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Objectives

  • Distinguish the function and general course of the pyramidal and extrapyramidal motor pathways.
  • Describe the organization of primary motor cortex.
  • Describe the roles of the higher motor areas, and list the sequence of events in a movement from initial planning through to execution.

Pyramidal

  • Pyramidal cells originate from the motor cortex; some of the largest neurons in the body
    • They ultimately make connections with alpha motor neurons
  • Extrapyramidal tract synapse mainly with interneurons in the spinal cord
  • Corticobulbar tract runs to the face
  • Together, corticospinal and corticobulbar deal are pyramidal cell projections
    • Because it controls the face, it comes form face regions in cortex (lateral side of motor gyrus)
    • Branch off at the level of the pons to form the nuclei for each of the cranial nerves
    • Similar to corticospinal tract (similar idea).
  • The corticobulbar tract fibres together with corticospinal fibres make the pyramidal tract. Corticobulbar fibres originate in the face representation region of motor cortex. The fibres branch off and decussate in the pons and brainstem, to innervate motor nuclei of the cranial nerves. Some of these nuclei receive bilateral innervation, others just contralateral.
    • Upper part of face is bilaterally innervated; but in our body in general, everything is contralateral (with crossing over)

Corticospinal tract: lateral and anterior

  • Most fibres cross over at the level of the pyramids to form the lateral corticospinal tract (pyramidal decussation)
    • But a proportion of the fibres don't cross here, but continue at the ipsilateral side (medially, rather than laterally) and continue down to cross over at the level of their corresponding spinal nerve
    • The medial ones = axial skeletal functions (posture)
      • Minimum of 4 synapses - so a slower pathway than the lateral pathway
    • The lateral ones = limb function
      • Very direct (few synapses) - for fractions of a second fine motor control (e.g. fingers, voluntary fine movements)
  • The corticospinal fibres for limbs travel mainly in the lateral corticospinal tract, nearly all of which decussate in the medulla. ~10-20% of fibres, mainly for postural muscles, travel in the anterior corticospinal tract and decussate at the spinal level of termination.


Classes of motor pathway – both are vital

  • Medial parts – bilaterally symmetric
    • Posture (note that as it runs down, it also contributes to medial brainstem pathways
  • Lateral parts – not bilaterally similar
    • Limb movements
  • Corticospinal tract
    • Travels from the cortex to the spine and contains motor output
    • Contained in the pyramids
  • Pyramidal is not all corticospinal, however

Motor pathways

  • Pyramidal tracts and extrapyramidal tracts
  • The pyramidal tract and corticospinal tract has both motor cortical and somatosensory fibres.
    • The sensory fibres are to provide an early warning signal that you're about to contact something because of reaching out etc

Brainstem motor pathways

  • The brainstem / extrapyramidal pathways have primary responsibility for posture.
  • They derive input from various sources, but are ultimately under descending cortical control.

Brainstem spinal pathways

  • See table
  • These are subconscious modulating regions to make sure you can balance, react to stimuli etc, so that the motor cortex can be the CEO and direct complex movements, rather than having to deal with all the simple things (e.g. balance etc)
  • Note that vestibulospinal pathway is mainly input by the otolith organ (which detects sense of gravity)

Summary of tracts

  • Lateral pathways – movement of limbs:
    • Corticospinal tract
    • Rubrospinal tract (from red nucleus - no big role in man)
  • Ventromedial pathways – maintaining posture
    • Tectospinal tract (superior colliculus)
    • Vestibulospinal tract
    • Pontine reticulospinal tract
    • Medullary reticulospinal tract
  • The pyramids have 106 axons
    • 2/3 of these are motor, 1/3 are somatosensory
  • motor tells sensory what is about to happen so that it is not unexpected

Ventromedial Tracts

  • Vestibulospinal tract – vestibular nuclei
    • Anti-gravitycentre
    • Keeps us standing upright, stabilise head
    • Otolith organs – tell us lying down vs standing up
    • Talks to leg extensors and postural muscles (neck, back) o If cut these tracts, can get decerebrate rigidity
  • Tectospinal tract – superior colliculus
    • Allows us to orient towards visual/tactile/auditory stimulus o Neck/shoulder muscles
  • Reticulospinal tract – reticular formation
    • Somatosensory, cerebellum, vestibular
    • Axial, limbs
    • Help us overcome vestibular activity to do useful things
    • Medial tract is formed by the pons (hence posture), the lateral tract is formed from the medulla (hence limb actions - overcoming antigravity movements e.g. move and loosen up the anti-gravity movements made by the vestibulospinal pathway)
  • Postural compensation
    • Rubrospinal
      • Fine control in carnivores
      • Little significance in man – corticospinal tract instead
    • Lateral tracks come from the motor cortex and allow us to reset alpha and gamma motor neurons
  • Note that the motor cortex can work to override reticular formation signals
  • If you lose your cerebral input (i.e. reticular formation and above), then you have only the vestibular nucleus controlling your muscles: you get decerebrate rigidity (because the descending unlocking control is lost).

Motor pathways in spinal cord

  • Note mislabelling: the front one in front of the pontine reticulospinal tract is the anterior corticospinal tract

Primary motor cortex

  • Located at Brodmann 4
  • Also has a topographic organisation
    • Size of area depends on complexity of muscle tasks
  • Primary motor cortex neurons
    • Some are simple with one joint
    • Many are complicated with multiple joints
  • The primary motor cortex generalises and coordinates movement
  • It thinks in terms of directions
  • “neurons work together as a population”
  • Topographic organisation (that a certain area of brain controls a certain area of the body)


Plays an important role in:

  • direct output to lower motor neurons
  • output to brainstem motor nuclei to exert indirect control of lower motor neurons (talk through its lackeys)
  • integration of information from other motor areas

Lesions may cause:

  • paralysis or weakness, on the opposite side of the body, and restricted by topographic organization
  • hyper-reflexia and hypertonia, which together constitute spasticity
    • e.g. clasp-knife effect. This is because your brainstem nuclei and reflexes are regulating all the movement.
  • damage to corticospinal tract causes loss of fractionated movement, but may be retraining

Topographic organisation of the primary motor cortex

  • Inside and outside the face are represented separately
  • There is disproportionately large representation of the hands and face (obviously)
  • Motor Homunculus

Movement direction coded as a population vector in motor cortex

  • Single neurons in primary motor cortex do not generally control one muscle, but instead are associated with a movement. One neuron may provide drive to up to seven muscles.
  • The pooled activity of all the cortical neurons associated with limb movement appears to specify the direction of movement for the limb.
  • Diagram: Response rates of a number of cortical motor neurons during a reaching movement in eight different directions. Response is shown as a vector along its preferred movement axis. The arrows are the average of all neuronal responses during one movement.
  • No one neuron is responsible for one movement - it's represented by a group of neurons

Organization of higher motor areas

  • Top 3 = making plans
  • Loops between the basal ganglia and the cortex and the pons/cerebellum and the cortex are via the thalamus
  • Basal ganglia are affected in Parkinson's/Huntington
  • Motor programs you lay down are in part in the cerebellum and in part in the cortex
  • Difficult to isolate what does what.

Secondary motor areas

Secondary motor cortical areas:

  • each have their own body representation (i.e. the hand region of the primary motor cortex for hand maps to the secondary motor cortex for hand)
  • are active before a movement is intiated, and before primary motor cortex
  • have extensive connections, especially inputs from sensory association areas
  • lesions do not cause weakness but lead to difficulty with co-ordinated fine movements
  • Supplementary area=
    • more corticospinal
    • finger movements
  • Pre-motor area
    • more reticulospinal output
    • orienting, and axial muscles (postural)

Primary motor cortex summary

  • Has direct output to lower motor neurons
    • Outputs to brainstem motor nuclei for this purpose
    • Integrates information from other motor areas
  • Lesions can cause:
    • Paralysis/weakness on contralateral side depending on topographic organisation
    • Hyper-reflexia/hypertonia – spasticity
  • If we were to cut motor neurons, muscular input would only come from interneurons and spindles
  • Thus, the reflex becomes dominant and causes spasticity
  • If the alpha motoneurons are cut, the muscles can’t contract and we get flaccid paralysis
  • Damage to corticospinal tract causes loss of fractionated movement
    • This may be retrain-able

Summary of motor system diagram

  • Note the secondary motor cortex has been left out for simplicity

Higher cortical motor areas

  • Basal ganglia, the premotor cortex, the supplementary motor cortex, and corticospinal tract all work together with variable input to create higher motor actions
    • Eg: we are already planning to reach out for water before the cortex is aware we are thirsty
  • Experiment, monitor brain, push a button when thirsty before reaching for water
  • Secondary motor areas
    • Each area has a specific body representation
    • Are active before movement is initiated and before the primary motor cortex is made aware o Have extensive connections including sensory associations
    • Lesions don’t cause weakness but can lead to difficulty in coordination of fine movements
  • Supplementary area
    • More corticospinal output
    • Finger movements
  • Pre-motor area
    • More reticulospinal output
    • Orienting and axial muscles