L15 - Motor Cortex General - descending pathways: how we control our body - motor = somatic muscles and movement (it is a descending motor output pathway) - two types of movement: goal-driven/voluntary and automated (learnt set of reflexes e.g. writing) - movements require different combinations of two sets of pathways: these pathways combine to give goal-driven or automated movement skilled pathways: distal somatic muscles (fingers, hands) basic pathways: proximal somatic muscles Control - hierarchy: cortex controlling everything below - descending pathway is mainly excitatory - higher structures controlling SC (brainstem, cortex) are giving excitatory signals to the descending pathway - glutamate or aspartate NT s are used - descending pathway synapsing on alpha cell is more likely to be involved in skilled movement (signal going directly to muscle) gamma cell is more likely to be involved in proximal basic movement (activates gamma reflex loop) - descending pathway is also producing inhibition (switching off movement) - can activate inhibitory interneurons which causes it to inhibit postsynaptic cells (gamma or alpha cells) which stops contraction or stops activity of muscle (this mainly occurs in reflexes) 1
e.g. inhibiting stretch reflex: descending pathway from cortex activates inhibitory neuron to relax muscles and switch off stretch reflex - no descending pathway: would cause exaggerated reflexes e.g. too much tension/ spasticity in muscles shows that the descending pathway is not switching off the stretch reflex Basic Pathway - AKA medial set (lies more medially in SC) - phylogenetically order (not too sophisticated but crucial for life) - involved in basic, gross movements - uses proximal muscles e.g. trunk - maintain posture and locomotion - can be involved in goal-driven AND automated movement - basic movements can be part of voluntary or involuntary systems (e.g. postural reflexes) - two main centres where this basic pathway originates from - brainstem vestibulospinal tract (comes from vestibular nuclei): gets input from inner ear (hearing/balance) > vestibular nuclei in medulla > triggers vestibulospinal tract reticulospinal tract (comes from reticular formation): activates/controls posture in terms of muscle tone (stretch reflex) > involved in basic movements of posture and locomotion - hypothalamus (higher up than brainstem in hierarchical order): controls viscera and movement behaviour in response to env (especially in emergency situations) hypothalamoreticular tract: controls reticular formation hypothalamospinal tract: goes straight to SC > they both have element of control on the 2 basic brainstem pathways - most of basic pathways go first to gamma cells to activate the proximal muscles and then reach alpha cells - hypothalamus: is controlled by cortex 2
Skilled Pathway - AKA lateral set (lie lateral in SC) - phylogenetically newer - superimposed on template made by basic pathways - give us quality of life - finer movements - involve distal muscles (finger, hands) - can be associated with goal-driven movements and automated movements (e.g. writing) 3
- most of skilled movements are voluntary Wednesday, 27 April 2016 - corticospinal tract: mainly involved in skilled movements comes from mainly primary motor cortex but also secondary motor cortex, primary somatosensory cortex, higher-order cortex (cingulate cortex) all come from layer 5 in cerebral cortex excitatory, glutamergic or aspartate motor cortex has hommunculus map: head towards lateral fissure, lower limbs towards medial side (proportional to how skilled the muscles are - fingers and hands are largest) 1 million cells in corticospinal tract goes along long white matter pathway (corticospinal cell > goes into corona radiata > extends down between thalamus and basal ganglia in internal capsule > descends into brainstem in cerebral peduncle > ends up in pyramid in medulla > 90% of fibres cross to contralateral side (decussation) and form lateral corticospinal tract in lateral faniculus, the other 10% stay on the same side and form ventral corticospinal tract > those 10% then cross over to ventral white commissure > synapse mainly on alpha cells and some gamma cells causing contraction internal capsule and corona radiata: have fibres from many different tracts pyramids of medulla: only have fibres of corticospinal tract terminations are mainly on alpha cells and some gamma cells many fibres also terminate in dorsal horn (which receives sensory info): related to the corticospinal tract fibres coming from cingulate cortex and primary somatosensory cortex > for sensory gating sensory gating: dampening down all of unnecessary afferents of the body and amplifying one sensory touch (used for attention focusing) > non-motor function of corticospinal tract corticospinal tract is involved in skilled movements lesions: skilled movements never recover - rubrospinal tract: involved in skilled movements helps with corticospinal tract 4
starts from red nucleus in midbrain > crosses at ventral tegmental decussation > descends mainly on alpha cells to help corticospinal tract in skilled movements Cortex of Movement - plans, programmers, selects, commands - 4 main layers - primary motor cortex (M1) precentral gyrus, paracentral lobule generates simple movement (flexion, extension etc.) output centre 5
lesions to M1: paralysis and loss of control of reflexes - secondary motor cortex (M2) premotor and supplementary motor cortex caudal frontal lobe plans and programs of movement active when imagining a movement (thinking of movement) maintains posture damage to M2: complex movements do not occur but simple movements are fine and also posture/muscle tone loss key sign of M2 lesion: forced grasp reflex - prefrontal cortex rostral frontal lobe moral and social centre (higher order cortex) damage: lose sensory of morality and social accountability (antisocial behaviour) - parietal cortex caudal parietal lobe focusing attention in env (e.g. sensory gating) damage: people lose attention, neglect syndrome (see things in visual field but don't pay attention to them e.g. only paying attention to right side, shaving on right side only etc.), problem with recognition Summary e.g. picking up bottle of water - focus attention on bottle: parietal cortex - subcortical area helping parietal cortex is thalamus (pulvinar): helps focus on details of the bottle - M2: plans all the different movements that you could use the pick up the bottle - basal ganglia helps M2 plan the motor programs 6
- prefrontal cortex selects which motor program to use - M1 executes the movement so you pick up the bottle (corticospinal tract - output centre) - identification > execution steps - goal-driven movement: cortex does sorting out and activates skilled and basic movements to carry out the goal-driven movement - automated movement: cortex only activates the movement e.g. learning to ride bicycle cerebellum is very active during learning of automated movement (coordination) the info that has been sorted out by cerebellum is stored in basal ganglia cortex choses winch motor system needs to be used from basal ganglia basal ganglia then selects the basic and skilled movements to execute the movement 7