Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Marieb Chapter 13 Part C Copyright © 2010 Pearson Education, Inc. Innervation of Skeletal Muscle • Takes place at a neuromuscular junction • Neurotransmitter ? • NT binds to receptors, resulting in: • Movement of Na+ and K+ across the membrane • Depolarization of the muscle cell • An graded potential, which triggers an action potential Copyright © 2010 Pearson Education, Inc. Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus 1 Action potential arrives at axon terminal of motor neuron. Sarcolemma of the muscle fiber 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Ca2+ Ca2+ 3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Axon terminal of motor neuron ACh Copyright © 2010 Pearson Education, Inc. Junctional folds of sarcolemma Sarcoplasm of muscle fiber Na+ 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Mitochondrion Synaptic cleft Fusing synaptic vesicles 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. Synaptic vesicle containing ACh ACh K+ Degraded ACh Na+ Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. K+ Acetylcholinesterase Figure 9.8 Innervation of Visceral Muscle and Glands • Autonomic motor endings and visceral effectors are simpler than somatic junctions • Branches form synapses via varicosities • Acetylcholine and norepinephrine act indirectly via second messengers • Visceral motor responses are slower than somatic responses Copyright © 2010 Pearson Education, Inc. Varicosities Autonomic nerve fibers innervate most smooth muscle fibers. Smooth muscle cell Synaptic vesicles Copyright © 2010 Pearson Education, Inc. Mitochondrion Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction). Figure 9.27 Reflexes • Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus • Learned (acquired) reflexes result from practice or repetition, • Example: Copyright © 2010 Pearson Education, Inc. Reflex Arc • Components of a reflex arc (neural path) —site of stimulus action 1. —transmits afferent impulses to the CNS 2. 3. —either monosynaptic or polysynaptic region within the CNS 4. —conducts efferent impulses from the integration center to an effector organ 5. —muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting What is this an example of? Copyright © 2010 Pearson Education, Inc. Stimulus Skin 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Copyright © 2010 Pearson Education, Inc. Figure 13.14 Spinal Reflexes • Spinal somatic reflexes • Integration center is in the spinal cord • Effectors are skeletal muscle • Why do they exist? Copyright © 2010 Pearson Education, Inc. Stretch and Golgi Tendon Reflexes • For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary • tell muscle length • tell muscle and tendon tension Copyright © 2010 Pearson Education, Inc. Secondary sensory endings Efferent (motor) fiber to muscle spindle Primary sensory endings Muscle spindle Intrafusal muscle fibers Sensory fiber Golgi tendon organ Copyright © 2010 Pearson Education, Inc. Tendon Figure 13.15 Muscle Spindles • Excited by stretch of muscle and muscle spindle • Stretch causes an increased rate of impulses in sensory fibers • Motor fibers then cause muscle contraction • What kind of feedback? Copyright © 2010 Pearson Education, Inc. Stretch Reflexes • Maintain muscle tone in large postural muscles • Cause muscle contraction in response to increased muscle length (stretch) Copyright © 2010 Pearson Education, Inc. Stretch Reflexes • How a stretch reflex works: • Stretch activates the muscle spindle • Sensory neurons synapse directly with motor neurons in the spinal cord • Motor neurons cause the stretched muscle to contract • All stretch reflexes are monosynaptic and ipsilateral Copyright © 2010 Pearson Education, Inc. Stretch Reflexes • Reciprocal inhibition also occurs—Sensory fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles • Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax • Can you think of another example? Copyright © 2010 Pearson Education, Inc. Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. The events by which muscle stretch is damped 1 When muscle spindles are activated 2 The sensory neurons synapse directly with motor neurons (red), which excite the stretched by stretch, the associated sensory muscle. Afferent fibers alsosynapse with neurons (blue) transmit afferent impulses interneurons (green) that inhibit motor at higher frequency to the spinal cord. neurons (purple) controlling antagonistic muscles. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 2 The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 3a 3b 3b Patella Muscle spindle Spinal cord (L2–L4) Hamstrings (flexors) Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 3a The motor neurons (red) send + – Excitatory synapse Inhibitory synapse activating impulses to the quadriceps causing it to contract, extending the knee. 3b The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2) Golgi Tendon Reflexes • Polysynaptic reflexes • Help to prevent damage due to excessive stretch • Important for smooth onset and termination of muscle contraction Copyright © 2010 Pearson Education, Inc. Golgi Tendon Reflexes • Too much contraction? • Contraction activates Golgi tendon organs • Afferent impulses are transmitted to spinal cord • Contracting muscle relaxes and the antagonist contracts (reciprocal activation) • Information transmitted simultaneously to the cerebellum is used to adjust muscle tension Copyright © 2010 Pearson Education, Inc. 1 Quadriceps strongly 2 Afferent fibers synapse contracts. Golgi tendon organs are activated. with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc. 3a Efferent impulses 3b Efferent to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. impulses to antagonist muscle cause it to contract. Figure 13.18 Flexor and Crossed-Extensor Reflexes • Flexor (withdrawal) reflex • Initiated by a painful stimulus • Causes automatic withdrawal of the threatened body part • Ipsilateral and polysynaptic Copyright © 2010 Pearson Education, Inc. Flexor and Crossed-Extensor Reflexes • Crossed extensor reflex • Occurs with flexor reflexes in weight-bearing limbs to maintain balance • Consists of an ipsilateral flexor reflex and a contralateral extensor reflex • The stimulated side is withdrawn (flexed) • The contralateral side is extended Copyright © 2010 Pearson Education, Inc. + Excitatory synapse – Inhibitory synapse Let’s Combine The Two! Interneurons Efferent fibers Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Copyright © 2010 Pearson Education, Inc. Arm movements Flexor inhibited Extensor stimulated Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Figure 13.19 Also A Flexor-Crossed Extensor Reflex! Copyright © 2010 Pearson Education, Inc. Superficial Reflexes • Elicited by gentle cutaneous stimulation • Depend on central or spinal reflex arcs Copyright © 2010 Pearson Education, Inc. Superficial Reflexes • Plantar reflex • Stimulus: stroking lateral aspect of the sole of the foot • Response: downward flexion of the toes • Tests for function of corticospinal tracts Copyright © 2010 Pearson Education, Inc. Superficial Reflexes • Babinski’s sign • Stimulus: same as in previous slide • Response: dorsiflexion of big toe and fanning of toes • Present in infants due to incomplete myelination • In adults, indicates corticospinal or motor cortex damage Copyright © 2010 Pearson Education, Inc. Plantar vs. Babinski Reflex Normal or Abnormal? Copyright © 2010 Pearson Education, Inc. Superficial Reflexes • Abdominal reflexes • Cause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin • Vary in intensity from one person to another • Absent when corticospinal tract lesions are present Copyright © 2010 Pearson Education, Inc.