Brachial Plexus Formed by ventral rami of C5–C8 and T1 It gives rise to the nerves that innervate the upper limb Skin, Shoulder Joint, Muscles

(a) Roots (rami C5 – T1), trunks, divisions, and cords Roots (ventral rami): C4 Dorsal scapular C5 Nerve to subclavius C6 Suprascapular Upper Posterior divisions C7 Middle Trunks Lateral C8 Lower Cords Posterior T1 Long thoracic Medial Medial pectoral Lateral pectoral Axillary Upper subscapular Musculo- cutaneous Lower subscapular Thoracodorsal Radial Medial cutaneous nerves of the arm and forearm Median Ulnar (a) Roots (rami C5 – T1), trunks, divisions, and cords Anterior divisions Posterior divisions Trunks Roots Figure 13.9 (a)

Musculocutaneous nerve Ulna Radius Ulnar nerve Median nerve Axillary nerve Anterior divisions Posterior divisions Trunks Roots Humerus Radial nerve Musculocutaneous nerve Ulna Radius Ulnar nerve Median nerve Radial nerve (superficial branch) Dorsal branch of ulnar nerve Superficial branch of ulnar nerve Digital branch of ulnar nerve Muscular branch Median nerve Digital branch (c) The major nerves of the upper limb Figure 13.9 (c)

Lumbar Plexus Arises from L1–L4 Innervates the thigh, abdominal wall, and psoas muscle Femoral Nerve- Quadriceps Obturator Nerve- adductor muscles

Ventral rami: Iliohypogastric L1 Ilioinguinal Femoral L2 Lateral femoral cutaneous Iliohypogastric Ilioinguinal Obturator L3 Genitofemoral Anterior femoral cutaneous Lateral femoral cutaneous Saphenous L4 Obturator Femoral L5 Lumbosacral trunk (a) Ventral rami and major branches of the lumbar plexus (b) Distribution of the major nerves from the lumbar plexus to the lower limb Figure 13.10

Sacral Plexus Arises from L4–S4 Serves the buttock, lower limb, pelvic structures, and perineum Sciatic nerve Longest and thickest nerve of the body Innervates the hamstring muscles, adductor magnus, and most muscles in the leg and foot Composed of two nerves: tibial and common fibular

Ventral rami and major branches of the sacral plexus Superior gluteal L5 Lumbosacral trunk S1 Inferior gluteal Common fibular S2 Tibial S3 Posterior femoral cutaneous S4 Pudendal S5 Sciatic Co1 Ventral rami and major branches of the sacral plexus Figure 13.11 (a)

(b) Distribution of the major nerves from Superior gluteal Inferior gluteal Pudendal Sciatic Posterior femoral cutaneous Common fibular Tibial Sural (cut) Deep fibular Superficial fibular Plantar branches (b) Distribution of the major nerves from the sacral plexus to the lower limb Figure 13.11 (b)

Reflexes Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, Example: driving skills

1 2 3 4 5 Stimulus Skin Interneuron Receptor Sensory neuron Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Figure 13.14

endings (type II fiber) Efferent (motor) fiber to muscle spindle Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle  Efferent (motor) fiber to extrafusal muscle fibers Primary sensory endings (type Ia fiber) Extrafusal muscle fiber Muscle spindle Intrafusal muscle fibers Connective tissue capsule Sensory fiber Golgi tendon organ Tendon Figure 13.15

Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time Time (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber. (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Figure 13.16a, b

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b Figure 13.17 (2 of 2)

Superficial Reflexes Plantar reflex Stimulus: stroking lateral aspect of the sole of the foot Response: downward flexion of the toes Tests for function of corticospinal tracts

Superficial Reflexes Babinski’s sign Stimulus: as above Response: dorsiflexion of hallux and fanning of toes Present in infants due to incomplete myelination In adults, indicates corticospinal or motor cortex damage