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Chapter Opener 13 © 2013 Pearson Education, Inc.
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Figure 13.1 Place of the PNS in the structural organization of the nervous system.
Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division © 2013 Pearson Education, Inc.
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Table 13.1 General Sensory Receptors Classified by Structure and Function (1 of 3)
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Table 13.1 General Sensory Receptors Classified by Structure and Function (2 of 3)
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Table 13.1 General Sensory Receptors Classified by Structure and Function (3 of 3)
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Thalamus Cerebellum Pons Medulla Spinal cord
Figure Three basic levels of neural integration in sensory systems. Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular formation Cerebellum Pons Circuit level (processing in ascending pathways) 2 Medulla Spinal cord Free nerve endings (pain, cold, warmth) Muscle spindle Receptor level (sensory reception and transmission to CNS) 1 Joint kinesthetic receptor © 2013 Pearson Education, Inc.
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Figure 13.3 Map of referred pain.
Lungs and diaphragm Heart Gallbladder Liver Appendix Stomach Pancreas Small intestine Ovaries Colon Kidneys Urinary bladder Ureters © 2013 Pearson Education, Inc.
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Figure 13.4 Structure of a nerve.
Endoneurium Perineurium Nerve fibers Blood vessel Fascicle Epineurium (a) Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels (b) © 2013 Pearson Education, Inc.
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Figure 13.4a Structure of a nerve.
Endoneurium Perineurium Nerve fibers Blood vessel Fascicle Epineurium © 2013 Pearson Education, Inc.
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Figure 13.4b Structure of a nerve.
Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels © 2013 Pearson Education, Inc.
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Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve.
Endoneurium Schwann cells The axon becomes fragmented at the injury site. 1 Droplets of myelin Fragmented axon Site of nerve damage Macrophages clean out the dead axon distal to the injury. Schwann cell Macrophage 2 Aligning Schwann cells form regeneration tube Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. 3 Fine axon sprouts or filaments Schwann cell New myelin sheath forming The axon regenerates and a new myelin sheath forms. 4 Single enlarging axon filament © 2013 Pearson Education, Inc.
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Endoneurium Schwann cells 1
Figure Regeneration of a nerve fiber in a peripheral nerve. (1 of 4) Endoneurium Schwann cells The axon becomes fragmented at the injury site. 1 Droplets of myelin Fragmented axon Site of nerve damage © 2013 Pearson Education, Inc.
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2 Schwann cell Macrophage
Figure Regeneration of a nerve fiber in a peripheral nerve. (2 of 4) Macrophages clean out the dead axon distal to the injury. 2 Schwann cell Macrophage © 2013 Pearson Education, Inc.
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Aligning Schwann cells form regeneration tube Axon sprouts,
Figure Regeneration of a nerve fiber in a peripheral nerve. (3 of 4) 3 Aligning Schwann cells form regeneration tube Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Fine axon sprouts or filaments © 2013 Pearson Education, Inc.
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The axon 4 Schwann cell New myelin regenerates and a sheath forming
Figure Regeneration of a nerve fiber in a peripheral nerve. (4 of 4) Schwann cell New myelin sheath forming The axon regenerates and a new myelin sheath forms. 4 Single enlarging axon filament © 2013 Pearson Education, Inc.
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Figure 13.6 Location and function of cranial nerves.
Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Temporal lobe Optic chiasma Optic tract Oculomotor nerve (III) Infundibulum Facial nerve (VII) Trochlear nerve (IV) Trigeminal nerve (V) Vestibulocochlear nerve (VIII) Abducens nerve (VI) Glossopharyngeal nerve (IX) Vagus nerve (X) Cerebellum Accessory nerve (XI) Medulla oblongata Hypoglossal nerve (XII) Cranial nerves I – VI Sensory function Motor function PS* fibers Cranial nerves VII – XII Sensory function Motor function PS* fibers I II III IV V VI Olfactory Optic Oculomotor Trochlear Trigeminal Abducens Yes (smell) Yes (vision) No Yes (general sensation) No Yes No Yes VII VIII IX X XI XII Facial Vestibulocochlear Glossopharyngeal Vagus Accessory Hypoglossal Yes (taste) Yes (hearing and balance) No Yes Some Yes No *PS = parasympathetic © 2013 Pearson Education, Inc.
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Figure 13.6a Location and function of cranial nerves.
Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Temporal lobe Optic chiasma Optic tract Oculomotor nerve (III) Infundibulum Facial nerve (VII) Trochlear nerve (IV) Vestibulocochlear nerve (VIII) Trigeminal nerve (V) Glossopharyngeal nerve (IX) Abducens nerve (VI) Vagus nerve (X) Cerebellum Accessory nerve (XI) Medulla oblongata Hypoglossal nerve (XII) © 2013 Pearson Education, Inc.
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Figure 13.6b Location and function of cranial nerves.
I – VI Sensory function Motor function PS* fibers Cranial nerves VII – XII Sensory function Motor function PS* fibers I II III IV V VI Olfactory Optic Oculomotor Trochlear Trigeminal Abducens Yes (smell) Yes (vision) No Yes (general sensation) No Yes No Yes VII VIII IX X XI XII Facial Vestibulocochlear Glossopharyngeal Vagus Accessory Hypoglossal Yes (taste) Yes (hearing and balance) No Yes Some Yes No *PS = parasympathetic © 2013 Pearson Education, Inc.
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Table 13.2 Cranial Nerves (1 of 14)
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Table 13.2 Cranial Nerves (2 of 14)
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Table 13.2 Cranial Nerves (3 of 14)
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Table 13.2 Cranial Nerves (4 of 14)
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Table 13.2 Cranial Nerves (5 of 14)
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Table 13.2 Cranial Nerves (6 of 14)
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Table 13.2 Cranial Nerves (7 of 14)
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Table 13.2 Cranial Nerves (8 of 14)
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Table 13.2 Cranial Nerves (9 of 14)
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Table 13.2 Cranial Nerves (10 of 14)
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Table 13.2 Cranial Nerves (11 of 14)
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Table 13.2 Cranial Nerves (12 of 14)
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Table 13.2 Cranial Nerves (13 of 14)
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Table 13.2 Cranial Nerves (14 of 14)
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Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical
Figure Spinal nerves. Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical enlargement Intercostal nerves Thoracic nerves T1 – T12 Lumbar enlargement Lumbar plexus Lumbar nerves L1 – L5 Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve Co1 © 2013 Pearson Education, Inc.
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Figure 13.8 Formation of spinal nerves and rami distribution.
Gray matter White matter Ventral root Dorsal and ventral rootlets of spinal nerve Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve. Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Intercostal nerve Sympathetic trunk ganglion Dorsal root ganglion Dorsal root Ventral root Branches of intercostal nerve Lateral cutaneous Anterior cutaneous Sternum Cross section of thorax showing the main roots and branches of a spinal nerve. © 2013 Pearson Education, Inc.
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Figure 13.8a Formation of spinal nerves and rami distribution.
Gray matter White matter Dorsal and ventral rootlets of spinal nerve Ventral root Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve. © 2013 Pearson Education, Inc.
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Figure 13.8b Formation of spinal nerves and rami distribution.
Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Intercostal nerve Sympathetic trunk ganglion Dorsal root ganglion Dorsal root Ventral root Branches of intercostal nerve Lateral cutaneous Anterior cutaneous Sternum Cross section of thorax showing the main roots and branches of a spinal nerve. © 2013 Pearson Education, Inc.
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Figure 13.9 The cervical plexus.
Ventral rami Segmental branches Hypoglossal nerve (XII) Ventral rami: Lesser occipital nerve C1 Greater auricular nerve C2 Transverse cervical nerve C3 Ansa cervicalis C4 Accessory nerve (XI) C5 Phrenic nerve Supraclavicular nerves © 2013 Pearson Education, Inc.
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Table 13.3 Branches of the Cervical Plexus
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Figure 13.10 The brachial plexus.
Anterior divisions Posterior divisions Trunks Roots Roots (ventral rami): Axillary nerve C4 Dorsal scapular C5 Nerve to subclavius Suprascapular C6 Upper Posterior divisions C7 Middle Trunks Lateral C8 Lower Cords Posterior T1 Long thoracic Medial Medial pectoral Humerus Lateral pectoral Axillary Upper subscapular Radial nerve Musculo- cutaneous Lower subscapular Musculo cutaneous nerve Radial Thoracodorsal Median Medial cutaneous nerves of the arm and forearm Ulnar Ulna Radius Roots (rami C5–T1), trunks, divisions, and cords Ulnar nerve Median nerve Radial nerve (superficial branch) Major terminal branches (peripheral nerves) Roots (ventral rami) Cords Divisions Trunks Dorsal branch of ulnar nerve Musculocutaneous Anterior Upper C5 Superficial branch of ulnar nerve Lateral Posterior Median C6 Digital branch of ulnar nerve Medial Anterior Ulnar Middle C7 Posterior Muscular branch Radial C8 Posterior Anterior Median nerve Axillary Lower T1 Digital branch Posterior Flowchart summarizing relationships within the brachial plexus The major nerves of the upper limb Musculocutaneous nerve Lateral cord Posterior cord Axillary nerve Medial cord Radial nerve Median nerve Ulnar nerve © 2013 Pearson Education, Inc. Cadaver photo
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Figure 13.10a The brachial plexus.
Anterior divisions Posterior divisions Trunks Roots Roots (ventral rami): C4 Dorsal scapular C5 Nerve to subclavius C6 Suprascapular Upper Posterior divisions C7 Middle Trunks C8 Lateral Lower Cords T1 Posterior Long thoracic Medial Medial pectoral Lateral pectoral Axillary Upper subscapular Musculo- cutaneous Lower subscapular Thoracodorsal Radial Medial cutaneous nerves of the arm and forearm Median Ulnar Roots (rami C5–T1), trunks, divisions, and cords © 2013 Pearson Education, Inc.
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Figure 13.10b The brachial plexus.
Major terminal branches (peripheral nerves) Roots (ventral rami) Cords Divisions Trunks Anterior Musculocutaneous C5 Upper Lateral Posterior Median C6 Medial Anterior Ulnar Middle C7 Posterior Radial C8 Posterior Anterior Axillary Lower T1 Posterior Flowchart summarizing relationships within the brachial plexus © 2013 Pearson Education, Inc.
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Figure 13.10c The brachial plexus.
Axillary nerve Humerus Radial nerve Musculocutaneous nerve Ulna Radius Ulnar nerve Median nerve Radial nerve (superficial branch) Dorsal branch of ulnar nerve Superficial branch of ulnar nerve Digital branch of ulnar nerve Muscular branch Median nerve Digital branch The major nerves of the upper limb © 2013 Pearson Education, Inc.
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Figure 13.10d The brachial plexus.
Musculocutaneous nerve Lateral cord Posterior cord Axillary nerve Medial cord Radial nerve Median nerve Ulnar nerve Cadaver photo © 2013 Pearson Education, Inc.
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Table 13.4 Branches of the Brachial Plexus
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Figure 13.11 The lumbar plexus.
Ventral rami Ventral rami: L1 Iliohypogastric Ilioinguinal L2 Iliohypogastric Femoral Lateral femoral cutaneous Ilioinguinal L3 Genitofemoral Obturator Lateral femoral cutaneous Anterior femoral cutaneous L4 Saphenous Obturator L5 Femoral Lumbosacral trunk Ventral rami and major branches of the lumbar plexus Distribution of the major nerves from the lumbar plexus to the lower limb © 2013 Pearson Education, Inc.
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Figure 13.11a The lumbar plexus.
Ventral rami Ventral rami: L1 L2 Iliohypogastric Ilioinguinal L3 Genitofemoral Lateral femoral cutaneous L4 Obturator L5 Femoral Lumbosacral trunk Ventral rami and major branches of the lumbar plexus © 2013 Pearson Education, Inc.
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Figure 13.11b The lumbar plexus.
Iliohypogastric Ilioinguinal Femoral Lateral femoral cutaneous Obturator Anterior femoral cutaneous Saphenous Distribution of the major nerves from the lumbar plexus to the lower limb © 2013 Pearson Education, Inc.
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Table 13.5 Branches of the Lumbar Plexus
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Figure 13.12 The sacral plexus.
Ventral rami Ventral rami: L4 Superior gluteal Superior gluteal L5 Inferior gluteal Lumbosacral trunk Pudendal S1 Inferior gluteal Sciatic Posterior femoral cutaneous S2 Common fibular Tibial S3 Common fibular Posterior Femoral cutaneous Tibial Sural (cut) S4 Pudendal Deep fibular S5 Sciatic Co1 Superficial fibular Ventral rami and major branches of the sacral plexus Gluteus maximus Plantar branches Piriformis Inferior gluteal nerve Distribution of the major nerves from the sacral plexus to the lower limb Common fibular nerve Tibial nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Cadaver photo © 2013 Pearson Education, Inc.
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Figure 13.12a The sacral plexus.
Ventral rami Ventral rami: L4 Superior gluteal L5 Lumbosacral trunk S1 Inferior gluteal S2 Common fibular Tibial S3 Posterior femoral cutaneous S4 Pudendal S5 Sciatic Co1 Ventral rami and major branches of the sacral plexus © 2013 Pearson Education, Inc.
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Figure 13.12b The sacral plexus.
Superior gluteal Inferior gluteal Pudendal Sciatic Posterior femoral cutaneous Common fibular Tibial Sural (cut) Deep fibular Superficial fibular Plantar branches Distribution of the major nerves from the sacral plexus to the lower limb © 2013 Pearson Education, Inc.
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Figure 13.12c The sacral plexus.
Gluteus maximus Piriformis Inferior gluteal nerve Common fibular nerve Tibial nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Cadaver photo © 2013 Pearson Education, Inc.
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Table 13.6 Branches of the Sacral Plexus
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Figure 13.13 Map of dermatomes.
C2 C3 C2 C4 C3 C5 C6 C4 C7 C8 C5 T1 T2 C5 T1 T2 T3 T3 T4 T2 T5 T4 T2 T6 T5 T7 T6 T8 T9 T7 T10 C5 T8 C5 C6 C6 T11 T9 C7 T12 C7 C6 T10 C6 S1 L1 C8 L2 T11 L3 C8 L4 S2 S3 L5 C6 L1 T12 L1 C6 S4 C7 S2 C7 S5 C8 S3 C8 L2 L2 S1 S2 S2 S1 L3 L3 L1 L5 L2 L5 L4 L4 L3 L5 L5 L4 S1 S1 L4 L4 L5 L5 S1 Anterior view Posterior view © 2013 Pearson Education, Inc.
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Figure 13.13a Map of dermatomes.
C2 C3 C4 C5 T1 T2 T3 T2 T4 T2 T5 T6 T7 T8 C5 T9 C5 T10 C6 C6 T11 T12 C6 L1 L1 C6 C7 S2 C7 C8 S3 C8 L2 L2 L3 L3 L4 L4 L5 L5 S1 S1 Anterior view © 2013 Pearson Education, Inc.
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Figure 13.13b Map of dermatomes.
C2 C3 C4 C5 C6 C7 C8 T1 T2 C5 T3 T4 T5 T6 T7 T8 T9 T10 C6 C6 T11 C7 T12 C7 S1 L1 C8 L2 L3 C8 L4 S2 S3 L5 S4 S5 S1 S2 S2 S1 L1 L5 L2 L5 L3 L4 L4 L4 L5 L5 S1 Posterior view © 2013 Pearson Education, Inc.
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Figure 13.14 Hierarchy of motor control.
Precommand level Precommand Level (highest) • Cerebellum • Cerebellum and basal nuclei • Basal nuclei • Programs and instructions (modified by feedback) Projection Level (middle) • Motor cortex (pyramidal pathways) and brain stem nuclei (vestibular, red, reticular formation, etc.) • Conveys instructions to spinal cord motor neurons and sends a copy of that information to higher levels Projection level • Primary motor cortex • Brain stem nuclei Segmental Level (lowest) • Spinal cord Segmental level • Contains central pattern generators (CPGs) • Spinal cord Structures involved Sensory input Reflex activity Motor output Levels of motor control and their interactions © 2013 Pearson Education, Inc.
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Figure 13.14a Hierarchy of motor control.
Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by feedback) Projection Level (middle) • Motor cortex (pyramidal pathways) and brain stem nuclei (vestibular, red, reticular formation, etc.) • Conveys instructions to spinal cord motor neurons and sends a copy of that information to higher levels Segmental Level (lowest) • Spinal cord • Contains central pattern generators (CPGs) Sensory input Reflex activity Motor output Levels of motor control and their interactions © 2013 Pearson Education, Inc.
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Figure 13.14b Hierarchy of motor control.
Precommand level • Cerebellum • Basal nuclei Projection level • Primary motor cortex • Brain stem nuclei Segmental level • Spinal cord Structures involved © 2013 Pearson Education, Inc.
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Figure 13.15 The five basic components of all reflex arcs.
Stimulus Skin Interneuron 1 Receptor 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross scetion) © 2013 Pearson Education, Inc.
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Figure 13.16 Anatomy of the muscle spindle and tendon organ.
Flower spray endings (secondary sensory endings) Efferent (motor) fiber to muscle spindle Anulo- spiral endings (primary sensory endings) Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Muscle spindle Intrafusal muscle fibers Capsule (connective tissue) Sensory fiber Tendon organ Tendon © 2013 Pearson Education, Inc.
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Figure 13.17 Operation of the muscle spindle.
How muscle stretch is detected The purpose of α-γ coactivation Muscle spindle Intrafusal muscle fiber Sensory fiber Extrafusal muscle fiber Time Time Time Time Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory fiber. Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. If only α motor neurons were activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes. But normally α-γ coactivation occurs. Both extrafusal and intrafusal muscle fibers contract. Tension is maintained in the muscle spindle and it can still signal changes in length. © 2013 Pearson Education, Inc.
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Figure 13.17a Operation of the muscle spindle. (1 of 2)
How muscle stretch is detected Muscle spindle Intrafusal muscle fiber Sensory fiber Extrafusal muscle fiber Time Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory fiber. © 2013 Pearson Education, Inc.
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Figure 13.17a Operation of the muscle spindle. (2 of 2)
How muscle stretch is detected Time Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. © 2013 Pearson Education, Inc.
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Figure 13.17b Operation of the muscle spindle. (1 of 2)
The purpose of α-γ coactivation Time If only α motor neurons were activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes. © 2013 Pearson Education, Inc.
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Figure 13.17b Operation of the muscle spindle. (2 of 2)
The purpose of αγ coactivation Time But normally α-γ coactivation occurs. Both extrafusal and intrafusal muscle fibers contract. Tension is maintained in the muscle spindle and it can still signal changes in length. © 2013 Pearson Education, Inc.
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Figure 13.18 Stretch Reflex © 2013 Pearson Education, Inc.
The events by which muscle stretch is damped The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Sensory fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 2 When stretch activates muscle spindles, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. 1 Sensory neuron Cell body of sensory neuron + Initial stimulus (muscle stretch) + – Spinal cord Muscle spindle Antagonist muscle Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). 3b The patellar (knee-jerk) reflex—an example of a stretch reflex 2 Quadriceps (extensors) + 3a 3b + 3b – 1 + + Patella Muscle spindle Spinal cord (L2–L4) Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. 1 1 Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 2 The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a 3a The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b 3b + Excitatory synapse – Inhibitory synapse © 2013 Pearson Education, Inc.
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Initial stimulus The events by which muscle stretch is damped
Figure Stretched muscle spindles initiate reflex, causing contraction of the stretched muscle and inhibition of its antagonist (1 of 2) The events by which muscle stretch is damped The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Sensory fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 2 When stretch activates muscle spindles, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. 1 Sensory neuron Cell body of sensory neuron + + Initial stimulus (muscle stretch) – Spinal cord Muscle spindle Antagonist muscle Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). 3b © 2013 Pearson Education, Inc.
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The patellar (knee-jerk) reflex—an example of a stretch reflex
Figure Stretched muscle spindles initiate reflex, causing contraction of the stretched muscle and inhibition of its antagonist (2 of 2) The patellar (knee-jerk) reflex—an example of a stretch reflex 2 + Quadriceps (extensors) 3a 3b 3b + – 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + Excitatory synapse – Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b © 2013 Pearson Education, Inc.
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Figure 13.19 The tendon reflex.
Afferent fibers synapse with interneurons in the spinal cord. 2 Quadriceps strongly contracts.Tendon organs are activated. 1 + + Interneurons Quadriceps (extensors) – + Spinal cord Tendon organ Hamstrings (flexors) 3a 3b Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse – Inhibitory synapse © 2013 Pearson Education, Inc.
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Figure 13.20 The crossed-extensor reflex.
+ Excitatory synapse Interneurons – Inhibitory synapse + + – + + – Afferent fiber Efferent fibers Efferent fibers Extensor inhibited Flexor inhibited Flexes Flexor stimulated Arm movements Extensor stimulated Extends Site of stimulus: A noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. © 2013 Pearson Education, Inc.
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