motor systems

what are reflexes

involuntary movements that happen unconsciously and consist of organised patterns of muscle contraction and relaxation

what are rhythmic movements

movements like breathing, chewing or running, often involve alternating contractions and relaxations of muscles on either side of the body. Controlled by circuits primarily in the spinal cord

what are voluntary movements

Self-initiated conscious movements to accomplish a goal, they get more accurate with practice and rely on both feed forward and back control

how to gains play a role in movement

motor stability, filtering, motor correction, adaptation and learning

what does reducing gains do

increase stability, filters feedback

what does increasing gains do

facilitate online motor control (real time adjustments made by the nervous system), enhanced movement adaptation

what is high gain

rapid movement to correct errors, vulnerable to environmental changes and long delays between sensation and action, prone to over correction so can lead to oscillations

what is low gains

slow movement to correct errors, less oscillatory behaviour. Most feedback control is low gain

what is the purpose of sensory gain

to allow animals to fine tune the impact that feedback information has on motor behavioural output

what are the three types of muscle

smooth, cardiac and skeletal

properties of smooth muscle

30-200 um length, mono-nucleated, non-striated, involuntary control, lines respiratory, cardiovascular, digestive and reproductive tract

properties of cardiac muscle

50-100um length, mainly mono-nucleated, striated, lining of the heart, self contractible

properties of skeletal muscle

up to 0.3m length, multi-nucleated, striated, attatched to skeleton, voluntary contraction

break down of the anatomy of muscles

muscle - fascicles (bundle of fibres) - muscle fibres (cell) - myofibrils (organelle)

properties of myofibrils

smallest contractile unit in a muscle fibre, surrounded by sarcoplasmic reticulum, interdigitated thick and thin filaments bounded by Z discs (sarcomeres) each contains 20,000 sarcomeres

what is a thin filament

composed of F actin arranged as a helix plus tropomyosin and troponin

what are thick filaments

250 myosin with globular heads

bands of a sarcomere

I = thin, H = thick, A = overlap, Z = end

what happens to the z bands during muscle contraction

z lines pull together shortening the myofibril

describe the sliding filament mechanism of contraction

at rest troponin-tropomyosin complexes block actin binding sites, myosin heads are ADP bound, Ca in sarcoplasm is low. When muscle fibres activate action potentials travel down the t tubules, Ca is released from the cisternae of the SPR, Ca binds to troponin confirmational shape changes expose the actin binding sites, cross bridges form, mechanical energy from ATP dephosphorylation pulls filaments into overlap, ADP is shed and relaxation occurs, ATP binds again detaching it from actin

role of Ca in muscle contraction

action potential intiatiated at NMJ, travels across surface of muscle fibres, depolarisation of transverse tubules within muscle, Ca released from cisternae of SPR, diffuses across myofibrils, binds to troponin enabling cross bridges. release of ca is rapid (20-50ms) reuptake is also rapid (80-200ms)

what is a twitch

low frequency of AP cause twitches, limited Ca release, enough time for relaxation

what are tetani

high frequency of AP, more Ca released, less time for relaxation causes summation, used for heavy lifting, sustained tetanus associated with disease

what are length-tension relationships dictated by

the number of actin-myosin cross bridge connections available

what are red muscles

anti-gravity/postural muscles, slow twitch fibres, resistant to fatigue, aerobic metabolism

what are pale muscles

muscles for short bursts of activity, mix of fast twitch and slow twitch fibres. fast twitch fibres can be fatigue resistance or fatigable

how are type 1 and 2 miscle fibres distributed

approximately 50:50, randomly distributed

properties of type 1 muscle fibres

found in red muscle, slow contraction (50-110ms twitch) small force, resistant to fatigue, recruited first during contraction

properties of type 2a muscle fibres

fast contraction time (25-45ms) intermediate force, resistant to fatigue, intermediate recruitment order

properties of type 2b muscle fibres

very fast contraction (<10ms) high force easy to fatigue recruited last during contraction

where do upper motor neurons originate

motor/premotor cortex and axons extend down to brain stem or spinal cord

where to lower motor neurons originate

spinal cord or motor nuclei of cranial nerves in the brain stem, axons move down to skeletal muscles and some glands

properties of upper mn

pyramidal cells, glutamatergic transmission, various pathways

properties if lower mn

located in ventral horn of spinal cord and cranial nerve nuclei, cholinergic transmission, large neurons, extensive dendritic trees

the cell bodies of lower mn converge input from where

sensory fibres, interneurons and descending pathways

motor pool definition

the group of mn’s supplying an individual muscle, arranged in longitudinal columns of neurons spanning several spinal cord segments

what is the proximal-distal rule

medial mn pools in the ventromedial section of the dorsal horn innervate proximal muscles like the axial and shoulder. Lateral mn pools in the dorsolateral section of the dorsal horn and innervate distal muscles like the wrists and fingers

motor unit definition

the motor neuron and the skeletal muscle fibres that it innervates

MU properties

basic unit of contraction, CNS controls a MU not a single muscle fibre, MU’s contain 10s to 1000s of muscle fibres

the strength of contraction is controlled by what

firing rates of motor units (summation of twitches and tetani) and recruitment of motor units

what is the size principle

recruitment order of MU’s for contraction is correlated with size of the motor neurons cell body, small MNs recruited first

what are small MNs recruited first in muscle contraction

small surface area = high density of synaptic inputs and high electrical input resistance. EPSP = current x input resistance so small MNs have large EPSPs which are more likely to exceed threshold

how does the size principle allow for a graduated muscle response

small MNs recruited first firing more action potentials initially giving a weak force over a long period of time, if more force is needed large MN can be recruited and begin firing APs producing a sudden large amount of force for a small amount of time

types of lower MN

alpha and gamma

properties of alpha lower MN

supply extrafusal muscle fibres, large soma, fast, myelinated controls force generation (voluntary movement)

properties of gamma lower MNs

supply muscle spindle fibres, smaller soma, slower, myelinated, controls muscle spindle responsiveness (proprioception)

what do small alpha MNs innervate

type S MUs with type 1 muscle fibres

what do intermediate alpha MNs innervate

type FR MUs with type 2a muscle fibres

what do large alpha MNs innervate

type FF MUs with type 2b muscle fibres

advantages of the size principle

non-fatigable muscle fibres are used for most tasks, increments in contractile force allows for distinction between fine movements and gross movements

classifications of MU diseases

dysfunction of the MN cell body (MN diseases), of MN axons (peripheral neuropathies) of the synapse between MNs and muscle fibres (neuromuscular diseases), of the muscle fibres (myopathies)

what is Gullian-Barrè syndrome

a rapid onset muscle weakness caused by the immune system damaging the PNS a type of peripheral neuropathy

symptoms of GBS

numbness and pain in distal limbs, ascending weakness of legs and arms, facial weakness, respiratory tract failure, paralysis, 80% patients make full recovery

causes of GBS

bacterial/viral infection, B cells aberrantly produce antibodies that target myelin

GBS treatment

intravenous immunoglobulins, plasma exchange, rehab

what is myasthenia gravis

a neuromuscular junction disease resulting in a failure transmission of signal at the NMJ

types of Myasthenia gravis

autoimmune (antibodies produced against own ACh receptors in the junctional fold), congenital (transfer of AChR antibodies across placenta ), inherited (genetic mutation)

clinical features of myasthenia gravis

abnormal muscle fatigue during prolonged contraction (most often face muscles), remission and relapse phases, no sign of denervation or muscle wasting, treated with drugs preventing ACh degradation

what is Duchenne muscular dystrophy

a myopathy that causes progressivr muscle degeneration, X linked recessive caused by mutation if dystophin gene, primarily affects boys presenting before 5 as awkward walking, Gowers sign, paralysis later in life

what is dystrophin and how is it affected in DMD

muscle cell membrane associated problem, links actin cytoskeleton to extracellular matrix providing mechanical support and membrane stabilisation. Mutant dystrophin cause fibres to experience mechanical stress meaning they are susceptible to damage and degeneration

treatments for DMD

no true treatments, clinical trials show some improvement in motor function when an antisense oligonucleotide is injected into muscle

what does the spinal cord contain in relation to the CNS

relay neurons, axons of sensory neurons and cell bodies of motor neurons

how is the spinal cord organised

top to bottom = cervical (1-8), thoracic (1-12), lumber (1-5), sacral (1-5)

what does a c4-6 injury cause

tetraplegia

what does a T6-L1 injury cause

paraplegia

how is a spinal vertebrae organised

white matter (tracts), grey matter (cell bodies) organised into 10 layers. Layers 1-6 are mainly second order sensory neurons from the posterior root (dorsal horn), layers 7-8 are interneurons, layer 9 is motor neurons from the anterior root (ventral horn)

how are interneurons in the spinal cord classified

electrophysiological or developmental genetic

evaluation of electrophysiologicsl classification of interneurons

advantages- links to specific functional motifs, caveats- expressions of functional motifs can be flexible depending on behavioural states

evaluation of developmental genetics to classify interneuons

advantages - based on neurodevelopment, can be used to identify distinct cell types, caveats - cannot map neuronal function to distinct genetic sub-types

types of interneurons classified by electrophysiology

Renshaw cells, 1a inhibitory, 1b inhibitory

what are renshaw cells

interneurons receiving input from motor neuron collateral, causing recurrent inhibition, important for descending control

what are type 1a inhibitory interneurons

interneurons that receive input from 1a afferents from muscle spindle fibres, causing reciprocal inhibition, they prevent synergistic and antagonistic muscles from working against each other

what are 1b inhibitory interneurons

interneuron that recieved input from 1b afferents from the golgi tendon organs causing non-reciprocal inhibition, important for the tendon reflex

how do developmental genetics characterise interneurons

11 progenitor domains give rise to the so called cardinal classes (dorsal 1-6, ventral 0-3 and motor neurons) dorsal 1-5 are sensory, d6 v0-3 and MN are motor. Renshaw cells are V1, 1a derive from V1 and V2b

what are the ascending tracts

dorsal column and spinothalamic

what is the dorsal column tracts

transmitting from the skin, muscles and golgi tendon receptors giving information about fine touch, tactile discrimination, vibrations and control of fine movement

what is the spinothalamic tracts

carries nociceptive, temperature , crude touch and pressure from our skin to the somatosensory area of the thalamus, responsive for our quick withdraw reaction to a painful stimulus

what are the descending tracts

corticospinal - lateral and ventral are pyramidal, there are extrapyramidal tracts like the reticulospinal, vestibulospinal, tectospinal

where do descending tracts originate

upper motor neurons in motor and premotor cortrx

where do the lateral and anterior descending tracts crossover

lateral - pyramids in the medulla oblongata, anterior - spinal cord

what do the pyramidal tract control

voluntary contralateral movements, lateral = extremity muscles, upper motor neurons innervate single muscles or small sets for fine motor control, ventral = axial muscles

how were extrapyramidal tracts discovered

epileptic motor activity cannot be stopped by disturbing the pyramids

what do extrapyramidal tracts control

control of learned/automatic and/or involuntary movements, muscle tone, posture and posture adjustments, reflexes and orientating responses

what is the reticulospinal tract involved in

control of posture and gross movement like locomotion and reaching. Neurons branch extensively contacting many motor pools controlling synergistic muscles

what is the rubrospinal tract involved in

originates in the red nucleus of the midbrain and transmits signals from the cerebellum and motor cortex controlling muscle tone

what are the vestibulospinal tracts involved in

receive information from vestibulococlear cranial nerve about angular and linear head accelerations. Originates from the vestibular nuclei in the pons. The medial tract stabiles head position, the lateral tract controls antigravity extensor muscles

what is the tectospinal tract involved in

originating in the superior colliculus which is an important centre of multisensory integration, controls coordinated orientation of the neck, head and eyes according to visual and auditory stimuli

which descending pathways are cortical

corticospinal and corticobulbar

what descending pathways are brainstem paths

vestibulospinal, reticulospinal, tectospinal and rubrospinal

how are brainstem pathways organised

ventromedial pathways (vestibulospinal, reticulospinal, tectospinal) control proximal muscles, lateral pathways (rubrospinal) control distal muscles

how does the myotatic reflex work

the tendon is tapped causing an elongation of the muscle, this is detected by 1a afferents which excites the MN for that same synergistic muscle causing contraction and also excites 1a inhibitory neurons at the antagonist muscle

how does the polysynaptic withdrawal reflex work ipsilaterally

nociceptive A delta fibres excite spinal sensory neurons, spinal sensory neurons indirectly promote flexor muscle contraction through excitatory interneurons and indirectly inhibits extensor muscles through inhibitory interneurons

how does the polysynaptic withdrawal reflex work contralaterally

nociceptive fibres excite spinal sensory neurons, these neurons contant commissural neurons which help maintain balance and support, commissural neurons relax flexor muscles and contract extensor muscles through interneurons

how is the tendon reflex modulated by behavioural state

the activity of the 1b inhibitory and excitatory interneurons are controlled by descending pathways according to behavioural stayes like walking vs resting

what is a neurodegenerative disease

a disease that selectively kills neurons and progresses over time, once neurons die they do not regenerate in adulthood

what is ALS

a neurodegenerative disease the effects the whole motor units , upper and lower motor neurons degenerate across whole pathways leading to muscle wasting and eventually paralysis. It is fatal within 2-5 years of symptomatic onset

what is spinal onset als

onset begins in the limbs and spreads inwards to the spine, causes spasticity, twitching, weakness in legs arms hands and feet, poor grip strength

what is bulbar onset als

inset if muscle weakness occurs in the neck and spreads down due to degeneration of the corticobulbar tract. Causes wasting, weakness and twitches as well as difficulty swallowing, spasms of the vocal cords, slurred speech. Worse prognosis due to respiratory infections

cognitive symptoms of als

50% have episodic memory impairments, 15% develop frontotemporal dementia, atrophy of the frontal and temporal loves, personality changes and language disorders

general genetics of als

5-10% of cases are hereditary (FALS) the rest are sporadic (SALS), it is a very heterogeneous disease and some mutations can cause both ALS and FTD