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Behavior: Definition

Purposeful, goal-directed body movements resulting from controlled skeletal muscle activity


Subcortical Motor Structures

  • Basal Ganglia: outputs back to cortex; modifies or changes grip force exerted by muscles under control of the cortex
  • Cerebellum: computational machine


Path of Movement stimuli to movement

  1. visual information locates target  EXAM
  2. Frontal-lobe motor areas (pre-frontal cortex) plan the movement and issue the command
  3. Spinal cord carries the information to the limb
  4. Motor neurons carry the message to the muscles
  5. Basal ganglia judges grip force (if hand movement) 
  6. Cerebellum corrects movement errors
  7. Spinal cord carries sensory info to brain
  8. Sensory cortx receives info: move accomplished


Cortical Path for Movement 

  1. Posterior cortex provides sensory info to frontal cortex
  2. Prefrontal cortex plans movement in advance of the behavior
  3. Premotor cortex organizes movement sequences
  4. Motor cortex porduces specific movements (then modified by brainstem)  EXAM (eg where do visually guided behaviors originate?)

Note: M1 in anterior parietal lobe next to posterior frontal lobe so close - no need for white matter tract


Hierarchy of Motor Control

  1. Posterior Cortex: provides information to Frontal Lobe (sensory/perceptual info to plan goals)
  2. Prefrontal Cortex: (most anterior part frontal cortex) - cognitive activity that may result in behavor or inhibition of behavior damage: disinhibition
  3. Motor Cortices: a) Primary b) Premotor c) Supplementary; all in frontal lobes and project directly to spinal cord via corticospinal tract
  4. Brain stem: integrates visual and vestibulary info with somatosensory input to modify movement: a) RAS b) Vestibular Nuclei - position c) Inferior olivary complex -coordination via projections to cerebellum



Basal Ganglia, Cerebellum & Movement

  • Basal Ganglia: large cluster of neurons and gets input from many cortical areas; projects to thalamus then to cortex re motor planning; modulates muscle force
  • Cerebellum: input from the spinal cord, projects to brainstem and thalamus, improves movement accuracy: compares descending motor command with info about resulting motor action
  • damage: wide stance walking - can't maintain coordinated movements to preven falling


Central Pattern Generators

  • local circuits of interneurons in spinal cord and brainstem that drive rhythmic patterns of movement
  • organize muscle movements (e.g. alternating stepping movements) even in absence of input from the cerebral cortex
  • one central pattern generator sent to same muscles in both limbs can create mirror images
  • generates movement activity spontaneously via input from peripheral sensory neurons that adjust as needed


Motor Neurons

  • Upper motor neurons: layer V and VI of primary motor cortex; heavily myelinated - project to spinal segments &synapse with neurons in grey matter of sp. cord segment assoc. with that partic. muscle it is designed to control
  • Lower motor neurons: in grey matter of spinal cord; project out of sp. cord via ventral roots and synapse with target muscles (efferent)


Cortico-motoneuronal System 

  • Hand control in primates: Direct, rapid and monosynaptic connections between the upper and lower motoneurons
  • Relatively Independent Finger Movements(RIFMs)
  • Some primates can only move fingers together like a claw (fewer cortico-motorneurons)
  • Extremely fast and no need for feedback (no interneurons)


Damage to Motorneurons

  • Upper motorneuron: 
    • weak  or absent voluntary movements
    • increased muscle tone (rigidity)
    • altered reflexes
  • Lower motorneuron:
    • reduced muscle tone
    • weak stretch reflex
    • atrophy of affected muscles
    • fibrillation 


Sliding Filament Theory

Two myofibrils: actin and myosin

Contraction: rotate the cross-bridges of the myosin along the Actin strands causing them to slide along one another (like a rowing boat)


Muscle Wasting

if no synapsing of muscle and lower motoneuron at the neuromuscular junction - if not the muscle cannot retain its normal muscle tone (not always purposeful, can have resting rate of production of synapses)


Neuromuscular Junction

  • 'synapse of pre-synaptic membrane of motoneuron and the postsynaptic membrane of muscle fiber 
  • motoneuron releases acetylcholine, which binds to nicotinic ACh receptors which depolarizes the muscle fiber and causes a cascade that results in muscle contraction


Monosynaptic Reflex

  • direct connection between a sensory and a motor neuron - only one synapse 
  • these are rare
  • example: knee jerk reflex 
  • there is no interneuron in the circuit


Polysynaptic Reflex

  • more than one synapse because an interneuron lies between the incoming sensory neuron (in the integrating centre in grey matter) and the outgoing motor neuron in the circuit
  • more flexibility in the response
  • e.g.: need to flex bicep but to do so need to inhibity the stretch of the tricep which would fight against it so add an inhibitory interneuron to connect the bicep motor neuron to the tricep motor neuron and it will help to override the tricep stretch reflex


Reflex Arc

  • circuit for connecting inputs to outputs
  • sensory neuron makes an excitatory connection to a motor neuron so that when the sensory neuron stimulated it activates the motor neuron
  • if muscle being overstretched the sensory neuron will alert the motor neuron to contract the muscle


5 Components Reflex Arc

  1. Somatic Receptors (skin, muscles, tendons)
  2. Afferent nerve fibres - carry signals from somatic receptors to dorsal horn (sp. cord) or brainstem
  3. Integrating Center - synapse of the neurons
  4. Efferent nerve fibres - carry motor info from spinal cord via ventral route to signal muscles
  5. Effector muscle - innervated by efferent nerve fibre, carries out the response

Need all 5 for a controlled reflex arc; fixed innervation ratios but can modify with experience and each muscle ratio varies depending on degree of control exerted over the muscle by the nervous system


Stretch Receptors

Two types:

  1. In series with the muscle - when the muscle contracts it puts force on the golgi tendon organ then it signals the level of force of the muscle
  2. In parallel with the muscle - it can't tell the force, only the length of the muscle - muscle spindles

The control of muscle is necessary or else you would seize or convulse


Muscle Agonists/Antagonists

  • Opponent pairs
  • Agonists: muscles that work together
  • Input: heavily myelinated large caliber axons project to spinal cord and synapse with myelinated lower motorneurons, exit ventral root & go to muscle
  • Feedback: from muscle and tendon in via dorsal root from muscle spindles (length) and golgi tendon organs (force) - feeds into position of body part


Flacidity, Hypokinesias, Hyperkinesias

  1. Flacidity: floppy limb due to damage to lower motor neurons (disconnected from sp. cord)
  2. Hypokinesias: decreased ability to produce body movement  - anormal basal ganglia activity (Parkinsonianism) 
  3. Hyperkinesias: exaggerated unwanted motor movements (Tourette's, Huntington's Chorea) - also associated with basal gangia


Innervation Ratio

  • average # of muscle fibres that are innervated by a single motor neuron 
  • low ratio: small muscles for fine motor skills (3:1 for extra ocular muscles). For every 3 muscle fibres there is 1 motor neuron axon that synapses to the 3
  • large ratio: for power muscles - e.g. calve has 2000:1 ( every 2000 muscle fibres there's 1 motor neuron that synapses so not much control)


Corticospinal Tract

  • Corticospinal tract: primary motor pathway CNS
  • originates in precentral gyrus (M1) plus other cortical areas (corticospinal pathway)
  • Axons go from layer V M1 into the internal capsule then forms cerebral peduncles then the decussation of the pyramids (white matter structures of medulla) 
  • Lateral Corticospinal Tract: M1 to decussate in pyramids then descend contralaterally for fine muscle control (limbs, digits)
  • Ventral Corticospinal Tract: M1, descend ipsilaterally to decussate on spinal segment (larger muscles of trunk)


Steps for Neural Control of a Muscle

  1. Lower motorneuron excites muscle
  2. The neuron dumps Ach into synaptic cleft of neuromuscular junction
  3. Ach binds to nicotinic receptors on the postsynaptic membrane of the muscle
  4. Muscle contracts
  5. Muscle spindle is strained and it sends afferent signal into the spinal cord via dorsal root
  6. This creates FEEDBACK


Motor Cortical Magnification Factor

  • volume of cortex devoted to a body part is NOT proportional to its size but to the complexity of its behavioural repertoire (what it can do)
  • Smaller the skeletal muscles, larger amount of cortex devoted to controlling it (facial muscles, oral cavity, hands)


Upper Motor Neurons

  • Originate in Layer V of primary motor cortex
  • 2 separate tracts:
    • Corticospinal Tract: Pyramidal Tracts; pass through pyramids in medulla, terminate in ventral horn and synapse with lower motor neurons - fine limb movement
    • Corticobulbar Tract: originates in M1; terminates in pons and medulla (voluntary control of facial and jaw muscles, swallowing, tongue movem) - output via cranial nerves to muscles of face - speech, eating, facial gestures


Penfield's Montreal Procedure

  • stimulated areas in S1 and M1
  • Created the "homunculus" as a learning aid
  • Large areas of S1 devoted to tip tongue, tip index finger, tip thumb
  • Large areas of M1 devoted to control of thumb and forefinger, tongue, lips


Movement Sequences

  • dictionary of movement sequences - lexicon
  • 3 aspects of volitional behavior:
    • part of body to be moved - from proprioceptive feedback from receptors, joints, etc (low level)
    • spatial location to which movement directed (visual feedback - high level)
    • Function to be achieved (high level feedback - whether you have achieved your goal)
  • Feedback loops in all 3 and each different


Mirror Neurons

  • neurons that fire when we see others make a movement
  • Can be used to imitating and understanding others' actions
  • they encode a complete action
  • are mirror cells in premotor area (which plans & organizes the behaviour before execution)
  • generally located in left hemisphere
  • important for gesturing and verbal language
  • import for recog of emotion (facial motor patterns)


Brainstem and Motor Control

  • Sends info re: posture, balance, control of autonomic nervous system
  • (reflexive and inate) eating, drinking, standing upright, walking, grooming
  • gateway into consciousness & planning and motor control 
  • NON-VOLITIONAL - so not interrupted, reflexive (even if decorticated, animal still walk or groom)
  • eating: hypothalamus (re cessation) so if slow down, hypth will kick in & stop you


Basal Ganglia & Movement

  • Control of movement force
  • How?
    • receives signal from motor cortex, limbic cortex & nigrostriatal dopamine pathway (and sends them back via the Thalamus) - so, no direct connection between B.G. and spinal cord
    • sends signals to motor cortex and substantia nigra in brainstem