Muscle, Reflexes, Cortical Control

Transcription

1 Muscle, Reflexes, Cortical Control

2 Movement Command Chain Sensory feedback Intention to move Activity in cerebral cortex, basal ganglia, and cerebellum Motor cortex firing Impulses in descending motor pathways Motoneuron excitation Impulse propagation to axon terminals Neuromuscular transmission Impulse propagation along muscle fibre plasmalemma Impulses into T-tubules Ca2+ release from SR Engagement of cross-bridges Movement (or Force) Mc Comas (1998)

3 Spinal Cord Automatic & stereotyped responses reflexes rhythmic motor patterns Can function without brain Spinal interneurons same circuits as voluntary movement Pathways converge on a motor neurons final common path

4 Motor neurons To execute a motor program is necessary to activate efferent neurons that innervate skeletal muscle. These neurons originate in the ventral horn of the spinal cord and are often referred to as lower motor neurons. Germann and Stanfield

5 Motor Units an a-motoneuron and all the muscle fibres it innervates Different size of units, different functions Fibre composition Consistent within a motor unit Provide the basis of the: All or none principle

6 The motor unit All muscle fibres in a single motor unit are controlled by firing of a unique motor neurone The use of each fibre in the unit is the same and all the fibres in the unit have the same properties Different motor units that have different firing patterns have different properties

7 The Reflex Arc A reflex arc consists of: - a sensory element (receptor) - an afferent (sensory) nerve - a central processing unit - an efferent (command) nerve - an effector (a muscle). Central processing unit Efferent nerve The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Muscle Afferent nerve Receptor

8 Components of the reflex latency: Afferent conduction delay Central processing delay Efferent conduction delay Efferent nerve Muscle T e Central processing unit Reaction T c The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Afferent nerve T a Muscle spindle Stim T a + T c + T e Latency Time

9 Primary ending Ia monosynaptic motor neuron Stretch reflex

10 H-Reflex Experiments Experiments with an electrical stimulation of a muscle nerve. The stimulus is applied to both afferent and efferent fibers. Stim Central processing unit Efferent nerve Afferent nerve Muscle The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Muscle spindle

11 H Reflex from Gastrocnemius muscle

12 H-Reflex and M-Response (I) Afferent fibers are the first to react to a slowly increasing electrical stimulus. They induce a reflex muscle contraction (H-reflex). EMG St EMG H-reflex Time Later, efferent fibers become excited and induce a direct muscle contraction (M-response). St EMG M-response H-reflex Time Time St

13 Pathway for the H reflex Electrical stimulus over peripheral nerve Response measured in the muscle innervated by the nerve

14 Types of neural fibres Fibres with the lowest threshold have the greatest conduction velocity Large diameter fibres have greatest conduction velocity

15 H-Reflex and M-Response (II) EMG St EMG St EMG H-reflex M-response Time M-response H-reflex Time M-response Further increase in the strength of the stimulation leads to an increase in the M- response and suppression of the H-reflex. St Time

16 Recruitment curve of H reflex and M wave in soleus Responses to 8 stimulus intensities Variations in the amplitude of H reflex and M wave with change in stimulus intensity

17 Changes in the Amplitude of the H- Reflex and M-Response with the Amplitude of the Stimulus A H,M H M A ST Peak-to-peak amplitude of the H-reflex and the M- response depends on the strength of the stimulation applied to a muscle nerve (AST). Note the nonmonotonic H- curve and a monotonic increase in the M- response. Threshold

18 H and M wave interaction When an action potential is generated in an efferent axon, the action potential is propagated both toward the neuromuscular junction and back toward the soma of the motor neuron The orthodromic AP (towards neuromuscular junction) will generate an M wave The antidromic AP (towards the motor neuron) will reduce the responsiveness of the motoneuron to incoming 1a afferent input Axon hillock α-motoneuron Orthodromic action potential Antidromic action potential Efferent fiber Afferent fiber

19 The hierarchy: Cortical Areas: Sensory Motor System Primary motor cortex M1 Premotor cortex Supplementary motor area S-I and S-II PPC Sub-cortical Areas: Cerebellum Basal Ganglia Muscles Receptors

20 Descending Tracts Hans Kuypers: classification of descending tracts Lemon, 2008

21 Supraspinal reflex Feedback delay 80 ms A4 A3a Sensorimotor cortex Thalamus Pyramidal tract Medulla Ia MN Muscle Muscle

22 Multiple loops Asanuma73 Cutaneous, proprioceptive Short latency spinal Long latency cortical

23 Sources of afferent information: the receptors Muscle spindle Golgi tendon organ Joint receptors Cutaneous receptors

24 Golgi tendon organ Muscle spindle afferents efferents Other proprioceptors joint vestibular Muscle receptors

25 Intrafusal fibres Spindle 4-10 mm long Centre noncontractile In parallel with extrafusal fibres Sensory axons Motor axons gamma Respond to length (change)

26 Golgi tendon organ Muscle/tendon junction Series with muscle 1 mm long Single Ib (large myelinated fibre) Respond to muscle contraction (force) Protects against excessive activity

27 Cutaneous receptors Receptive field I : sharp II : large Merkel s discs Meissner corpuscle Adaptation SA : slow FA : fast Ruffini endings Pacinian corupuscle

28 How we get from skin & muscles to the brain: Cortex Midbrain Brain Stem Spinal Cord Peripheral receptors in skin and muscles

29 Germann & Stanfield, fig 8.17 Afferent and Efferent Latency?