Chapter 13 Part D The Peripheral Nervous System

Chapter 13 Part D The Peripheral Nervous System and Reflex Activity © Annie Leibovitz/Contact Press Images © 2017 Pearson Education, Inc. Power. Point® Lecture Slides prepared by Karen Dunbar Kareiva Ivy Tech Community College

Part 3 – Motor Endings and Motor Activity 13. 9 Peripheral motor endings connect nerves to their effectors • Motor endings: PNS elements that activate effectors by releasing neurotransmitters © 2017 Pearson Education, Inc.

Innervation of Skeletal Muscle • Takes place at neuromuscular junction • Neurotransmitter acetylcholine (ACh) is released when nerve impulse reaches axon terminal, diffuses across synaptic cleft, and attaches to ACh receptors on the sarcolemma at the junction © 2017 Pearson Education, Inc.

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 membrane, which triggers opening of voltagegated Na+ channels – Results in an action potential that propagates along sarcolemma, stimulating muscle contraction © 2017 Pearson Education, Inc.

Innervation of Visceral Muscle and Glands • Autonomic motor endings and visceral effectors are simpler than somatic junctions • Branches form synapses en passant (synapses in passing) with effector cells via varicosities (knoblike swellings) • Acetylcholine and norepinephrine act indirectly via second messengers • Visceral motor responses are slower than somatic responses © 2017 Pearson Education, Inc.

13. 10 There are three levels of motor control • Cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities • Complex motor behavior depends on complex patterns of control; three levels are: – Segmental level – Projection level – Precommand level © 2017 Pearson Education, Inc.

The Segmental Level • Lowest level of motor hierarchy – Consists of reflexes and automatic movements • Segmental circuits activate networks of ventral horn neurons to stimulate specific groups of muscles • Central pattern generators (CPGs): networks of oscillating inhibitory and excitatory neurons, which set crude rhythms and patterns of movement © 2017 Pearson Education, Inc.

The Projection Level • Consists of: – Upper motor neurons: initiate direct (pyramidal) pathways to produce voluntary skeletal muscle movements – Brain stem motor nuclei: oversee indirect pathways to control reflex and CPG-controlled motor actions • Projection motor pathways send information to lower motor neurons and keep higher command levels informed of what is happening © 2017 Pearson Education, Inc.

The Precommand Level • Neurons in cerebellum and basal nuclei regulate motor activity – Precisely start or stop movements – Coordinate movements with posture – Block unwanted movements – Monitor muscle tone © 2017 Pearson Education, Inc.

The Precommand Level (cont. ) • Precommand areas are at the highest level of motor hierarchy; they control the outputs of the cortex and brain stem motor centers and provide proper timing and patterns to execute desired movements • Cerebellum – Acts on motor pathways through projection areas of brain stem – Acts on motor cortex via thalamus to fine-tune motor activity © 2017 Pearson Education, Inc.

The Precommand Level (cont. ) • Basal nuclei – Receive input from all cortical areas and send output to premotor and prefrontal cortical areas via the thalamus – Involved in more complex aspects of motor control © 2017 Pearson Education, Inc.

Part 4 – Reflex Activity 13. 11 The reflex arc enables rapid and predictable responses • Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to stimulus (most can be modified by conscious effort) – Examples: maintain posture, control visceral activities • Learned (acquired) reflex: response that results from practice or repetition – Example: driving skills © 2017 Pearson Education, Inc.

Components of a Reflex Arc • Components of a reflex arc (neural path) 1. Receptor: site of stimulus action 2. Sensory neuron: transmits afferent impulses to CNS 3. Integration center: either monosynaptic or polysynaptic region within CNS 4. Motor neuron: conducts efferent impulses from integration center to effector organ 5. Effector: muscle fiber or gland cell that responds to efferent impulses by contracting (muscle) or secreting (gland) © 2017 Pearson Education, Inc.

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) © 2017 Pearson Education, Inc.

Figure 13. 40 The five basic components of all reflex arcs. Stimulus Skin 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) © 2017 Pearson Education, Inc.

13. 12 Spinal reflexes are somatic reflexes mediated by the spinal cord • Spinal reflexes occur without direct involvement of higher brain centers – Brain is “advised” of most spinal reflex activity and may have an effect on it • Testing of somatic reflexes important clinically to assess condition of nervous system – Exaggerated, distorted, or absent reflexes indicate degeneration or pathology of specific nervous system regions – Most commonly assessed reflexes are stretch, flexor, and superficial reflexes © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes • To smoothly coordinate skeletal muscle activity, nervous system must receive input regarding: – Length of muscle: information from muscle spindles – Amount of tension in muscle: information from tendon organs © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) Functional anatomy of muscle spindles – Composed of 3– 10 modified skeletal muscle fibers called intrafusal muscle fibers enclosed in a connective tissue capsule • Central regions of intrafusal fibers lack myofilaments and are noncontractile • Contractile regions at ends contain actin and myosin myofilaments – Regular effector fibers of muscle referred to as extrafusal muscle fibers © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) Functional anatomy of muscle spindles (cont. ) • Two types of afferent endings send sensory inputs to CNS: – Anulospiral endings (primary sensory endings) • Endings of large axons that wrap around spindle center • Stimulated by rate and degree of stretch – Flower spray endings (secondary sensory endings) • Formed by smaller axons at spindle ends • Stimulated only by degree of stretch © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) Functional anatomy of muscle spindles (cont. ) • Contractile end regions of spindle are innervated by gamma ( ) efferent fibers – Arise from small motor neurons in ventral horn of spinal cord – Maintain spindle sensitivity – Distinct from alpha ( ) efferent fibers of large alpha ( ) motor neurons that stimulate extrafusal muscle fibers to contract © 2017 Pearson Education, Inc.

Figure 13. 41 Anatomy of the muscle spindle and tendon organ. Flower spray endings (secondary sensory endings) Anulospiral endings (primary sensory endings) Muscle spindle Capsule (connective tissue) Efferent (motor) fiber to muscle spindle Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers Sensory fiber Tendon organ © 2017 Pearson Education, Inc. Tendon

Stretch and Tendon Reflexes (cont. ) Functional anatomy of muscle spindles (cont. ) – Muscle spindles are stretched (and excited) in two ways: • External force lengthens entire muscle (such as when carrying a heavy weight) • Activation of motor neurons stimulates ends of intrafusal fibers to contract, stretching middle of spindle (internal stretch) • Stretching results in increased rate of impulses to spinal cord © 2017 Pearson Education, Inc.

Figure 13. 42 a Operation of the muscle spindle. How muscle stretch is detected Muscle spindle Intrafusal muscle fiber Sensory fiber Extrafusal muscle fiber © 2017 Pearson Education, Inc. 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.

Stretch and Tendon Reflexes (cont. ) Functional anatomy of muscle spindles (cont. ) • During voluntary skeletal muscle contraction, the muscle shortens. If intrafusal fibers didn’t contract along with extrafusal fibers, the spindle would go slack and cease generating action potentials (become useless) • Situation avoided via coactivation in which motor impulses are simultaneously sent to large extrafusal fibers and to muscle spindle intrafusal fibers. This maintains tension and sensitivity of spindle during muscle contraction. © 2017 Pearson Education, Inc.

Figure 13. 42 b Operation of the muscle spindle. The purpose of - coactivation © 2017 Pearson Education, Inc. Time If only 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 - coactivation occurs. Both extrafusal and intrafusal muscle fibers contract. Tension is maintained in the muscle spindle and it can still signal changes in length.

Stretch and Tendon Reflexes (cont. ) Stretch reflex – Brain sets muscle’s length and stretch reflex ensures that it stays at that length – Example: patellar reflex (knee-jerk reflex) keeps knees from buckling when body stands upright – Stretch reflex important for maintaining muscle tone and adjusting it reflexively (especially in large postural muscles) © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) Stretch reflex (cont. ) – How stretch reflex works: • Stretch activates sensory neurons of muscle spindle • Sensory neurons synapse directly with motor neurons in spinal cord • Extrafusal fibers of the stretched muscle are stimulated and muscle contracts – Reciprocal inhibition: afferent fibers synapse with interneurons that inhibit motor neurons of antagonistic muscles © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) Stretch reflex (cont. ) – All stretch reflexes are monosynaptic (involve a single synapse) and ipsilateral (involve motor activity on same side of body) – Stretch reflexes are only monosynaptic reflexes in the body – The part of the reflex arc that inhibits motor neurons serving the antagonistic muscles is polysynaptic – Positive result on clinical stretch reflex test proves that sensory and motor connections between muscle and spinal cord are intact, and the strength of response indicates degree of excitability of spinal cord © 2017 Pearson Education, Inc.

Clinical – Homeostatic Imbalance 13. 23 • 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 • Reflexes are hyperactive when lesions of corticospinal tract reduce inhibitory effect of brain on spinal cord (as in cases of stroke) © 2017 Pearson Education, Inc.

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 1 When stretch activates muscle spindles, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron 2 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. Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle (stretched) Antagonist muscle 3 a Efferent impulses of alpha motor neurons 3 b Efferent impulses of alpha motor neurons to cause the stretched muscle to contract, which resists or reverses the stretch. antagonist muscles are reduced (reciprocal inhibition). © 2017 Pearson Education, Inc.

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) 1 3 a 3 b 3 b Patella Muscle spindle (stretched) Spinal cord (L 2–L 4) 1 Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons 3 a The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3 b The interneurons (green) make inhibitory Excitatory synapse Inhibitory synapse © 2017 Pearson Education, Inc. synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps.

Stretch and Tendon Reflexes (cont. ) • Adjusting muscle spindle sensitivity – When neurons are stimulated by brain, spindle is stretched, and contraction force is maintained or increased – If neurons are inhibited, spindle becomes nonresponsive, and muscle relaxes – Ability to modify the stretch reflex is important: as speed and difficulty of a movement increase, the brain increases motor output to make muscle spindles more sensitive. Example: gymnast on balance beam. © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) • Tendon reflex – Involves polysynaptic reflexes – Helps prevent damage due to excessive stretch – Important for smooth onset and termination of muscle contraction © 2017 Pearson Education, Inc.

Stretch and Tendon Reflexes (cont. ) • Tendon reflex (cont. ) – Muscles relax and lengthen in response to tension; effect is opposite of stretch reflex – Contraction or passive stretch activates tendon reflex – Afferent impulses transmitted to spinal cord • Reciprocal activation: contracting muscle relaxes; antagonist contracts – Tendon organs help prevent muscles and tendons from tearing when they are subjected to potentially damaging stretching force © 2017 Pearson Education, Inc.

The Flexor and Crossed-Extensor Reflexes • Flexor (withdrawal) reflex is initiated by painful stimulus – Causes automatic withdrawal of threatened body part – Ipsilateral and polysynaptic • Many different muscles may be called into play, so needs to be polysynaptic – Protective and important to survival – Brain can override • Example: Knowing a finger stick for blood test is coming, brain overrides pulling arm away © 2017 Pearson Education, Inc.

The Flexor and Crossed-Extensor Reflexes (cont. ) • Crossed extensor reflex: complex spinal reflex that often accompanies flexor reflex in weight-bearing limbs; important in maintaining balance – Consists of ipsilateral withdrawal reflex and contralateral extensor reflex; flexor withdrawal response on same side of body, and extensor reflex on opposite side – Example: Stepping barefoot on broken glass causes injured foot to quickly withdraw and opposite leg to support sudden shift in weight © 2017 Pearson Education, Inc.

Figure 13. 43 The crossed-extensor reflex. Excitatory synapse Interneurons Inhibitory synapse Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Fle s xe Flexor inhibited Arm movements Extensor stimulated ds ten Ex Site of stimulus: A noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. © 2017 Pearson Education, Inc. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated.

Superficial Reflexes • Superficial reflexes are elicited by gentle cutaneous stimulation (light touch on skin) • Clinically important reflexes depend on upper motor pathways and cord-level reflex arcs • Best known: – Plantar reflex – Abdominal reflex © 2017 Pearson Education, Inc.

Superficial Reflexes (cont. ) – Plantar reflex • Tests integrity of cord from L 4 to S 2. Normal = toes flex downward (curl); abnormal reflex is Babinski’s sign © 2017 Pearson Education, Inc.