Peripheral Nervous system © 2016 Pearson Education, Inc.

13.6 Peripheral Motor Endings 2 13.6 Peripheral Motor Endings Motor endings: PNS elements that activate effectors by releasing neurotransmitters These element innervate skeletal muscle, visceral muscle, and glands © 2016 Pearson Education, Inc.

3 Innervation of Skeletal Muscle Takes place at neuromuscular junction Neurotransmitter acetylcholine (ACh) is released when nerve impulse reaches axon terminal © 2016 Pearson Education, Inc.

4 Innervation of Skeletal Muscle (cont.) ACh binds to receptors, resulting in: Movement of Na+ and K+ across membrane Depolarization of muscle cell An end plate potential, spreads to adjacent areas of sarcolemma, which triggers opening of Na+ voltage-gated channels Results in an action potential, which leads to muscle contraction © 2016 Pearson Education, Inc.

5 Focus Figure 9.1 neuromuscular junction, Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Action potential arrives at axon terminal of motor neuron. 1 Ca2+ Voltage-gated Ca2+ channels open. Ca2+ enters the axon terminal, moving down its electrochemical gradient. 2 Ca2+ Synaptic vesicle containing ACh Axon terminal of motor neuron Synaptic cleft Fusing synaptic vesicles Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis. 3 ACh Junctional folds of sarcolemma ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma. 4 Sarcoplasm of muscle fiber ACh binding opens ion channels in the receptors that allow simultaneous passage of Na + into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential. 5 Na+ K+ Postsynaptic membrane ion channel opens; ions pass. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction. 6 ACh Degraded ACh Ion channel closes; ions cannot pass. Na+ Acetylcholin- esterase K+ © 2016 Pearson Education, Inc.

13.6 Peripheral Motor Endings 6 Part 4 – Reflex Activity 13.6 Peripheral Motor Endings Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to stimulus Examples: maintain posture, control visceral activities Can be modified by learning and conscious effort Learned (acquired) reflexes result from practice or repetition – cerebellum, premotor cortex Example: driving skills, basketball etc. © 2016 Pearson Education, Inc.

7 Components of a Reflex Arc Components of a reflex arc (neural path) Receptor: site of stimulus action Sensory neuron: transmits afferent impulses to CNS Integration center: either monosynaptic or polysynaptic region within CNS Motor neuron: conducts efferent impulses from integration center to effector organ Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting © 2016 Pearson Education, Inc.

8 Components of a Reflex Arc (cont.) Reflexes are classified functionally as: Somatic reflexes*** Activate skeletal muscle Autonomic (visceral) reflexes Activate visceral effectors (smooth or cardiac muscle or glands) © 2016 Pearson Education, Inc.

9 Figure 13.15 The five basic components of all reflex arcs. Stimulus Unipolar nerve Skin 1 Interneuron Receptor 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) © 2016 Pearson Education, Inc.

10 Table 11.1, pg.393 © 2016 Pearson Education, Inc.

11 13.9 Spinal Reflexes Spinal reflexes occur without direct involvement of higher brain centers Brain is still advised of spinal reflex activity and may have an effect on the reflex Testing of somatic reflexes important clinically to assess condition of nervous system If exaggerated, distorted, or absent, may indicate degeneration or pathology of specific nervous system regions Most commonly assessed reflexes are stretch, flexor, and superficial reflexes © 2016 Pearson Education, Inc.

12 Stretch and Tendon Reflexes To smoothly coordinate skeletal muscle, nervous system must receive proprioceptor input regarding: Length of muscle Information sent from muscle spindles Amount of tension in muscle Information sent from tendon organs © 2016 Pearson Education, Inc.

13 Figure 13.16 Anatomy of the muscle spindle and tendon organ.  Efferent (motor) fiber to muscle spindle Flower spray endings (secondary sensory endings) α Efferent (motor) fiber to extrafusal muscle fibers Anulospiral endings (primary sensory endings) Extrafusal muscle fiber Muscle spindle Intrafusal muscle fibers Capsule (connective tissue) Sensory fiber Tendon organ Tendon © 2016 Pearson Education, Inc.

14 Stretch and Tendon Reflexes (cont.) Functional anatomy of muscle spindles (cont.) Muscle spindles are stretched (and excited) in two ways External stretch: external force lengthens entire muscle Internal stretch:  motor neurons stimulate spindle ends to contract, thereby stretching spindle Stretching results in increased rate of impulses to spinal cord © 2016 Pearson Education, Inc.

15 Stretch and Tendon Reflexes (cont.) Stretch reflex Brain sets muscle’s length via stretch reflex Example: knee-jerk reflex is a stretch reflex that keeps knees from buckling when you stand upright Stretch reflexes maintain muscle tone in large postural muscles and adjust it reflexively Causes muscle contraction on side of spine in response to increased muscle length (stretch) on other side of spine © 2016 Pearson Education, Inc.

16 Stretch and Tendon Reflexes (cont.) Stretch reflex (cont.) How stretch reflex works: Stretch activates muscle spindle Sensory neurons synapse directly with  motor neurons in spinal cord  motor neurons cause extrafusal muscles of stretched muscle to contract © 2016 Pearson Education, Inc.

17 Stretch and Tendon Reflexes (cont.) Stretch reflex (cont.) Reciprocal inhibition also occurs—afferent fibers synapse with interneurons that inhibit  motor neurons of antagonistic muscles Example: In patellar reflex, stretched muscle (quadriceps) contracts, and antagonists (hamstrings) relax © 2016 Pearson Education, Inc.

18 Clinical – Homeostatic Imbalance 13.9 Stretch reflexes can be hypoactive or absent if peripheral nerve damage or ventral horn injury has occurred Reflexes are absent in people with chronic diabetes mellitus or neurosyphilis and during coma Stretch reflexes can be hyperactive if lesions of corticospinal tract reduce inhibitory effect of brain on spinal cord © 2016 Pearson Education, Inc.

19 Focus Figure 13.1-1 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 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 (stretched) Antagonist muscle 3a Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). 3b © 2016 Pearson Education, Inc.

20 Focus Figure 13.1-2 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. The patellar (knee-jerk) reflex—an example of a stretch reflex 2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle (stretched) Spinal cord (L2–L4) Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. 1 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 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 Excitatory synapse Inhibitory synapse © 2016 Pearson Education, Inc.