L4 NEUR3101 Spinal Control PDF

NEUR3101 Motor control Lecture 4 Spinal control of movement and locomotion Reflexes controlled by muscle and cutaneous afferents A/Prof Ingvars Birznieks  Chapter 16, Purves et al., Neuroscience (6th Ed) Chapters 35-36, Kandel et al., Prin...

NEUR3101 Motor control Lecture 4 Spinal control of movement and locomotion Reflexes controlled by muscle and cutaneous afferents A/Prof Ingvars Birznieks  Chapter 16, Purves et al., Neuroscience (6th Ed) Chapters 35-36, Kandel et al., Principles of Neural Science (5th Ed) Chapter 13, Bear et al., Neuroscience: exploring the brain (4rd Ed)  Learning Outcomes You should be able to: Describe the spinal reflex and its components Describe how reflexes are modulated accordingly to the task Describe various muscle afferent reflex pathways ( e.g., Ia, Ib) Describe different classes of muscle receptors, and demonstrate their role in motor control Demonstrate how rhythmic movements are controlled and describe the role of central pattern generators (CPGs)  Spinal reflex What is a spinal reflex? An involuntary response to activation of a sensory receptor that is mediated through spinal pathways. The concept of a “reflex” is changing Once perceived as “hard-wired” (i.e. always get the same response to a given stimulus) now even the simplest reflex is viewed as highly modifiable Now the concept of reflex modulation predominates - a reflex response depends on the context/task being performed - reflexes are incorporated with the voluntary motor command Spinal reflex arc  Sensory systems controlling reflexes Proprioception Proprioception from Latin proprius, meaning "one's own", "individual" and perception, is the sense of the relative position of neighbouring parts of the body position of limbs and other body parts in space strength of effort being employed in movement Specialised mechanoreceptors – proprioceptors (“receptors for self”) Muscle spindles Golgi tendon organs Joint receptors To be able provide this information signals from specialised proprioceptors have to be integrated with signals from other receptor types and sensory systems: Vestibular sensory system Skin (cutaneous) mechanoreceptors may provide proprioceptive information to signal body part location by sensing pattern of skin stretch Visual system (also plays an important role continuously calibrating the proprioceptive system)  Proprioception Muscle spindles Extrafusal muscle fibres – true force producing fibres of the muscle. Intrafusal muscle fibres – part of the sensory organ - muscle spindles. Keep sensory elements stretched to be able to maintain sensitivity to changes in stretch regardless of the overall muscle length. Primary endings or group Ia afferents – show rapidly adapting responses to changes in muscle length. Provide information about velocity of the movement. Secondary endings or group II afferents – produce sustained response to muscle length, thus largely provide information about extent of muscle stretch. ɤ-motor neurons activate intrafusal muscle fibres and by changing tension significantly impact on sensitivity of muscle spindles. α-motor neurons activate extrafusal (force producing) muscle fibres The highest density of muscle spindles is in extraocular muscles, intrinsic muscles of the hand and muscles of the neck. Figure 9.7 Muscle spindles are not present in middle ear muscles.  Co-activation of α and γ motor neurons Muscle spindles respond to muscle stretch, but muscle contraction shortens muscle rather than stretches it Figure 16.11  Functions of spinal reflexes Rapidly respond to perturbations Allow very fast initiation of corrective responses following an unexpected perturbation Examples: Stretch reflex, Cutaneous reflex, Flexor withdrawal reflex Contribute to the motor control and movement adjustments Take care of the details of movement execution to unload higher control centers Reflexes can be highly organised and modulated accordingly to the task A perturbation of one arm causes an excitatory reflex response in the contralateral elbow extensor muscle when the contralateral limb is used to prevent the body from moving forward by grasping a table. The same stimulus produces an inhibitory response in the muscle when the contralateral hand holds filled cup. Kandel Schwartz & Jessel (2000). Principles of Neural Science 4/e McGraw-Hill.  Muscle stretch reflex Stretch reflex also known as “myotatic”, “deep tendon”, “knee-jerk” or “patellar” reflex is a monosynaptic reflex with biological function to maintain muscle at the desired length. The afferent neuron is connected to a muscle spindle, which detects stretch in a muscle. The efferent neuron is a motor neuron, which causes muscle to twitch. Figure 13.7  Different ways to evoke a spinal reflex Rotation of joint (most natural stretch reflex) Tendon tap Electrical stimulation (H-Reflex)  Muscle stretch reflex Biological function of the stretch reflex is to maintain muscle at a desired length. From the control point of view stretch reflex is a feedback control mechanism. Deviation from a desired length is detected by muscle spindles. The increase or decrease in stretch of muscle spindles alter their discharge rate, which directly translates into excitation of α-motor neurons and muscle contraction. The induced muscle contraction will return muscle to the desired length and limb to its initial position restoring muscle spindle activity to a background level. During neurological testing the input is mostly from Ia muscle spindle afferents. Normally muscles are always under some degree of stretch, this reflex circuit mediated by group II muscle spindle afferents is responsible for the steady level of muscle tension in muscle called muscle tone. Figure 16.10 Spinal reflexes as part of the motor command The central nervous system can take advantage of the functional organisation of spinal cord to reduce processing requirements of higher centres. Gamma drive to muscle spindles allows stretch reflex contribute to muscle activity during slow contractions. Kandel Schwartz & Jessel (2000). Principles of Neural Science 4/e McGraw-Hill.  Gain of the stretch reflex The gain of the stretch reflex depends on excitability of α-motor neurons sensitivity of muscle spindles regulated by γ-motor neurons The level of γ-motor neuron activity referred to as γ- bias or gain can be adjusted by upper motor neuron pathways as well by a local circuitry. The gain of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the muscle spindle. Under various demands of voluntary and involuntary movement α and γ motor neurons are often co-activated by higher centres to prevent muscle spindles from being unloaded or overstretched/ overactivated. γ motor neuron activity can be modulated independently of α-motor neuron activity if the context of movement requires it. In general γ motor neuron activity is high if movement is relatively difficult and demands rapid and precise execution.  Fusimotor set Muscle spindle sensitivity adjusted for different conditions or behavioural “states” 1) differential control of static (γs) and dynamic (γd) gamma activity 2) more complex tasks have increased γ drive to increase sensitivity “state” rest sit stand slow fast imposed paw beam walk walk mov’t shake walk γd γs Kandel Schwartz & Jessel (2000). Principles of Neural Science 4/e McGraw-Hill.  Proprioception Golgi tendon organs (GTO) Golgi tendon organs are formed by branches of group Ib afferents distributed among collagen fibres that form tendons. They provide information about muscle tension. GTOs are arranged in series with a small number (10-20) of extrafusal muscle fibres. Population of afferents provide accurate sample of tension which exists in a whole muscle. Figure 9.7 Golgi tendon organs - a negative feedback system to regulate muscle tension Golgi tendon organ circuit is a negative feedback system to regulate muscle tension contacts Ib inhibitory interneurons in local circuits Golgi tendon organ circuit counteracts small changes in muscle tension by increasing or decreasing the inhibition of α-motor neurons. Golgi tendon organ control system tends to maintain a steady level of force, counteracting effects that diminish muscle force, for example, fatigue. It plays a protective role at large forces. Ib inhibitory interneurons receive modulatory synaptic inputs from various sources including upper motor neurons, joint receptors, muscle spindles and cutaneous receptors.  In summary Muscle spindle system is a feedback control system that monitors and maintains muscle length and thus keeps limbs in a desired position. Golgi tendon organ system is a feedback control system that monitors and maintains muscle force. Protective reflexes Protective reflexes: flexion reflex pathways Triggered by cutaneous nociceptors (Aδ myelinated fibres) Polysynaptic pathway Excitation of ipsilateral flexors and inhibition of extensors Inhibition of contralateral flexors and excitation of extensors, thus providing postural support during withdrawal Descending pathways regulate suppression of the reflex Following damage to descending pathways and after removing inhibition other types of stimuli can trigger the flexion reflex Figure 16.14  Recurrent inhibition and Renshaw cell Identified in the 1940’s by Renshaw Renshaw cell receives monosynaptic excitatory input from motoneuron axon collaterals Renshaw cell output is monosynaptic inhibition back to homonymous and synergistic motoneurones (feedback) It is called “Recurrent inhibition”  Renshaw cell-function Regulation of motoneurone firing by higher level inputs Inputs from supraspinal inputs can inhibit and the brain facilitate the Renshaw cell this is a mechanism to control how motoneurones respond to inputs (gain control) when Renshaw cell is inhibited, the motoneurone is more responsive to inputs when Renshaw cell is excited, the motoneurone is inhibited and becomes less responsive to inputs - decreased gain means - smaller output for same input Rothwell JC (1994) Control of Human Voluntary Movement. 2nd ed, Chapman and Hall, Fig 5.11  Rhythmic movements Sequence of coordinated motor actions performed in repetitive manner: locomotion, flying, breathing, swallowing and vomiting Lamprey locomotion Feline locomotion Human locomotion Why differentiate from other movements? The basic pattern for rhythmic movements is thought to be controlled by oscillating circuits within the spinal cord: CENTRAL PATTERN GENERATORs (CPGs) Signals from higher centres (supraspinal) and peripheral receptors (afferent feedback) influence the basic pattern produced by the CPG to produce a flexible functional output. Advantages of spinal control.  Spinal cord circuitry and locomotion Central pattern generators (CPGs) are biological neural networks that produce rhythmic patterned outputs. CPGs don’t require sensory input to be initiated, but their activity could be modified by sensory feedback. They are fully capable of controlling the timing and coordination of complex patterns of movement and adjusting them in response to altered circumstances without coordinating input from the brain. Cats after transection of the spinal cord at thoracic level can walk on a treadmill making coordinated movements with their hind limbs. Speed of locomotor movements is determined by the speed of the treadmill, suggesting that the stretch reflex is a part of a motor program. When dorsal roots are also transected and thus there is no sensory input, locomotion can be induced by intravenous injection of L-DOPA (dopamine precursor). In humans CPGs have not been successfully activated to restore continuous natural walking pattern. Warning may be Figure 16.15 https://www.youtube.com/watch?v=sK7nKweiDro (cat) unpleasant to watch! https://www.youtube.com/watch?v=oPerfpxYJ1U (rat; CPG + spinal cord regeneration)  CPGs in intact animals Central pattern generators play a broad role in all animals: besides various types of locomotion (on the ground, in the water, in the air) they might also be involved in generation of respiratory and swallowing patterns. In intact animals, spinal CPGs are activated by tonic (non-rhythmic) descending drive from the brain. The CPG produces the basic oscillating pattern. Central pattern generators show amazing variability and adaptability: Sensory information shapes the basic oscillating pattern to make the motor output appropriate for the environment. Signals from the brain can modify the motor output to meet goals or refine the CPG command.  Locomotion in the leech The leech propels itself through the water by sequential contraction and relaxation of the body wall musculature of each segment. The segmental ganglia in the ventral mid line coordinate swimming, with each ganglion containing a population of limited number of neurons. Electrical recordings from the ventral (EMGV) and dorsal (EMGD) longitudinal muscles in the leech and the corresponding motor neurons show a reciprocal pattern of excitation for the dorsal and ventral muscles of a given segment.