The Peripheral Nervous System and Reflex Activity: Part D 13 The Peripheral Nervous System and Reflex Activity: Part D

Motor Endings PNS elements that activate effectors by releasing neurotransmitters

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

Figure 9.8 Axon terminal of neuromuscular junction 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+ Synaptic vesicle containing ACh Ca2+ 3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Axon terminal of motor neuron Mitochondrion Synaptic cleft Fusing synaptic vesicles 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. ACh Junctional folds of sarcolemma Sarcoplasm of muscle fiber Na+ K+ Postsynaptic membrane ion channel opens; ions pass. 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. ACh Degraded ACh Na+ Postsynaptic membrane ion channel closed; ions cannot pass. 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. 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

their neurotransmitters into a wide synaptic Varicosities Autonomic nerve fibers innervate most smooth muscle fibers. Smooth muscle cell Synaptic vesicles 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

• Programs and instructions (modified by feedback) 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 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

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

Neurons in the cerebellum and basal nuclei 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

Precommand Level Cerebellum Basal nuclei Acts on motor pathways through projection areas of the brain stem Acts on the motor cortex via the thalamus Basal nuclei Inhibit various motor centers under resting conditions

• Programs and instructions (modified by feedback) 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 Figure 13.13a

(b) Structures involved Precommand level • Cerebellum • Basal nuclei Projection level • Primary motor cortex • Brain stem nuclei Segmental level • Spinal cord (b) Structures involved Figure 13.13b

Learned (acquired) reflexes result from practice or repetition, Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, Example: driving skills

Reflex Arc Components of a reflex arc (neural path) Receptor—site of stimulus action Sensory neuron—transmits afferent impulses to the CNS Integration center—either monosynaptic or polysynaptic region within the CNS Motor neuron—conducts efferent impulses from the integration center to an effector organ Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

1 2 3 4 5 Stimulus Skin Interneuron Receptor Sensory neuron Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Figure 13.14

Spinal somatic reflexes 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

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

Muscle Spindles Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule 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

Muscle Spindles 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

endings (type II fiber) Efferent (motor) fiber to muscle spindle Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle  Efferent (motor) fiber to extrafusal muscle fibers Primary sensory endings (type Ia fiber) Extrafusal muscle fiber Muscle spindle Intrafusal muscle fibers Connective tissue capsule Sensory fiber Golgi tendon organ Tendon Figure 13.15

Stretch causes an increased rate of impulses in Ia fibers Muscle Spindles Excited in two ways: External stretch of muscle and muscle spindle Internal stretch of muscle spindle: Activating the  motor neurons stimulates the ends to contract, thereby stretching the spindle Stretch causes an increased rate of impulses in Ia fibers

Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber 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. Figure 13.16a, b

Muscle Spindles Contracting the muscle reduces tension on the muscle spindle Sensitivity would be lost unless the muscle spindle is shortened by impulses in the  motor neurons – coactivation maintains the tension and sensitivity of the spindle during muscle contraction

Time Time (c) Only motor neurons 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. (d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length. Figure 13.16c, d

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 Reflexes 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

Stretch Reflexes 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

Cell body of sensory neuron 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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a 3b Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Figure 13.17 (1 of 2)

Cell body of sensory neuron 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 by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Figure 13.17 (1 of 2), step1

Cell body of sensory neuron 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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Figure 13.17 (1 of 2), step 2

Cell body of sensory neuron 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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle 3a Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Figure 13.17 (1 of 2), step 3a

Cell body of sensory neuron 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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a 3b Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Figure 13.17 (1 of 2), step 3b

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse 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 Figure 13.17 (2 of 2)

The patellar (knee-jerk) reflex—a specific example of a stretch reflex Quadriceps (extensors) 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament + – Excitatory synapse Inhibitory synapse Figure 13.17 (2 of 2), step 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 + – Excitatory synapse Inhibitory synapse Figure 13.17 (2 of 2), step 2

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 3a 1 Patella Muscle spindle Spinal cord (L2–L4) Tapping the patellar ligament excites muscle spindles in the quadriceps. 1 Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 3a The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. + – Excitatory synapse Inhibitory synapse Figure 13.17 (2 of 2), step 3a

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse 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 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

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

Quadriceps (extensors) Hamstrings (flexors) 1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3b Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse – Inhibitory synapse Figure 13.18

Quadriceps (extensors) Hamstrings (flexors) 1 Quadriceps strongly contracts. Golgi tendon organs are activated. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Figure 13.18, step 1

Quadriceps (extensors) Hamstrings (flexors) 1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Figure 13.18, step 2

Quadriceps (extensors) Hamstrings (flexors) 1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. + Excitatory synapse – Inhibitory synapse Figure 13.18, step 3a

Quadriceps (extensors) Hamstrings (flexors) 1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3b Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse – Inhibitory synapse 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

Flexor and Crossed-Extensor Reflexes 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

+ Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers Afferent fiber Efferent fibers Extensor inhibited Flexor inhibited Arm movements Flexor stimulated Extensor stimulated Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Figure 13.19

Superficial Reflexes Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord-level reflex arcs

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

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

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

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

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