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

Review from Muscle Innervations PNS elements activate effectors by releasing neurotransmitters How is a muscle innervated?

Review from Muscle Innervations PNS elements activate effectors by releasing neurotransmitters How is a muscle innervated? Impulse Ach released Ach binds to the receptors Induces movement of Na and K Depolarization 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

Differences: Innervation of Visceral Muscle and Glands 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

Exam Prep: Levels of Motor Control Segmental level Projection level Precommand level

(modified by feedback) Precommand Level (highest) (modified by feedback) • Cerebellum & basal nuclei • Programs and instructions sent for processing Internal feedback Feedback Projection Level (middle) • Motor cortex • Brain stem nuclei • Signals to spinal cord (efferent neurons) • Send same info to the 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

Lowest level of the motor hierarchy Central pattern generators (CPGs) Segmental Level Lowest level of the motor hierarchy Central pattern generators (CPGs) segmental circuits activate networks of ventral horn neurons stimulate specific groups of muscles Controls Locomotion Specific, oft-repeated motor activity

Projection motor pathways Projection Level Consists of: Upper motor neurons Functions Induce voluntary skeletal muscle movements Brain stem motor areas oversee Reflex control CPG-controlled motor actions (segmental level) Projection motor pathways Inform the higher command levels

What? Cerebellar neurons and basal nuclei Functions: Precommand Level What? Cerebellar neurons and basal nuclei Functions: Posture and monitoring muscle tone Performing unconscious planning and discharge in advance of willed movements Example? Regulate motor activity Precisely start or stop movements Block unwanted movements With what disorder are these associated?

• 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

Reflexes What is the difference between an intrinsic reflex and an acquired reflex? Inborn (intrinsic) reflex Learned (acquired) reflexes

Reflexes What is the difference between an intrinsic reflex and an acquired reflex? Inborn (intrinsic) reflex Rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes Result from practice or repetition, Example: driving skills, riding a bike

Review of the Reflex Arc Components of a reflex arc (neural path)

Review of the Reflex Arc Components of a reflex arc (neural path) Receptor Sensory neuron Integration center Motor neuron Effector

Review of the Reflex Arc Components of a reflex arc (neural path) Receptor site of stimulus action Sensory neuron afferent impulses → CNS Integration center internal processing Motor neuron effector impulses → effector organ Effector target contracts or secretes

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

Take note which reflexes are: Types of Reflexes Spinal Muscle spindle Stretch Golgi tendon Flexor Crossed-extension Superficial Take note which reflexes are: polysynaptic monosynaptic ipsilateral contralateral

Spinal somatic reflexes Spinal Reflexes Spinal somatic reflexes Integration center spinal cord Effectors skeletal muscle Important for assessing spinal damage and general function of the NS

Stretch and Golgi Tendon Reflexes Function Important for skeletal muscle activity Proprioceptor input is necessary Muscle spindles muscle length Golgi tendon organs muscle/tendon tension

Maintain muscle tone (large postural muscles) Stretch Reflexes Maintain muscle tone (large postural muscles) Cause muscle contraction A response to increased muscle length (stretch) Example: patellar stim. → quads/hamstrings respond All stretch reflexes are: Monosynaptic involve one synapse Ipsilateral same side of body

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)

Polysynaptic reflexes Golgi Tendon Reflexes Polysynaptic reflexes Prevent damage from excessive stretch Produces muscle relaxation Smooth onset/termination of muscle contraction

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

Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex Initiated by a painful stimulus Withdrawal of the threatened body part Ipsilateral and polysynaptic Crossed extensor reflex Targets weight-bearing limbs to maintain balance Consists of two simultaneous reflexes Ipsilateral withdrawn (flexed) Contralateral 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 “Sensitive” reflexes Plantar reflex Elicited by gentle cutaneous stimulation Plantar reflex Stimulus: foot Response: downward flexion of toes Tests: corticospinal tracts Babinski’s sign Stimulus: same Response: fanning of toes Infants: incomplete myelination Adults: corticospinal or motor cortex damage

Superficial Reflexes Abdominal reflexes Response contraction of abdominal muscles 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 https://www.youtube.com/watch?v=4oo1oDQSfPs

Developmental Aspects of the PNS Distribution and growth of spinal nerves correlate with the segmented body plan Sensory receptors atrophy with age muscle tone lessens due to loss of neurons decreased numbers of synapses per neuron slower central processing Peripheral nerves remain viable throughout life unless subjected to trauma