• *4 and/or external forces length Dictated solely by gravity No appreciable Lengthening Shortening and/or external forces Joint angle change Consistent with gravity and/ changes INNI •II• or other external forces Direction of No appreciable In direction of applied In direction of exter- Either no motion or passive body part change motion as a result of gray- muscular force nal force (resistance) ity and/or other external Motion forces Against immovable Against gravity With gravity and/or Passive; relaxation Description object or matched and/or other external other external force external force force (resistance) (resistance) No force, all resistance Applied muscle (resistance) force versus Consistent with inertia of resistance Prevents motion; Causes motion Controls motion applied external forces or pressure (force) the speed of gravity Speed relative to applied, but no gravity or applied resulting motion Either zero or acceleration resistance includ- consistent with applied ing inertial forces Static; fixating Dynamic shortening; Dynamic lengthening; external forces positive work negative work (0) Acceleration/ Passive motion by force deceleration Force = resistance Force > resistance Force < resistance from gravity and/or other external forces Descriptive symbol Equal to speed of Faster than the inertia Slower than the speed applied resistance of the resistance of gravity or applied Practical inertial forces application Zero acceleration Acceleration Deceleration 70 11 (=) (+) (-) Prevents external Initiates movement or Slows down the rate forces from causing speeds up the rate of of movement or stops movement movement movement, \"braking action\" to the confusion, a variety of terms and descrip- isokinetics. Isokinetics is not another type of tive phrases are used by different authorities to contraction, as some authorities have mistakenly describe these phenomena. Table 2.2 attempts to described; rather, it is a specific technique that provide an exhaustive explanation of the various may use any or all of the different types of con- types of contraction and resulting joint move- tractions. Isokinetics is a type of dynamic exercise ments. The varying terminology utilized in defin- usually using concentric and/or eccentric muscle ing and describing these actions is included. Ap- contractions in which the speed (or velocity) of pendix 5 provides an algorithm for determining if movement is constant and muscular contraction a muscle or muscle group is contracting and, if so, (ideally, maximum contraction) occurs throughout the type of contraction. the movement. Biodex, Cybex, and other types of apparatuses are engineered to allow this type of Various exercises may use any one or all of exercise. these contraction types for muscle development. Development of exercise machines has resulted Students well educated in kinesiology should in another type of muscle exercise known as be qualified to prescribe exercises and activities www.mhhe.com/floyd18e 43
Chapter for the development of large muscles and of mus- example, the quadriceps muscles are antagonists cle groups in the human body. They should be to the hamstrings in knee flexion. 2 able to read the description of an exercise or ob- serve an exercise and immediately know the most Stabilizers important muscles being used. Terms describing how muscles function in joint movements follow. Stabilizers surround the joint or body part and con- tract to fixate or stabilize the area to enable another Role of muscles limb or body segment to exert force and move. Known as fixators, they are essential in establish- When a muscle contracts, it simply attempts to pull ing a relatively firm base for the more distal joints the bones to which both of its ends are attached to work from when carrying out movements. In a toward each other. Usually this does not happen, biceps curling example, the muscles of the scapula however, because one of the bones is usually more and glenohumeral joint must contract in order to stable than the other. As a result, the less stable maintain the shoulder complex and humerus in a bone moves toward the more stable bone. When a relatively static position so that the biceps brachii muscle that is capable of performing multiple ac- can more effectively perform the curls. tions contracts, it attempts to perform all of its ac- tions unless other forces, such as those provided Synergist by other muscles, prevent the undesired actions. Muscles that assist in the action of an agonist but Agonist FIG. 2.3 are not necessarily prime movers for the action, known as guiding muscles, assist in refined move- Agonist muscles, when contracting concentrically, ment and rule out undesired motion. Synergist cause joint motion through a specified plane of muscles may be either helping synergists or true motion. Any concentrically contracting muscle that synergists. Helping synergists have an action in causes the same joint motion is an agonist for the common but also have actions antagonistic to each motion. However, some muscles, because of their other. They help another muscle move the joint in relative location, size, length, or force generation the desired manner and simultaneously prevent capacity, are able to contribute significantly more undesired actions. An example involves the ante- to the joint movement than other agonists. These rior and posterior deltoid. The anterior deltoid acts muscles are known as prime or primary mov- as an agonist in glenohumeral flexion, while the ers or as muscles most involved. Agonist muscles posterior deltoid acts as an extensor. Helping each that contribute significantly less to the joint motion other, they work in synergy with the middle deltoid are commonly referred to as assisters or assistant to accomplish abduction. True synergists contract movers. Consensus among all authorities regard- to prevent an undesired joint action of the agonist ing which muscles are primary movers and which and have no direct effect on the agonist action. The are weak assistants does not exist in every case. finger flexors are provided true synergy by the wrist This text will emphasize the primary movers. The extensors when one is grasping an object. The fin- remaining agonists or assistants, when listed, will ger flexors originating on the forearm and humerus be referred to as weak contributors to the motion are agonists in both wrist flexion and finger flexion. involved. As an example, the hamstrings (semi- The wrist extensors contract to prevent wrist flexion tendinosus, semimembranosus, biceps femoris) sar- by the finger flexors. The allows the finger flexors to torius, gracilis, popliteus, and gastrocnemius are all utilize more of their force in flexing the fingers. agonists in knee flexion, but most kinesiologists regard only the hamstrings as the prime movers. Neutralizers Antagonist FIG. 2.3 Neutralizers counteract or neutralize the action of other muscles to prevent undesirable movements Antagonist muscles have the opposite concentric such as inappropriate muscle substitutions. They action from agonists. Referred to as contralateral contract to resist specific actions of other muscles. muscles, antagonists are located on the oppo- As an example, when only the supination action site side of the joint from the agonist and work of the biceps brachii is desired, the triceps brachii in cooperation with agonist muscles by relax- contracts to neutralize the flexion action of the ing and allowing movement; but when contract- biceps brachii. ing concentrically, they perform the joint motion opposite to that of the agonist. Using the previous Force couples Force couples occur when two or more forces are pulling in different directions on an object, 44 www.mhhe.com/floyd18e
Middle Chapter trapezius 2 Upward rotation Serratus anterior Lower trapezius AB FIG. 2.4 • Force couples. A, When a person steers with two hands, the hands act as a force couple; B, Two force couples act on the scapula to rotate it upward. The middle trapezius and lower serratus anterior are excellent examples. The middle trapezius and lower trapezius also tend to act as a force couple, although their pulls are not in opposite directions. causing the object to rotate about its axis. Fig. 2.4, hip flexion and knee extension while kicking a A depicts a force couple consisting of one hand ball. In this example, the hamstrings are antago- on each side of a steering wheel. One hand pulls nistic and relax to allow the kick to occur. This the wheel up and to the right, and the other hand does not mean that all other muscles in the hip pulls it down and to the left. Coupling of mus- area are uninvolved. The preciseness of the kick cular forces in the body can result in a more ef- depends on the involvement of many other mus- ficient movement. Figure 2.4, B illustrates a force cles. As the lower extremity swings forward, its couple in which the middle trapezius, lower tra- route and subsequent angle at the point of con- pezius, and serratus anterior all attach at different tact depend on a certain amount of relative con- points on the scapula. Each muscle pulls on the traction or relaxation in the hip abductors, adduc- scapula from a different direction to produce the tors, internal rotators, and external rotators. These combined result of upward rotation. muscles act in a synergistic fashion to guide the lower extremity in a precise manner. That is, they Tying the roles of muscles together are not primarily responsible for knee extension and hip flexion, but they do contribute to the ac- When a muscle with multiple agonist actions con- curacy of the total movement. These guiding mus- tracts, it attempts to perform all its actions. Mus- cles assist in refining the kick and preventing ex- cles cannot determine which actions are appro- traneous motions. Additionally, the muscles in the priate for the task at hand. The resulting actions contralateral hip and pelvic area must be under actually performed depend upon several factors, relative tension to help fixate or stabilize the pel- such as the motor units activated, joint position, vis on that side in order to provide a relatively muscle length, and the relative contraction or re- stable pelvis for the hip flexors on the involved laxation of other muscles acting on the joint. In side to contract against. In kicking the ball, the certain instances, two muscles may work in syn- pectineus and tensor fascia latae are adductors ergy by counteracting their opposing actions to and abductors, respectively, in addition to flexors. accomplish a common action. The actions of adduction and abduction are neu- tralized by each other, and the common action of As discussed, agonist muscles are primarily re- the two muscles results in hip flexion. sponsible for a given movement, such as those of www.mhhe.com/floyd18e 45
Chapter From a practical point of view, it is not essen- Although not available to all students, cadaver dis- tial that individuals know the exact force exerted section of muscles and joints is an excellent way to 2 by each of the elbow flexors—biceps, brachi- further understand muscle action. alis, and brachioradialis—in chinning. It is impor- tant to understand that this muscle group is the For most of the skeletal muscles, palpation agonist or primary mover responsible for elbow is a very useful way to determine muscle action. joint flexion. Similarly, it is important to under- It is done through using the sense of touch to feel or examine a muscle as it contracts. Palpa- stand that these muscles contract concentrically tion is limited to superficial muscles but is helpful when the chin is pulled up to the bar and that in furthering an understanding of joint mechan- they contract eccentrically when the body is low- ics. Models such as long rubber bands may be ered slowly. Antagonistic muscles produce actions used to facilitate understanding of lines of pull opposite those of the agonist. For example, the and to simulate muscle lengthening or shortening muscles that produce extension of the elbow joint as joints move through various ranges of motion. are antagonistic to the muscles that produce flex- ion of the elbow joint. It is important to under- Electromyography (EMG) utilizes either surface stand that specific exercises need to be prescribed electrodes that are placed over the muscle or fine for the development of each antagonistic muscle wire/needle electrodes placed into the muscle. group. The return movement to the hanging po- As the subject then moves the joint and contracts sition at the elbow joint after chinning is elbow the muscles, the EMG unit detects the action po- joint extension, but the triceps and anconeus are tentials of the muscles and provides an electronic not being strengthened. A concentric contraction readout of the contraction intensity and duration. of the elbow joint flexors occurs, followed by an EMG is the most accurate way of detecting the eccentric contraction of the same muscles. presence and extent of muscle activity. Reversal of muscle function Electrical muscle stimulation is somewhat a re- verse approach of electromyography. Instead of A muscle group that is described to perform a electricity being used to detect muscle action, it is given function can contract to control the exact op- used to cause muscle activity. Surface electrodes posite motion. Fig. 2.3, A illustrates how the biceps are placed over a muscle, and then the stimula- is an agonist by contracting concentrically to flex tor causes the muscle to contract. The joint's ac- the elbow. The triceps is an antagonist to elbow tions may then be observed to see the effect of flexion, and the pronator teres is considered to be the muscle's contraction on it. a synergist to the biceps in this example. If the bi- ceps were to slowly lengthen and control elbow Lines of pull FIG. 2.5 extension, as in Fig. 2.3, E, it would still be the agonist, but it would be contracting eccentrically. Combining the knowledge of a particular joint's Fig. 2.3, B illustrates how the triceps is an agonist functional design and diarthrodial classification by contracting concentrically to extend the elbow. with an understanding of the specific location of The biceps is an antagonist to elbow extension in a musculotendinous unit as it crosses a joint is ex- this example. If the triceps were to slowly lengthen tremely helpful in understanding its action on the and control elbow flexion, as in Fig. 2.3, F, it would joint. For example, knowing that the rectus femo- still be the agonist, but it would be eccentrically ris has its origin on the anterior inferior iliac spine contracting. In both of these examples, the deltoid, and its insertion on the tibial tuberosity via the pa- trapezius, and various other shoulder muscles are tella, you can then determine that the muscle must serving as stabilizers of the shoulder area. have an anterior relationship to the knee and hip. Combining this knowledge with the knowledge Determination of muscle action that both joints are capable of sagittal plane move- ments such as flexion/extension, you can then The specific action of a muscle may be determined determine that when the rectus femoris contracts through a variety of methods. These include con- concentrically, it should cause the knee to extend sidering anatomical lines of pull, anatomical dis- and the hip to flex. section, palpation, models, electromyography, and electrical stimulation. Furthermore, knowing that the semitendinosus, semimembranosus, and biceps femoris all origi- With an understanding of a muscle's line of pull nate on the ischial tuberosity and that the semi- relative to a joint, one may determine the mus- tendinosus and semimembranosus cross the knee cle's action at the joint. (See lines of pull below.) posteromedially before inserting on the tibia, but that the biceps femoris crosses the knee postero- 46 www.mhhe.com/floyd18e
Anterior Quadriceps pull must be anterior to the joint. Additionally, you would know that the origin of the brachialis must Rectus femoris Vastus medialis Medial be somewhere on the anterior humerus and the Chapter .• Vastus lateralis 2insertion must be somewhere on the anterior ulna. —Vastus intermedius 0. Sartorius Consider all the following factors and their re- lationships as you study movements of the body Femur Intermuscular to gain a more thorough understanding. septum Biceps femoris 1. Exact locations of bony landmarks to which short head .• Adductor longus muscles attach proximally and distally and their relationship to joints Biceps femoris• Adductor 6revis long head 2. The planes of motion through which a joint is Gracilis capable of moving Lateral Adductor magnus Semimembranosus 3. The muscle's relationship or line of pull rela- tive to the joint's axes of rotation Semitendinosus 4. As a joint moves through a particular range of FIG. 2.5 • Lines of pull in relation to the left motion, the ability of the line of pull of a par- knee. Biceps femoris with a posterolateral ticular muscle to change and even result in the relationship enables it to externally rotate the muscle having a different or opposite action knee; semitendinosus and semimembranosus than in the original position have a posteromedial relationship enabling them to internally rotate the knee; hamstrings (biceps 5. The potential effect of other muscles' relative femoris, semitendinosus, and semimembranosus) contraction or relaxation on a particular mus- all have a posterior relationship enabling them to cle's ability to cause motion flex the knee; quadriceps muscles have an anterior relationship enabling them to extend the knee. 6. The effect of a muscle's relative length on its ability to generate force (See muscle length— laterally before inserting on the fibula head, you tension relationship, p. 58, and active and pas- may determine that all three muscles have pos- sive insufficiency, p. 61.) terior relationships to the hip and knee, which would enable them to be hip extensors and knee 7. The effect of the position of other joints on flexors upon concentric contraction. The specific the ability of a biarticular or multiarticular knowledge related to their distal attachments and muscle to generate force or allow lengthening the knee's ability to rotate when flexed would (See uniarticular, biarticular, and multiarticular allow you to determine that the semitendinosus muscles, p. 61.) and semimembranosus will cause internal rota- tion, whereas the biceps femoris will cause ex- Neural control of voluntary movement ternal rotation. Knowledge that the knee's axes of rotation are only frontal and vertical, but not sag- When we discuss muscular activity, we should re- ittal, enables you to determine that even though ally state it as neuromuscular activity, since muscle the semitendinosus and semimembranosus have cannot be active without nervous innervation. All a posteromedial line of pull and the biceps femo- voluntary movement is a result of the muscular ris has a posterolateral line of pull, they are not and the nervous systems working together. All capable of causing knee adduction and abduc- muscle contraction occurs as a result of stimula- tion, respectively. tion from the nervous system. Ultimately, every muscle fiber is innervated by a somatic motor neu- You can also apply this concept in reverse. For ron, which, when an appropriate stimulus is pro- example, if the only action of a muscle such as vided, results in a muscle contraction. Depending the brachialis is known to be elbow flexion, then upon a variety of factors, this stimulus may be you should be able to determine that its line of processed in varying degrees at different levels of the central nervous system (CNS). The CNS, for the purposes of this discussion, may be divided into five levels of control. Listed in order from the most general level of control and the most supe- riorly located to the most specific level of control and the most inferiorly located, these levels are the cerebral cortex, the basal ganglia, the cerebellum, the brain stem, and the spinal cord. www.mhhe.com/floyd18e 47
Chapter The cerebral cortex, the highest level of inhibition of desired neuromuscular actions and control, provides for the creation of voluntary functions in arousal or maintaining a wakeful state. 2 movement as aggregate muscle action but not as specific muscle activity. Sensory stimuli from the Finally, the spinal cord is the common path- body also are interpreted here, to a degree, for way between the CNS and the peripheral ner- the determination of needed responses. vous system (PNS), which contains all the re- maining nerves throughout the body. It has the At the next level, the basal ganglia control most specific control and integrates various sim- the maintenance of postures and equilibrium and ple and complex spinal reflexes, as well as corti- learned movements such as driving a car. Sensory cal and basal ganglia activity. integration for balance and rhythmic activities is controlled here. Functionally, the PNS can be divided into sensory and motor divisions. The sensory or afferent nerves The cerebellum is a major integrator of sensory bring impulses from receptors in the skin, joints, impulses and provides feedback relative to motion. muscles, and other peripheral aspects of the body to It controls the timing and intensity of muscle activity the CNS, while the motor or efferent nerves carry to assist in the refinement of movements. impulses to the outlying regions of the body. Next, the brain stem integrates all central The spinal nerves, illustrated in Fig. 2.6, also nervous system activity through excitation and provide both motor and sensory function for their Cervical plexus (C1-C4) Atlas (first cervical vertebra) Cervical nerves (8 pairs) Ansa cervicalis Cervical enlargement Lesser occipital nerve First thoracic vertebra Transverse cervical nerve St,praclavicular nerve Phrenic nerve Brachial plexus (C5-T1) Axillary nerve Radial nerve Musculocutaneous nerve Median nerve Ulnar nerve Thoracic nerves (12 pairs) Intercostal (thoracic)nerves Dura mater NEE of spinal cord Lumbar plexus (L1-IA) Lumbar enlargement Iliohypogastric nerve ad/ j Tl 2 llioinguinal nerve mipdre '71•1=1111./116'4-First lumbar vertebra Genitofemoral nerve P; Conus medullaris Lateral femoral cutaneous nerve Lumbar nerves (5 pairs) Femoral nerve Cauda equina Obturator nerve Sacral plexus (L5-54) Ilium Sciatic Common peroneal nerve Sacrum nerve Tibial nerve Sacral nerves (5 pairs) Posterior cutaneous femoral nerve —V S5 Coccygeal nerves (1 pair) Pudendal nerve Filum terminale N — Lumbosacral plexus FIG. 2.6 • Spinal nerve roots and plexuses. 48 www.mhhe.com/floyd18e
respective portions of the body and are named specific anatomical locations in the thorax. All of for the locations from which they exit the verte- bral column. From each side of the spinal column, the lumbar, sacral, and coccygeal nerves form the there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal lumbosacral plexus, which supplies sensation and Chapter nerve. Cervical nerves 1 through 4 form the cervi- motor function to the lower trunk and the entire cal plexus, which is generally responsible for sen- 2 sation from the upper part of the shoulders to the lower extremity and perineum. back of the head and front of the neck. The cervi- cal plexus supplies motor innervation to several One aspect of the sensory function of spinal muscles of the neck. Cervical nerves 5 through 8, along with thoracic nerve 1, form the brachial nerves is to provide feedback to the CNS regard- plexus, which supplies motor and sensory func- tion to the upper extremity and most of the scap- ing skin sensation. A defined area of skin sup- ula. Thoracic nerves 2 through 12 run directly to plied by a specific spinal nerve is known as a dermatome (Fig. 2.7). Regarding motor function of spinal nerves, a myotome is defined as a mus- cle or group of muscles supplied by a specific spi- nal nerve. Certain spinal nerves are also responsi- ble for reflexes. Table 2.3 summarizes the specific spinal nerve functions. Functions Cervical Head movement C6 nerves Diaphragm CB movement Thoracic nerves Neck and shoulder movement Lumbar nerves Upper limb 8 movement Sacral T1 nerves 2 3 4 5 6 Rib movement in breathing, 7 vertebral column movement, and 8 tone in postural 9 back muscles 10 11 12 —Hip movement L1 24 34 44 /ilk Lower limb movement B Coccygeal nerves A Posterior view FIG. 2.7 • Spinal cord and dermatomal map. A, Nerves and functions of the spinal cord (regions color-coded); B, Letters and numbers indicate the spinal nerves innervating a given region of skin. www.mhhe.com/floyd18e 49
TABLE 2.3 • Spinal nerve root dermatomes, myotomes, reflexes, and functional applications Chapter Nerve Dermatome afferent Myotome efferent Reflexes Functional application root (sensory) (motor) None 2 None Capital flexion and extension Cl Touch: Vertex of skull Upper neck muscles Sensation behind the ear and Touch: Temple, forehead, Upper neck muscles posterior skull occiput Capital and upper cervical C2 movements Cervical plexus Touch: Entire neck, Trapezius, splenius, None Scapula retraction, neck None extension posterior cheek, temporal capitis Sensation to cheek and side C3 area, under mandible of neck Touch: Shoulder area, Trapezius, levator Scapula retraction and clavicular area, upper scapulae elevation C4 scapular area Sensation to clavicle and upper scapula Touch: Deltoid area, Supraspinatus, Biceps brachii Shoulder abduction C5 anterior aspect of entire infraspinatus, deltoid, Sensation to lateral side of biceps brachii arm and elbow arm to base of thumb Touch: Anterior arm, radial Biceps, supinator, wrist Biceps brachii, Elbow flexion, wrist extension Sensation to lateral side of side of hand to thumb extensors brachioradialis forearm including thumb and C6 and index finger index fingers Touch: Lateral arm and Triceps brachii, wrist Triceps brachii Elbow extension, wrist flexion C7 forearm to index, long, flexors Sensation to middle of anterior forearm and long finger and ring fingers Brachial plexus Touch: Medial side of Ulnar deviators, thumb None Wrist ulnar deviation, thumb forearm to ring and extensors, thumb extension C8 little fingers adductors (rarely triceps) Sensation to posterior elbow and medial forearm to little Touch: Medial arm and Intrinsic muscles of None fingers forearm to wrist the hand except for T1 opponens pollicis and Abduction and adduction of abductor pollicis brevis fingers Sensation to medial arm and elbow Touch: Medial side of Intercostal muscles None Sensation to medial upper upper arm to medial elbow, None arm, upper chest, and T71 midscapular area pectoral and midscapular areas Sensation to chest, abdomen, and low back Touch: T3-T6, upper Intercostal muscles, T3- thorax; T5-T7, coastal abdominal muscles T12 margin; T8-T12, abdomen and lumbar region The basic functional units of the nervous sys- which transmit impulses to the neuron and cell tem responsible for generating and transmitting body; and an axon, an elongated projection that impulses are nerve cells known as neurons. transmits impulses away from neuron cell bodies. Neurons consist of a neuron cell body; one or As shown in Fig. 2.8, neurons are classified into more branching projections known as dendrites, three types, according to the direction in which 50 www.mhhe.com/floyd18e
TABLE 2.3 (continued) • Spinal nerve root dermatomes, myotomes, reflexes, and functional applications Nerve Dermatome afferent Myotome efferent Reflexes Functional application Chapter root (sensory) (motor) None Sensation to low back, over 2 Touch: Lower abdomen, Quadratus lumborum None trochanter and groin groin, lumbar region from Ll 2nd to 4th vertebrae, Iliopsoas, quadriceps Patella or knee Hip flexion upper and outer aspect of extensors Sensation to back, front of buttocks Psoas, quadriceps thigh to knee I Touch: Lower lumbar Hip flexion and knee region, upper buttock, extension Sensation to back, upper A anterior aspect of thigh buttock, anterior thigh and knee, medial lower leg Touch: Medial aspect of Ankle dorsiflexion, thigh to knee, anterior transverse tarsal/subtalar inversion L3 aspect of lower 1/3 of the Sensation to medial buttock, thigh to just below patella lateral thigh, medial leg, dorsum of foot, great toe Touch: Medial aspect of Tibialis anterior, extensor Patella or knee lower leg and foot, inner hallucis and digitorum extensors Great toe extension, transverse border of foot, great toe longus, peroneals tarsal/subtalar eversion L4 Sensation to upper lateral leg, anterior surface of the Lumbosacral plexus Touch: lateral border of Extensor hallucis and dig- None lower leg, middle three toes leg, anterior surface of itorum longus, peroneals, L5 lower leg, top of foot to gluteus maximus and Ankle plantarflexion, knee middle three toes medius, dorsiflexors flexion, transverse tarsal/ subtalar eversion • Touch: Posterior aspect of Gastrocnemius, soleus, Achilles reflex Sensation to lateral leg, lateral the lower 1/4 of the leg, gluteus maximus and foot, lateral two toes, plantar posterior aspect of the foot, medius, hamstrings, aspect of foot peroneals S1 including the heel, lateral Ankle plantarflexion and toe border of the foot and sole flexion Sensation to posterior thigh Touch: Posterior central Gastrocnemius, soleus, None and upper posterior leg strip of the leg from below gluteus maximus, Sensation to groin and S2 thegluteal fold to 3/4 of adductor region hamstrings Urinary and bowel control the way down the leg Sensation to saddle area, genitals, anus Touch: Groin, medial thigh Intrinsic foot muscles None to knee Bladder, rectum None Touch: Perineum, genitals, lower sacrum they transmit impulses. Sensory neurons trans- Proprioception and kinesthesis mit impulses to the spinal cord and brain from all parts of the body, whereas motor neurons trans- The performance of various activities is signifi- mit impulses away from the brain and spinal cord cantly dependent upon neurological feedback to muscle and glandular tissue. Interneurons are from the body. Very simply, we use the various central or connecting neurons that conduct im- senses to determine a response to our environ- pulses from sensory neurons to motor neurons. ment, as when we use sight to know when to lift www.mhhe.com/floyd18e 51
Cell body Dendrite feedback relative to the tension, length, and con- traction state of muscle, the position of the body Chapter Direction of Node of Ranvier and limbs, and movements of the joints. These conduction (neurofibril node) proprioceptors in combination with the other 2 sense organs of the body are vital in kinesthesis, ri Myelin sheath the conscious awareness of the position and A movement of the body in space. For example, if Axon standing on one leg with the other knee flexed, you do not have to look at your non—weight- Axon bearing leg to know the approximate number of terminal degrees that you may have it flexed. The proprio- ceptors in and around the knee provide informa- Muscle tion so that you are kinesthetically aware of your knee position. Muscle spindles and Golgi tendon B Cell body organs (GTO) are proprioceptors specific to the Dendrite Axon Direction of muscles, whereas Meissner's corpuscles, Ruffini's corpuscles, Pacinian corpuscles, and Krause's No\\nduction end-bulbs are proprioceptors specific to the joints Sensory and skin. receptor While kinesthesis is concerned with the con- Myelin sheath scious awareness of the body's position, proprio- ception is the subconscious mechanism by which Cell body Skin the body is able to regulate posture and move- Axon ment by responding to stimuli originating in the proprioceptors imbedded in the joints, tendons, C muscles, and inner ear. When we unexpectedly step on an unlevel or unstable surface, if we have FIG. 2.8 • Neuron anatomy. A, Motor neuron. good proprioception the muscles in and about Note the branched dendrites and the single long our lower extremity may respond very quickly by axon, which branches only near its tip; B, Sensory contracting appropriately to prevent a fall or in- neuron with dendritelike structures projecting from jury. This protective response of the body occurs the peripheral end of the axon; C, Interneuron without our having time to make a conscious de- (from the cortex of the cerebellum) with very highly cision about how to respond. branched dendrites. Muscle spindles (Fig. 2.9), concentrated pri- our hand to catch a fly ball. We are familiar with marily in the muscle belly between the fibers, are the senses of smell, touch, sight, hearing, and sensitive to stretch and rate of stretch. Specifically, taste. We are also aware of other sensations, such they insert into the connective tissue within the as pain, pressure, heat, and cold, but we often muscle and run parallel with the muscle fibers. take for granted the sensory feedback provided The number of spindles in a particular muscle var- by proprioceptors during neuromuscular activity. ies depending upon the level of control needed Proprioceptors are internal receptors located in for the area. Consequently, the concentration of the skin, joints, muscles, and tendons that provide muscle spindles in the hands is much greater than in the thigh. 52 www.mhhe.com/floyd18e When rapid stretch occurs, an impulse is sent to the CNS. The CNS then activates the motor neurons of the muscle and causes it to contract. All muscles possess this myotatic or stretch reflex, but it is most remarkable in the extensor muscles of the ex- tremities. The knee jerk or patellar tendon reflex is an example, as shown in Fig. 2.10. When the reflex hammer strikes the patellar tendon, it causes a quick stretch of the musculotendinous unit of the quadriceps. In response, the quadriceps fires and
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