Peripheral Nervous System and Lumbar Plexus

The lumbar plexus is formed by the anterior rami of the upper four lumbar nerves (L1-L4). It's located within the psoas major muscle.

Most branches of the lumbar plexus emerge from the lateral border of the psoas major muscle. These branches include the iliohypogastric, ilioinguinal, lateral cutaneous nerve of the thigh, and the femoral nerve.

The iliohypogastric and ilioinguinal nerves arise from L1 and supply the muscles of the anterior abdominal wall. The iliohypogastric nerve also supplies skin on the lower anterior abdominal wall, while the ilioinguinal nerve supplies skin in the groin and scrotum or labia.

The lateral cutaneous nerve of the thigh arises from L2-L3 and supplies the skin on the lateral surface of the thigh. It crosses the iliac fossa in front of the iliacus muscle and enters the thigh behind the inguinal ligament.

The femoral nerve is the largest branch of the lumbar plexus and originates from L2-L4. It runs between the psoas and iliacus muscles and enters the thigh behind the inguinal ligament. It supplies muscles like iliacus, pectineus, sartorius, and quadriceps femoris, as well as skin on the anterior thigh and medial leg and foot.

The obturator nerve emerges from the medial border of the psoas muscle and supplies muscles in the medial thigh, like adductors.

The genitofemoral nerve arises from L1-L2 and emerges from the anterior surface of the psoas muscle. It supplies the cremaster muscle and the skin of the scrotum or labia.

The branches of the lumbar plexus provide sensory and motor innervation to the lower abdomen, anterior thigh, and medial leg and foot.

Autonomic ganglia are clusters of nerve cells located along the efferent pathways of the autonomic nervous system, acting as relay points between preganglionic and postganglionic neurons.

Sympathetic ganglia are situated alongside the sympathetic trunk (chain of ganglia running down the spine) or in prevertebral positions. They are associated with the 'fight-or-flight' response.

Parasympathetic ganglia are found close to or within the walls of internal organs (viscera). They help maintain homeostasis and promote 'rest-and-digest' functions.

Preganglionic fibers are myelinated, small, and relatively slow-conducting B fibers. They carry signals from the central nervous system to the ganglia.

Postganglionic fibers are unmyelinated, smaller, and slower-conducting C fibers. They carry signals from the ganglia to target organs.

A nerve that runs down the front of the psoas muscle. Divides into genital and femoral branches, supplying the cremaster muscle and a small area of skin on the thigh. Also involved in the cremasteric reflex.

A nerve that arises from L2-L4, enters the pelvis, and passes through the obturator foramen into the thigh. Supplies muscles of the medial thigh compartment, including adductors, obturator externus, pectineus, and skin on the medial thigh.

A nerve that emerges from the lower border of the piriformis muscle. The largest nerve in the body, it divides into the tibial and common peroneal nerves.

A branch of the sciatic nerve that supplies muscles of the anterior compartment of the leg, specifically tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius, and extensor digitorum brevis. It also supplies skin over the cleft between the first and second toes.

A nerve that arises from L4-S3 and is the largest branch of the sacral plexus. It emerges from the lower border of the piriformis muscle and divides into the tibial and common peroneal nerves.

A nerve that emerges from the lower border of the piriformis muscle. The largest nerve in the body, it divides into the tibial and common peroneal nerves. It supplies muscles responsible for movement at the knee, ankle, and foot.

A branch of the sciatic nerve that divides into the deep peroneal nerve and the superficial peroneal nerve. In the thigh, it supplies the short head of the biceps femoris.

A plexus formed by the anterior rami of L4-S3 nerves, located on the posterior pelvic wall in front of the piriformis muscle. It supplies muscles of the gluteal region, thigh, and leg.

Postganglionic sympathetic fibers distribute to smooth muscle in blood vessel walls, sweat glands, and arrector pili muscles in the skin.

Myelinated axons exiting the spinal cord via the anterior nerve roots, forming pelvic splanchnic nerves (preganglionic fibers).

Preganglionic fibers travel up the sympathetic trunk to synapse in cervical ganglia, with postganglionic fibers joining cervical spinal nerves via gray rami communicantes.

They are the preganglionic fibers of the craniosacral outflow that synapse in peripheral ganglia close to the organs they innervate.

They are located close to the viscera they innervate, including the ciliary, pterygopalatine, submandibular, and otic ganglia.

Preganglionic fibers can travel down the sympathetic trunk to synapse in lumbar and sacral ganglia, with postganglionic fibers joining lumbar, sacral, and coccygeal spinal nerves.

Preganglionic fibers that don't synapse in sympathetic trunk ganglia form splanchnic nerves (greater, lesser, least). These nerves synapse in prevertebral ganglia.

They are located in nerve plexuses like the cardiac, pulmonary, myenteric (Auerbach), and mucosal (Meissner) plexuses.

The pelvic splanchnic nerves synapse in ganglia within the hypogastric plexuses.

Postganglionic fibers from prevertebral ganglia innervate smooth muscle and glands in the viscera.

They are nonmyelinated and relatively short compared to sympathetic postganglionic fibers due to the proximity of the ganglia to the effector organ.

Some preganglionic fibers skip the ganglia and directly synapse with chromaffin cells in the adrenal medulla.

They travel from the viscera to their cell bodies located in the sensory ganglia of the cranial nerves or the DRG of the sacral spinal nerves.

Chromaffin cells in the adrenal medulla are modified sympathetic postganglionic neurons that release epinephrine and norepinephrine into the bloodstream.

Afferent (sensory) fibers travel from visceral organs through sympathetic ganglia, without synapsing, to carry information to the central nervous system.

They either form local reflex arcs or travel to higher centers of the ANS, like the hypothalamus.

The superficial peroneal nerve supplies the fibularis longus and brevis muscles in the lateral compartment of the leg, and also provides sensory innervation to the lower anterior leg and the dorsum of the foot.

The tibial nerve innervates the hamstring muscles in the thigh, including the semitendinosus, biceps femoris (long head), and adductor magnus (hamstring part). It also supplies the muscles of the posterior leg compartment, such as the gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus. Finally, it branches into medial and lateral plantar branches to innervate the muscles of the sole of the foot.

The superior gluteal nerve emerges from the upper border of the piriformis muscle and innervates the gluteus medius, gluteus minimus, and tensor fasciae latae muscles.

The inferior gluteal nerve emerges from the lower border of the piriformis muscle and supplies the gluteus maximus muscle.

The posterior cutaneous nerve of the thigh emerges from the lower border of the piriformis muscle and provides sensory innervation to the skin of the buttock and back of the thigh.

The nerve to the quadratus femoris muscle supplies the quadratus femoris and inferior gemellus muscles.

The nerve to the obturator internus muscle supplies the obturator internus and superior gemellus muscles.

The pudendal nerve emerges from the pelvis through the greater sciatic foramen and enters the perineum through the lesser sciatic foramen. It supplies various structures in the perineum, including the external genitalia, anal sphincter, and pelvic floor muscles.