marieb chapter 13 part c

Copyright © 2010 Pearson Education, Inc.

Marieb Chapter 13 Part C


Innervation of Skeletal Muscle

• Takes place at a neuromuscular junction

• Neurotransmitter ?

• NT binds to receptors, resulting in:

• Movement of Na+ and K+ across the membrane

• Depolarization of the muscle cell

• An graded potential, which triggers an action potential

Copyright © 2010 Pearson Education, Inc. Figure 9.8

Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal of neuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

Mitochondrion

Synaptic cleft

Junctionalfolds of sarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

AChNa+

K+

Degraded ACh

Acetylcholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

Action potential arrives at axon terminal of motor neuron.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.

Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.

ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

1

2

3

4

5

6


Innervation of Visceral Muscle and Glands

• Autonomic motor endings and visceral effectors are simpler than somatic junctions

• Branches form synapses via varicosities

• Acetylcholine and norepinephrine act indirectly via second messengers

• Visceral motor responses are slower than somatic responses


Smoothmusclecell

Varicosities releasetheir neurotransmittersinto a wide synaptic cleft (a diffuse junction).

Synapticvesicles

Mitochondrion

Autonomicnerve fibersinnervatemost smoothmuscle fibers.

Varicosities


Reflexes

• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

• Learned (acquired) reflexes result from practice or repetition,

• Example:


Reflex Arc

• Components of a reflex arc (neural path)

1. —site of stimulus action

2. —transmits afferent impulses to the CNS

3. —either monosynaptic or polysynaptic region within the CNS

4. —conducts efferent impulses from the integration center to an effector organ

5. —muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

What is this an example of?


Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5


Spinal Reflexes

• Spinal somatic reflexes

• Integration center is in the spinal cord

• Effectors are skeletal muscle

•Why do they exist?


Stretch and Golgi Tendon Reflexes

• For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary

• tell muscle length

• tell muscle and tendon tension


Secondary sensoryendings

Efferent (motor)fiber to muscle spindle

Primary sensoryendings

Muscle spindle

Tendon

Sensory fiber

Golgi tendonorgan

Intrafusal musclefibers


Muscle Spindles

• Excited by stretch of muscle and muscle spindle

• Stretch causes an increased rate of impulses in sensory fibers

• Motor fibers then cause muscle contraction

• What kind of feedback?


Stretch Reflexes

• Maintain muscle tone in large postural muscles

• Cause muscle contraction in response to increased muscle length (stretch)


Stretch Reflexes

• How a stretch reflex works:

• Stretch activates the muscle spindle

• 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—Sensory 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

• Can you think of another example?

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 2

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with motor neurons (red), which excite the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

12

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2)

The patellar (knee-jerk) reflex—a specific example of a stretch reflex

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 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

• Too much contraction?

• Contraction 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


+ Excitatory synapse– Inhibitory synapse

Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord

Quadriceps(extensors)

Golgitendon

organHamstrings

(flexors)

1 2

3a 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

• 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


Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated

+ Excitatory synapse– Inhibitory synapse

Let’s Combine The Two!


Also A Flexor-Crossed Extensor Reflex!


Superficial Reflexes

• Elicited by gentle cutaneous stimulation

• Depend on central or spinal reflex arcs



• Plantar reflex

• Stimulus: stroking lateral aspect of the sole of the foot

• Response: downward flexion of the toes

• Tests for function of corticospinal tracts



• Babinski’s sign

• Stimulus: same as in previous slide

• Response: dorsiflexion of big toe and fanning of toes

• Present in infants due to incomplete myelination

• In adults, indicates corticospinal or motor cortex damage


Plantar vs. Babinski Reflex

Normal or Abnormal?



• 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

marieb chapter 13 part c

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