REFLEX SYSTEMS

1. REFLEX SYSTEMS Reflexes are an involuntary response to internal or external stimuli which are (generally) protective of the body. A partial list of reflexes follows: i) Auditory - Loud noise causes eyes to turn toward noise ii) Corneal - Light touch of cornea causes eye blink iii) Cough - Food or liquid in pharynx causes coughing to clear airway iv) Gag - Stimulation of back of tongue causes gag v) Pupillary - Light causes pupils to constrict vi) Accommodation - Change of focus point changes size of pupil vii) Muscle - To be discussed in detail viii) Movement - To be discussed in detail

2. BRAIN ORGANIZATION Cerebral Cortex Thalamus Red Nucleus Basal ganglia Medulla Pons Cerebellum Afferent axon (from Sensory Cell) Spinal Cord Efferent axon (to Muscle)

3. MUSCLE RECEPTORS The main job of muscle is for movement and/or force production. Muscle also must contain sensory cells to keep the brain (cerebellum) aware of where the body’s various parts are with respect to the environment There are 5 classes of receptors in muscles , tendons and joints: Free endings ( pain, temperature, etc.) Paciniform corpuscles (vibration, pressure) Joint receptors (state of segments forming the joint, pain) Golgi Tendon Organs (force) Muscle spindles (length, speed of stretch)

4. REFLEX SYSTEMS III Elements which comprise the MYOTATIC REFLEX LOOP (engineer’s view) - motorneuron efferent Stretch Synapse with Anterior Horn Cell Muscle Dynamics Muscle Spindle GIa afferent Mechanical Coupling - motorneuron efferent To analyze the ‘what’, ‘how’, and ‘when’ of this system, we would have to Model the muscle dynamics, spindle and anterior horn cell synapse Model the encoding and decoding of spike trains in neurons Recognize that the effects of other receptors and higher centers are neglected

5. REFLEX SYSTEMS II Interaction of inhibition and excitation on a motorneuron interneurones From agonist muscle spindles From antagonist muscle spindles IPSP EPSP Motorneuron IPSP Interneuron AP Motorneuron EPSP Axonal AP

6. OTHER SPINAL REFLEXES Cutaneous reflexes - generally elicit complex responses that are protective of the organism e.g., 1) flexion withdrawal is a total body response to remove painful stimuli 2) crossed extensor reflex enhances posture during 1) 3) extensor thrust caused by light pressure on the plantar foot 4) scratch reflex removes annoying stimuli by repetitive movements Locomotory reflexes Central Pattern Generators - from insects to’ higher’ animals

7. MODIFIABILITY OF REFLEXES Often, reflexes are considered to produce a repeatable pattern of neural signals. But all reflexes have some degree of modifiability. For example, consider gripping a fragile object as opposed to a heavy object. Or consider the Achilles’ tendon reflex: Subject Sitting Subject Standing Subject Partial Standing Stimulus Stimulus Stimulus Triceps Surae EMG Triceps Surae EMG Triceps Surae EMG mv TIME in msec

8. EFFECTS OF HIGHER CENTERS ON SPINAL LEVEL MECHANISMS Reflex circuits provide higher centers of the brain with a set of elementary patterns of coordination from relatively simple combinations (e.g., reciprocal inhibition at a single joint) to more complex patterns (e.g., flexion reflexes). Higher centers produce voluntary movements by activating appropriate reflex circuits. This allows higher centers of the brain to control very complex movement patterns with relatively simple descending systems. Example of complexity of a voluntary movement

9. SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES Downflow from the brain affects the excitability of motorneurons involved in peripheral reflexes. This can be easily seen by changing the excitability of the motorneuron pool during elicitation of the tendon jerk response. NOTE: In general tonic signals from the brain are inhibitory - i.e., try to keep the ‘status quo’ Other examples are seen when specific lesion conditions are examined. Consider the following experiments:

10. SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES II Normal subject lying prone with a foot tied in an oscillating boot Position Input Torque Output IEMG Activity

11. SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES IV Subject with hemiplegia (due to stroke) lying prone with affected foot tied in an oscillating boot Position Input Torque Output IEMG Activity RESULT: Increased fusimotor drive gives larger sensitivity to stretch

12. SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES V Subject with paraplegia lying prone with foot tied in oscillating boot Position Input Torque Output IEMG Activity RESULT: Inhibitory ’ downflow’ removed

13. MUSCLE SPINDLE REDUX Historically, Sherrington (~ 1925) first described the function of the muscle spindle and its role in control of motor function. He used the Bell-Magendie Law to deafferent an animal’s limb. He found that the animal could not use the limb after it was deafferented so this became the accepted scientific fact. About 50 years later (1970), it was found that with (almost) total BILATERAL DEAFFERENTATION, the limbs could be used quite well. So how does the CNS use information from muscle spindles? (i) controls motorneuron pool excitability when a command signal comes from the brain, if the motorneurons are close to their threshold, then the command will more likely be carried out (what would happen without gamma motor system???)

14. MUSCLE SPINDLE REDUX I (ii) spindle acts as a feedback element through the brain to keep track of the state of muscles ( e.g., fatigue) (iii) spindles act as learning elements that can adjust commands for voluntary functional acts Historically, (again) about 1955, the FOLLOW-UP SERVO THEORY (Marsden, Merton, Morton) postulated that movements occur by setting spindles through gamma motorneurons and then the feedback loops causing appropriate alpha motorneuron firing. Physiological evidence for this came from experiments on decerebrate cats in which turning their head, caused leg muscle contraction which disappeared if the dorsal roots were cut ( the afferent signals remained unchanged)

15. MUSCLE SPINDLE REDUX II Alpha - Gamma Motorneuron Co-activation Command signals are sent down from the brain to control muscle action as well as to set the stiffness (gain) of the reflex pathways. The ‘stiffness’ settings are learned, e.g., picking up heavy looking but actually light objects causes large, inappropriate movements

16. MUSCLE SPINDLE SUMMARY The muscle spindle reacts to a muscle’s overall length, speed of stretch (and maybe acceleration). Its response (i.e., the number of afferent Action Potentials produced) is controlled by a gamma efferent system which comes from the CNS separate from the force - producing alpha efferent system.