Chapter 13 (part d) provides knowledge of motor endings and motor activity. In this chapter, students will be able to compare and contrast the motor endings of somatic and autonomic nerve fibers, outline the three levels of the motor hierarchy, compare the roles of the cerebellum and basal nuclei in controlling motor activity.
PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 13 The Peripheral Nervous System and Reflex Activity: Part D Copyright © 2010 Pearson Education, Inc Motor Endings • PNS elements that activate effectors by releasing neurotransmitters Copyright © 2010 Pearson Education, Inc Review of Innervation of Skeletal Muscle • Takes place at a neuromusclular junction • Acetylcholine (ACh) is the neurotransmitter • ACh binds to receptors, resulting in: • Movement of Na+ and K+ across the membrane • Depolarization of the muscle cell • An end plate 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 Action potential arrives at axon terminal of motor neuron Sarcolemma of the muscle fiber Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal Ca2+ Ca2+ 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+ ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase Mitochondrion Synaptic cleft Fusing synaptic vesicles Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma 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 Review of Innervation of Visceral Muscle and Glands • Autonomic motor endings and visceral effectors are simpler than somatic junctions • Branches form synapses en passant 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 Levels of Motor Control • Segmental level • Projection level • Precommand level Copyright © 2010 Pearson Education, Inc Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by feedback) Internal feedback Feedback Projection Level (middle) • Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.) • Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels Segmental Level (lowest) • Spinal cord • Contains central pattern generators (CPGs) Sensory input Reflex activity Motor output (a) Levels of motor control and their interactions Copyright © 2010 Pearson Education, Inc Figure 13.13a Segmental Level • The lowest level of the motor hierarchy • Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles • Controls locomotion and specific, oft-repeated motor activity Copyright © 2010 Pearson Education, Inc Projection Level • Consists of: • Upper motor neurons that direct the direct (pyramidal) system to produce voluntary skeletal muscle movements • Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions • Projection motor pathways keep higher command levels informed of what is happening Copyright © 2010 Pearson Education, Inc Golgi Tendon Reflexes • Produce muscle relaxation (lengthening) in response to tension • Contraction or passive stretch 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 Afferent fibers synapse with interneurons in the spinal cord Quadriceps strongly contracts Golgi tendon organs are activated Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3a Efferent impulses + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc to muscle with stretched tendon are damped Muscle relaxes, reducing tension 3b Efferent impulses to antagonist muscle cause it to contract Figure 13.18 Quadriceps strongly contracts Golgi tendon organs are activated Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc Figure 13.18, step Quadriceps strongly contracts Golgi tendon organs are activated Afferent fibers synapse with interneurons in the spinal cord Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc Figure 13.18, step Afferent fibers synapse with interneurons in the spinal cord Quadriceps strongly contracts Golgi tendon organs are activated Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3a Efferent impulses + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc to muscle with stretched tendon are damped Muscle relaxes, reducing tension Figure 13.18, step 3a Afferent fibers synapse with interneurons in the spinal cord Quadriceps strongly contracts Golgi tendon organs are activated Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3a Efferent impulses + Excitatory synapse – Inhibitory synapse Copyright © 2010 Pearson Education, Inc to muscle with stretched tendon are damped Muscle relaxes, reducing tension 3b Efferent impulses to antagonist muscle cause it to contract Figure 13.18, step 3b 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 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 Superficial Reflexes • Elicited by gentle cutaneous stimulation • Depend on upper motor pathways and cordlevel reflex arcs Copyright â 2010 Pearson Education, Inc Superficial Reflexes ã 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: as above • Response: dorsiflexion of hallux and fanning of toes • Present in infants due to incomplete myelination • In adults, indicates corticospinal or motor cortex damage 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 Developmental Aspects of the PNS • Spinal nerves branch from the developing spinal cord and neural crest cells • Supply both motor and sensory fibers to developing muscles to help direct their maturation • Cranial nerves innervate muscles of the head Copyright © 2010 Pearson Education, Inc Developmental Aspects of the PNS • Distribution and growth of spinal nerves correlate with the segmented body plan • Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased numbers of synapses per neuron, and slower central processing • Peripheral nerves remain viable throughout life unless subjected to trauma Copyright © 2010 Pearson Education, Inc ... necessary • Muscle spindles inform the nervous system of the length of the muscle • Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons Copyright © 2010 Pearson... 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... stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions • Projection motor pathways keep higher command levels informed of what is