1. Reflexes • The typical pattern of a neural “circuit” is an arc. sensory afferents  integration in CNS  motor efferents  effector tissue • Reflexes are the simplest form of integration by CNS. • spinal cord and brain stem only • monosynaptic • polysynaptic • Somatic reflexes • effector tissue = skeletal muscle Fig. 13.21

2. Skeletal Muscle Proprioceptors • Muscle Spindles • detect muscle length • inhibit muscle stretch • Golgi Tendon Organs • detect muscle tension • inhibit over-contraction

3. Golgi Tendon Organ Type Ib fibers Fig. 13.23

4. Muscle Spindles fus- = spindle intrafusal fibers – specialized muscle fibers of the spindle extrafusal fibers – regular skeletal muscle fibers A Type Ia fiber A Type II fiber Fig. 13.20

5. Muscle Spindles • The (Ia) sensory nerve ending is a stretch receptor, wrapped around the center of an intrafusal fiber. • The intrafusal fibers have sarcomeres only on their ends. • A stimulation of intrafusal fibers will cause contraction of the ends and stretch of the middle. • There is always some tonic discharge by the A motor neurons. • Therefore, the stretch receptors are always slightly stimulated – warmed up and ready to respond to any stretch of the muscle. Fig. 6-3 Ganong

6. Stretch Reflex • a monosynaptic reflex • e.g., the knee-jerk reflex A • negative feedback • ipsilateral stimulation • of extension • (simultaneous inhibition • of flexion – polysynaptic) Fig. 13.21

7. via interneurons sensory neuron from skin  [interneuron(s)]  A motor neuron examples withdrawal (flexor) reflex crossed extensor reflex Polysynaptic Reflexes

8. Withdrawal Reflex + = stimulatory -= inhibitory • a flexor reflex • ipsilateral • stimulation of flexion • inhibition of extension Fig. 13.22

9. Crossed Extensor Reflex • an extensor reflex • contralateral • stimulation of extension • inhibition of flexion Fig. 13.22

10. Somatic Motor Pathways(from brain) • organizing principles • phylogenetic • lateral and medial motor systems • medial tracts are more primitive (see ventral/anterior columns) • control proximal muscles: posture and gross movements • lateral tracts are more advanced • control distal muscles: fine, skilled movements • clinical • pyramidal and extrapyramidal systems • pyramidal system (direct pathway) • extrapyramidal systems (indirect pathways)

11. Fig. 14.23 Somatic Motor System • The frontal lobe is a major control center for somatic motor functions. • precentral gyrus • 30-60% of cortical motor fibers originate here.

12. Fig. 13.6 Pyramidal System • direct pathway • The axon of one neuron travels from the cerebral cortex to lower motor neurons (such as the A motor neurons). • two paths • lateral corticospinal tracts • ventral corticospinal tracts

13. Fig. 12-2 Ganong • lateral corticospinal tracts • lateral motor system • major importance only in primates • precise control of voluntary movements in distal muscles • In humans, 80-90% of cortical motor fibers travel in these tracts. • cross over in the pyramids of the medulla • giving the “pyramidal” system its name • ventral corticospinal tracts • medial motor system • cross over (via interneurons) in the spinal cord, at the level of the lower motor neuron

14. Medullary Pyramids Fig. 14.27 Fig. 14.9