VII Neuroanatomy and Neurophysiology

• There are 3 levels of motor control
• Different from the sensory system:

  • Motor endings serve effectors rather than pick up by receptors
  • Descending efferent circuits instead of ascending afferent circuits
  • Motor behaviour instead of perception
• Segmental Level:

  • Reflexes and spinal cord circuits that control automatic movements
  • Ventral horn neurons are activated and they stimulate certain muscles
  • Locomotion and other repeated patterns are called central pattern generators (CPGs): networks of oscillating inhibitory and excitatory neurons that set rhythms
• Projection Level:

  • Neurons using direct and indirect motor pathways (descending projection fibers)
  • Upper motor neurons of motor cortex initiate direct (pyramidal pathways) to muscles
  • Brain stem motor nuclei oversee the indirect pathways; help to control reflex and CPG-controlled motor actions
  • In addition to descending pathways, also send internal feedback to higher command levels, continually informing them of what should be happening repeated patterns
  • Happens in spinal cord
  • Heading down on spinal cord precise control
• Precommand Level:

  • 2 systems of neurons in the cerebellum and basal nuclei that:
  • Start/stop movements
  • Coordinate movements with posture
  • Block unwanted movements
  • Monitor muscle tone
  • Highest level because they control the outputs - the actions of the 2 lower levels
  • Cerebellum: no access to spinal cord - communicates with projection areas of brain stem and motor cortex via the thalamus
  • Basal nuclei: receive inputs from all cortical areas and send output primarily to premotor and prefrontal cortical areas via the thalamus
• • Seem to be involved in more complex management compared to the cerebellum
  • For both, all of this happens BEFORE actual motor responses - hence called Precommand
  • Highest Level: Boss can't stimulate direct activity
• Descending Motor Pathways:

  • Efferent impulses from brain to spinal cord
  • Divided into 2 groups:
  • Direct Pathways = pyramidal tracts
  • Indirect Pathways = all the other ones
  • Motor pathways involve 2 neurons (upper & lower)
  • Upper motor neuron begins in motor cortex
  • Lower motor neuron is a spinal motor neuron or a cranial nerve motor neuron
• Direct (Pyramidal) Pathways:

  • Axons descend (corticospinal tracts) without synapsing from primary motor cortex (pyramidal neurons in precentral gyrus) through length of spinal cord
  • Regulate fast and fine (or skilled) movements - e.g. typing, playing a musical instrument
  • Synapsing with lower motor neuron occurs in anterior horn at level where that neuron will exit spinal cord to activate skeletal muscles
  • 90% of these neurons in lateral corticospinal pathway; 10% in the smaller anterior pathway
• Spinal Cord Injury and Paralysis:

  • Transecting spinal cord at any level results in total motor and sensory loss in regions inferior to the cut
  • Paralysis = loss of motor function
  • Paraplegia: transection between T1 and L1
  • Quadriplegia: transection in cervical region
  • Hemiplegia: usually due to a brain injury rather than the spinal cord
  • Flaccid paralysis: damage to ventral roots or anterior horns means that impulses do not reach muscles and there is no voluntary or involuntary control of muscles
• • Spastic paralysis: damage to only upper motor neurons of primary motor cortex; spinal neurons remain intact meaning that there is loss of voluntary control of muscles but muscles can still be stimulated irregularly via spinal reflexes
• Indirect Pathways:

  • Include brain stem, motor nuclei and all motor pathways not part of the pyramidal system
  • Includes rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts
  • Pathways are complex & multi-synaptic and regulate:
  • Axial muscles for balance and posture
  • Muscles controlling coarse limb movements
  • Head, neck and eye movement to follow objects in visual field
• Reflex Arcs:

  • Reflex arcs permit rapid and predictable responses
  • 2 types: inborn (intrinsic) and learned (acquired)
  • Inborn reflex:
  • Unlearned, unpremeditated and involuntary - help us maintain posture, avoid pain and control visceral activities without having to think about it
  • Learned reflex:
  • Results often from practice or repetition
  • Can modify an inborn reflex by learning and conscious effort
• Stretch and Tendon Reflexes:

  • Help you smoothly coordinate activity of skeletal muscles
  • Nervous system needs to know:
  • Length of the muscle (muscle spindles)
  • Tension in the muscle plus tendons (tendon organs)
  • Both types of proprioceptive information important for some spinal reflexes and also to provide feedback to cortex and cerebellum
• Superficial Reflexes to Test for Spinal Cord Damage:

  • Abdominal Reflexes:
  • Using a tongue depressor, stroke skin that is lateral, superior or inferior to the umbilicus
  • Should see movement of umbilicus toward site of stimulation due to reflex contraction of abdominal muscles
  • Tests integrity of T8 to T12
  • Plantar Reflex:
  • Draw blunt object from heel to toe along lateral aspect of plantar surface of foot
  • Should induce plantar flexion (except in infants less than 1 year)
• • Babinski’s sign (great toe dorsiflexes and remaining toes fan laterally) is indicative of damage to the primary motor cortex or L4 to S2 are damaged
• • The parasympathetic performs maintenance activities and conserves energy
  • The two divisions usually counterbalance each other
• The Autonomic Nervous System and Autonomic Reflexes:

  • Effectors:
  • Somatic: skeletal muscle; ANS: smooth muscle, cardiac muscle & glands
  • Pathways & Ganglia:
  • Somatic: thick, myelinated axon from spinal cord to skeletal muscle; rapid conduction of impulses (no ganglia)
  • ANS: two-neuron chain: preganglionic neuron originates in brain or spinal cord; preganglionic axon synapses with 2nd motor neuron (postganglionic) in ganglion outside CNS - the postganglionic axon to effector organ
  • The sympathetic system mobilizes the body during extreme situations