The Neuromuscular System: The Basics


Steps in EC coupling in skeletal muscle?

  1. Influx of calcium into presynaptic terminal and fusion of vesicles

  2. Release of ACh

  3. Binding to ACh to AChR

  4. Influx of sodium at the motor end plate 

  5. Propagation of AP bidirectionally

  6. Activation of DHPR (Dihydrolpyradine receptor) in T-tubules; conformational coupling with RyR

  7. Release of calcium from SR (CICR)

  8. Binding of calcium to troponin (conformational change in tropomyosin) 

  9. Crossbridge formation (actin and myosin; ATP)

  10. Crossbridge cycling (power stroke, release of ADP + Pi)

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Draw a monosynaptic reflex

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Draw a polysynaptic reflex

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Draw an Autonomic Reflex arc

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Compare Somatic & Autonomic Reflexes

Somatic Reflexes:

  • Sensory receptor responds to stimuli at an exteroreceptor or proprioceptor 
  • Motor neuron communicates directly (one cell path) with its effector 
  • Motor neuron cell body resides in the brain stem or ventral horn of spinal cord 
  • Somatic effector is skeletal muscle (motor units) 
  • Acetylcholine is the excitatory neurotransmitter at the neuromuscular junction but remember that some somatic reflexes i.e the Golgi or deep tendon reflexes are inhibitory 
  • Generally, somatic reflexes are consciously perceived, after the reflex has been initiated 

Autonomic Reflexes:

  • Sensory receptor responds to stimuli at an exteroreceptor or proprioceptor 
  • Motor neuron communicates in a two-cell pathwith effector: pre- and post-ganglionic autonomic neurons
  • Pre-ganglionic motor neuron cell body resides in the brainstem or ventral horn of the spinal cord whereas the post-ganglionic neuron cell body resides in the autonomic ganglion
  • Visceral effector is smooth muscle or glands
  • ACh or norepinephrine are the main neurotransmitters and may be excitatory or inhibitory
  • Generally, they are not perceived consciously 
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What is Voluntary Movement?

There are three key steps to voluntary movement:

  1. Planning movement - cortical association areas, cerebellum and basal nuclei

  2. Initiating movement - motor cortex

  3. Executing movement - cerebellum, brainstem

  • Sensory input is passed from the motor cortex to the motor association centres
  • Planning and decision making occurs in the basal nuclei and thalamus
  • Coordination and timing requires cerebellar input
  • Execution occurs via information being relayed down the corticospinal tract to the skeletal muscles and the extrapyramidal tract has an influence on posture, balance and gait
  • The sensory receptors provide continuous feedback
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What is Rhythmic Movement?

  • A combination of reflex & voluntary movements e.g. swimming and walking 

  • Initiated & terminated by the cerebral cortex 

  • Central Pattern Generators are networks of CNS interneurons which crossover to control the opposite sides of the body to the brain hemisphere

  • CPGs essentially maintain spontenous repetitive activity

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What is the Postural Reflex?

  • The Postural reflex maintains body position when standing & moving

  • The reflex requires continuous sensory input from the: 

  • Visual system

  • Vestibular system 

  • Muscles 

  • The reflex requires proprioception – the position of various body parts relative to one another

  • The postural reflex does not require integration in the cortex
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