Reflex Arc for Stretch Reflex Muscle Spindles Figure 13.2b, p. 419 - - PowerPoint PPT Presentation

Reflex Arc for Stretch Reflex

Figure 13.2b, p. 419

Muscle Spindles

Proprioceptors

muscle spindle

Figure 16.9a Neuroscience, 2nd Edition (2001)

Golgi tendon organ

Figure 16.11a Neuroscience, 2nd Edition (2001)

The Stretch Reflex in Action

Figure 16.9b Neuroscience, 2nd Edition (2001)

Reciprocal Innervation in the Stretch Reflex

• muscle spindle afferent directly excites somatic motor neuron to

same muscle

• muscle spindle afferent excites an inhibitory interneuron that

prevents activation of somatic motor neurons to antagonists muscle

What structure is hit with the reflex hammer? What muscles are stretched? What muscle action occurs as a consequence? Why do this test?

The Stretch Reflex is a Negative Feedback Loop to Maintain Muscle Length

• loss of descending inhibition from CNS in upper motor neuron disorders
• hypertonia; hyperreflexia; clonus
• positive Babinski sign

Plantar Reflex: Babinski Sign

normal adult response positive Babinski sign

• positive Babinski sign occurs in infants and in upper motor neuron disorders

Stretch Reflexes to Test

• patellar tendon (knee jerk)
• Achilles tendon (ankle reflex)
• triceps tendon
• finger flexors

Patient B

Diagnosis

• Is it peripheral neuropathy?
• If so, what is the cause?

Sensory Nerve Conduction Study: Recording Setup

ground recording electrode stimulating electrode

To determine conduction velocity:

• measure distance along nerve between negative pole of

stimulating electrode and positive pole of recording electrode

• divide by time between stimulus and start of recorded

response

Sensory Nerve Conduction Study

stimulus artifact recorded response stimulus start of recorded response

recorded response is due to the sum of all the action potentials in axons

• f the nerve under the

recording electrode

Electromyography

stimulate nerve and record from muscle What is the specific electrical activity in the muscle cells that is being measured in and EMG recording?

Motor Unit: Somatic Motor Neuron and all the Muscle Fibers it Innervates

EMG signal: summed APs of all the muscle fibers in the motor unit

Peripheral Neuropathy

demyelinating: decreased conduction velocity example: Guillain-Barré axonal: decreased amplitude (fewer axons firing action potentials) example: diabetic neuropathy polyneuropathy: multiple nerves affected; generalized disorder diabetic neuropathy: usually sensory-predominant, distal axonal polyneuropathy

Figure 36-14A in Principles of Neural Science (2000) 4th edition

Glove and Stocking Pattern

Terminology

paresthesia: abnormal sensation allodynia: pain in response to a non-painful stimulus HbA1c: percentage of glycated hemoglobin

• used to monitor glycemic control
• used to diagnose diabetes mellitus

Examination

Treatments for Diabetic Neuropathy

• glycemic control
• treat cardiovascular issues
• pain medications to control symptoms

Patient C

Possible causes of weakness:

• CNS damage (upper motor neuron disorder)
• peripheral neuropathy
• myopathy
• neuromuscular junction disorder

Ptosis

• due to weakness in skeletal muscles that retract the eyelid

Figure 12.12

• p. 389

Creatine Kinase: a Marker for Muscle Cells

Myasthenia Gravis

• autoimmune disorder
• antibodies to nicotinic acetylcholine receptor at NMJ

most common cause, but can be caused by other autoantibodies

• decreases the size of the muscle EPSP
• diagnosed by tests for antibodies, repetitive nerve

stimulation test

Figure 1 in Lee, I., et al “Weak and Winded” NEJM 381: 76-82

Decline in Amplitude of Muscle Response to Repetitive Nerve Stimulation

Repetitive Nerve Stimulation Test

Steps in Muscle Excitation

Muscle EPSP

Repetitive Nerve Stimulation Test

Treatment of Myasthenia Gravis

• pyridostigmine
• immunosuppressive drugs

“Diagnosing Myasthenia Gravis with an Ice Pack” NEJM (2016) 375: e39

• ice pack decreases acetylcholinesterase activity to improve symptoms

Ice Pack Test