The Nervous System: Nerve Plexuses, Reflexes, and Sensory and Motor Pathways. By: Avi Asraf Roger Yee Santiago Roybal Sasha Buz Valeria Muňoz Vincent Cottrill Nerve Plexuses • Cervical plexus – innervates the muscles of the neck and extends into the thoracic cavity to control the diaphragm. • Brachial plexus – innervates the shoulder girdle and upper limb • Lumbar plexus & Sacral plexus – supply the pelvic girdle and lower limb. ▫ *all designate the lumbosacral plexus. Reflexes • A reflex is an automatic motor response to a specific stimulus. • A reflex response usually removes or opposes the original stimulus. • Reflexes help maintain homeostasis by making rapid adjustments to the functions of organs/ organ systems. • Reflex arcs are an example of negative • feedback. Simple Reflex • The “wiring” of a single reflex is called a reflex arc. Simple Reflex • In the simplest reflex arc, a sensory neuron synapses directly on a motor neuron, which performs information processing function. This is also known as a monosynaptic reflex. • The sensory receptors in the stretch reflex are called muscle spindles. Simple Reflex • Stretch reflexes are important in maintaining normal posture and balance and in making automatic adjustments in muscle tone. • Doctors can use the sensitivity of the stretch reflex to test general conditions of the spinal cord, peripheral nerves, and muscles. • Example: knee jerk reflex Complex Reflexes • Polysyanptic reflexes include a longer delay between stimulus and response. • They can produce more involved responses because the interneurons can control several muscle groups simultaneously. • Withdrawal reflexes move stimulated parts of the body away from a source of stimulation. • The strongest withdrawal reflexes are caused by pain stimuli. • A flexor reflex is a withdrawal reflex affecting the muscles of a limb. Complex Reflexes • When a specific muscle contracts, opposing muscles are stretched. • Contraction of a flexor muscles should trigger in the extensors a stretch reflex that would cause them to contract, opposing the movement that is underway. • Interneurons in the spinal cord prevent such competitions through reciprocal inhibition. Integration and Control of Spinal Reflexes • Although reflexes are automatic, higher centers in the brain influence these response by stimulating or inhibiting the interneurons and motor neurons involved. • Stroking the side of an infants sole produces a fanning of toes known as the Babinski sign/positive Babinski reflex. • In adults, the toes curl, which is called Plantar reflex/negative Babinski reflex. Sensory and Motor Pathways Sensory Pathways • A sensation, the information gathered by a sensory receptor, arrives in the form of action potentials in an afferent (sensory) fiber. ▫ Posterior Column Pathway is an example of an ascending sensory pathway. The pathway of messages sent from the brain to the phalanges. Sensory and Motor Pathways Motor Pathways • The corticospinal pathway/pyramidal system provides conscious, voluntary control of skeletal muscles. • The medial and lateral pathways provide subconscious, involuntary control of muscle tone and movements of the neck, trunk, and limbs. ▫ These pathways were known as the extrapyramidal system because it was thought that they operated independently of and parallel to the pyramidal system. ▫ The corticospinal pathway begins at pyramidal cells of the cerebral cortex. Quick Review • 1) What is another name for the simplest reflex arc? • 2) Why are stretch reflexes important? • 3) What is the difference between the positive Babinski reflex and the negative Babinski reflex? • 4) What is a sensation? • 5) Where do corticospinal pathways begin?