Open and closed loop control

• Introduction Human Motion Control – Musculoskeletal system with proprioceptive feedback 2008-2009 – Role of sensors Proprioception • – – Peripheral nervous system – Sensors and • Information Processing – Organization & reflexes • Sensors in motor pathways – Joint capsule sensors – Muscle spindles – Golgi Tendon Organs Proprioception

Proprioception Proprioception Loss of proprioception

Have you ever said any of the following? Example loss of proprioception: Ian Waterman from proprius Latin for “one’s own” and - “I think I smell gas!” - No sensory feedback below the neck perception. - “Did you hear that sound?” - Not paralyzed! - “I have a headache.” - One of only 10 patients in the world. The body's awareness of position, posture, - “I just noticed my elbow is slightly flexed and movement and changes in equilibrium my hand is approximately 30 centimeters away from my face.”

The man who lost Proprioception happens unconsciously. his body

1 Sensors Central Nervous System (CNS) Structure of the CNS • 1011 neurons • Vestibulary system: Translational and rotational accelerations of the head • 104 per : • Visual system: Position and velocity information 1015 synapses • Tactile system: External force information • A-synchronous processing • Joint capsule receptors: Joint angle • Distributed information storage • Muscle spindles: Muscle length and contractile velocity • Golgi tendon organs: Muscle Force

Sensory integration Ω Central Nervous System

CNS Sensory and motor projections Sensory and motor projections The Brain in the cortex in the cortex

Cerebrum Higher brain functions like: speech, emotions, reasoning, movements, vision, memory, etc…

Cerebellum Brain stem Regulation and coordination Basic vital life functions of movement, posture and like breathing, heartbeat, balance. Sensory system Motor system blood pressure. Sensory system Motor system

2 CNS CNS Building block of the CNS: the neuron Spinal cord

3. Ventral root: motor output to muscles 5. Dorsal root ganglia: contains cell bodies of sensory neurons 6. Dorsal root: sensory input to spinal cord 9. Posterior horn: spinal 10. Anterior horn: alpha motoneurons 2. : myelinated 4. : cell bodies

Connections between CNS and PNS: Connections between neurons: Organisation of neurons: Neurons synapses nerve fibers • Neuron also called “nerve cell” • • Junction between two – receive incoming spikes. – diffusely branched neurons or between – one neuron generally has many dendrites. neuron and effector • • Between 1,000 - – transmits information. 10,000 synapses per – one neuron has one axon. – length up to one meter. neuron • Axon terminals • Electrical (few) or – communicate output to other neurons chemical (most) (or muscles). – One axon generally has multiple terminals

3 Types of somatic endings Types of somatic sensory nerve endings Neural integration: neuronal pools () (musculoskeletal system)

Muscle spindle muscle stretch and tendon force Touch, dynamic stretch velocity pressure

Deep pressure, Skin stretch Touch, static , temperature, vibration pressure crude touch

Neural integration: Neural integration Simplified structure of the CNS Motoric Pyramidal & Extrapyramidal tracts (CNS and PNS) • The somatic sensory system • the motor division

voluntary

Pyramidal & extrapyramidal tracts

automated

efferent afferent Pyramidal system or Extrapyramidal system or “selective control system” “automatic control system”

4 Information processing Reflexes Reflex loops

Association neuron • Short latency reflexes (about 50 ms): Receptor • Cortex complex – Spinal cord – Monosynaptic (excitation of same muscle that received ) – motor control – Act like delayed ‘stiffness’ and ‘viscosity’ (position and velocity • Lower brain feedback) – subconscious activities, coordinate functions • Medium latency reflexes (about 70 - 100 ms) – medulla, pons, cerebellum, basal ganglia – Spinal cord / Brain stem • Spinal cord – Coordinated reflex between antagonistic muscle pairs – Adequate response to perturbation – reflexes basic • Long latency reflexes Integrating center – Cerebellum Effector – Well-coordinated reflex responses for muscle system of one limb Reflex: involuntary response to a stimulus. Response is initiated before the subject is aware of it.

Monosynaptic reflex: patellar reflex Multisynaptic reflex: withdrawal reflex Reciprocal inhibition

1. Subject steps on • Antagonist muscle is inhibited tack. when agonist muscle is excited.

2. Leg flexes as • Increases effectiveness response to pain. 3. Opposite leg • Increases efficiency. extends for weight support.

1. Hammer tap stretches tendon ‰ muscle sensors stretch. 2. Sensory neuron excites spinal motor neuron of extensor. 3. Sensory neuron inhibits flexor motor neuron through /

5 Injuries to the CNS & PNS affecting Sensors in motor pathways reflexes Sensors in the joint capsule

• Joints – Stretch receptors

• Muscles – Golgi tendon organ –

Sensors in the joint capsule Sensors in the muscles Golgi Tendon Organ

• Mainly stretch receptors in ligaments • Golgi Tendon Organs: • Only signaling towards outer position of the – In series with the muscle fibers joint – strain in tendon ≈ force in muscle • Not appropriate for proportional control of joint angle • Muscle spindles: • Slow adaptation – parallel to muscle fibers – sensitive to length and contraction velocity • Presumed to have role in error – active ‘intra-fusal’ muscle fibers, passive sensory part 160x signaling for learning – innervated by γ motor neurons

6 Golgi Tendon Organ Muscle spindle Muscle spindle: Length and velocity feedback

• Strain in tendon Ω force • Ib • About 50 GTOs per muscle • Sensitive to a few motor-units (active force) • Less sensitive to passive muscle stretch • Contribution to position feedback: Compensates (slow) muscle dynamics • Comparable with pressure feedback in muscle length muscle velocity Muscle Ia afferent nerve hydraulic actuators γ γs-motor neuron Spindle II afferent nerve γd-motor neuron demo

Muscle spindle sensory part Mechanical equivalent of spindle Response muscle spindle α motor neuron

• : – length: 7-8 mm – velocity tendon Ia, II tendon γ motor neuron • : • Extra-fusal fibers innervated by: α motor neurons – length: 3-4 mm • Intra-fusal fibers innervated by: γ motor neurons – senses length • Sensor is sensitive to – stretch: II afferents – stretch and stretch velocity: Ia afferents stretch: II afferents • Goal of gamma-activation: keep muscle spindle pre- stretch & stretch velocity: Ia afferents tensed and regulate sensor gain.

7 Function γ-activation • Type Ia afferent – sensory information from the central annulospiral endings of Competitive summation in Ia fibers all the intrafusal fibres • excitatory effect (mono- or polysynaptic) on the alpha motorneurons of the same muscle (and its synergists) • Connection of nerve from nuclear chain and nuclear • inhibitory effects (via interneurons) on antagonist alpha motorneurons bag has no . • Spike trains do not merge. • ‘Winner takes all’: – Nerve ending with highest frequency overrules competitors by ‘antidromic inhibition’ – Max-operation • During posture: Length information • Type II afferent • During motion: – sensory information from the secondary endings, mainly associated with chain fibres. Velocity information • excites alpha motorneurons of the same muscle via polysynaptic • Adjustment of operating point (optimal sensitivity) pathways. • Adjustment feedback gain of reflex loop demo

Summary Muscle spindle Transmission speeds of neurons ‘Open-loop’ musculoskeletal models (measured in cat, humans about 20% slower) • Length and contraction velocity information Muscle length • γ-activation: Reflex gain, sensor sensitivity Group Diameter (µm) Transmission Sensor Stimulus -r speed (m/s) Muscle velocity • Length feedback: Contributes to additional Ia 12 – 20 70 – 100 muscle length & -r spindle velocity Mex ‘stiffness’ of muscle Ib 12 – 20 70 – 100 Golgi tendon force muscle Skeletal organ r — — dynamics inertia • Velocity feedback: Contributes to additional II 6 – 12 35 – 70 muscle length α F M θ spindle ‘viscosity’ of muscle 2 – 5 12 - 30 Pacini sensors pressure 0.5 – 2 0.5 – 2.5 free nerve nociceptive Passive • Time-delays in nervous system results in endings visco-elasticity additional dynamic behavior

Result depends on: •Inertia, passive visco-elasticity •Intrinsic muscle properties: length & velocity dependency muscle

8 Closed loop control Closed loop musculoskeletal models Conclusion

Km(u0) -r disturbance + + • Musculoskeletal system needs feedback Bm (u0) -r setpoint Central + position u Mex • Human nervous system comprises integrated α + Skeletal nervous muscle skeleton Hact r — — feedback systems inertia + - system + + + F M θ – Reflex loops at different levels of organization τ Golgi – some sensors are also feedback systems Ib τ visual τ -r • Feedback sensors vestibulary + Ia τ sensor Muscle – muscle spindles tactile τ -r – golgi tendon organis capsule II Spindle muscle spindle – joint capsule stretch sensors

Golgi tendon organ γs

γd

9