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Chapter 13 The Peripheral Nervous System Part D Shilla Chakrabarty, Ph.D. Copyright © 2010 Pearson Education, Inc. Motor Endings • Are PNS elements that activate effectors by releasing neurotransmitters • Found at neuromuscular junctions Terminals of somatic fibers innervating voluntary muscles forms elaborate neuromuscular junctions with their effector cells As each axon branch reaches its target, a single muscle cell, the ending splits into a cluster of axon terminals Axon terminals branch like a tree over the junctional folds of sarcolemma of muscle fiber Axon terminals contain mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine (ACh) When a nerve impulse reaches an axon terminal, Ach is released by exocytosis and a series of events is initiated. Copyright © 2010 Pearson Education, Inc. Events At A Neuromuscular Junction When A Nerve Impulse Arrives 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+ K+ Acetylcholinesterase Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. Review of Innervation of Visceral Muscle and Glands • Autonomic motor endings and visceral effectors (such as smooth and cardiac muscles and glands) are simpler than somatic junctions • Branches form synapses en passant via varicosities Varicosities are knoblike swellings containing mitochondria and synaptic vesicles, and appear like a string of beads • Autonomic synaptic vesicles typically contain acetylcholine or norepinephrine, both of which act indirectly via second messengers • Visceral motor responses are slower than somatic responses which directly open ion channels Copyright © 2010 Pearson Education, Inc. Innervations Of Smooth Muscles 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). Levels of Motor Control And Their Interactions 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 Copyright © 2010 Pearson Education, Inc. Reflex activity Motor output Levels of Motor Control: 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. Levels of Motor Control: Projection Level • Consists of: Upper motor neurons that direct the pyramidal system to directly 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. Levels of Motor Control: Precommand Level • Neurons in the cerebellum and basal nuclei Regulate motor activity Precisely start or stop movements Coordinate movements with posture Block unwanted movements Monitor muscle tone Perform unconscious planning and discharge in advance of willed movements Copyright © 2010 Pearson Education, Inc. Precommand Level • Cerebellum • Lacks direct connections to the spinal cord • Acts on motor pathways through projection areas of the brain stem • Acts on the motor cortex via the thalamus • Basal nuclei • Receive inputs from all cortical areas and send output to premotor and prefrontal cortical areas via thalamus • Inhibit various motor centers under resting conditions Copyright © 2010 Pearson Education, Inc. Levels of Motor Control: Segmental, Projection and Precommand Precommand level • Cerebellum • Basal nuclei Projection level • Primary motor cortex • Brain stem nuclei Segmental level • Spinal cord (b) Structures involved Copyright © 2010 Pearson Education, Inc. Reflexes • Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus • Learned (acquired) reflexes result from practice or repetition, Example: driving skills Copyright © 2010 Pearson Education, Inc. Reflex Arc • Reflexes occur over highly specific neural paths called reflex arcs • Components of a reflex arc (neural path) 1. Receptor—site of stimulus action 2. Sensory neuron—transmits afferent impulses to the CNS 3. Integration center—either monosynaptic or polysynaptic region within the CNS 4. Motor neuron—conducts efferent impulses from the integration center to an effector organ 5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting 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 • Testing of somatic reflexes is important clinically to assess the condition of the nervous system Copyright © 2010 Pearson Education, Inc. Stretch and Golgi Tendon Reflexes • For skeletal muscle activity to be smoothly coordinated, proprioceptor input is 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 Education, Inc. Muscle Spindles • Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule Secondary sensory endings (type II fiber) • Intrafusal fibers Noncontractile in their central regions (lack myofilaments) Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers • Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity Note: Extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibers Copyright © 2010 Pearson Education, Inc. Primary sensory endings (type Ia fiber) Muscle spindle Connective tissue capsule Efferent (motor) fiber to muscle spindle Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers Sensory fiber Golgi tendon organ Tendon Operation Of The Muscle Spindle Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time Time Time Time (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber. (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. (c) Only motor (d) Coactivation. neurons activated. Both extrafusal and Only the extrafusal intrafusal muscle muscle fibers contract. fibers contract. The muscle spindle Muscle spindle becomes slack and no tension is mainAPs are fired. It is tained and it can unable to signal further still signal changes length changes. in length. Action potentials generated in sensory fibers are shown as black lines in yellow bars Copyright © 2010 Pearson Education, Inc. Stretch Reflexes • Maintain muscle tone in large postural muscles • Cause muscle contraction in response to increased muscle length (stretch) How a stretch reflex works: • Stretch activates the muscle spindle • IIa 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 • Reciprocal inhibition also occurs—IIa 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 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 alpha motor neurons (red), which excite extrafusal fibers by stretch, the associated sensory of the stretched muscle. Afferent fibers also neurons (blue) transmit afferent impulses synapse with 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 3a Efferent impulses of alpha motor neurons 3b Efferent impulses of alpha motor cause the stretched muscle to contract, which resists or reverses the stretch. neurons to antagonist muscles are reduced (reciprocal inhibition). Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 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) 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), step 3b 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 • 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. 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 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 © 2010 Pearson Education, Inc. Superficial Reflexes: Plantar Reflex • 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.