There are 31 spinal nerve pairs:

• 8 cervical spinal nerve pairs (C1-C8)

• 12 thoracic spinal nerve pairs (T1-12)

• 5 lumbar spinal nerve pairs (L1-L5)

• 5 sacral spinal nerve pairs (S1-S5)

• 1 coccygeal spinal nerve pair (Co1)

Recall that the spinal cord ends at L2, which means that many of the distal spinal nerves will have to travel a comparatively long distance to their intervertebral foramen.

Spinal nerves - Intervertebral foramina

Spinal nerves typically exit the vertebral column through intervertebral foramina inferior to the same-numbered vertebra. For example, the T7 spinal nerve exits the column between the T7 & T8 vertebrae. Another example includes the L5 spinal nerve exits the vertebral column between L5 & S1 vertebrae. Recall that from there most of these spinal nerves will send fibers to plexuses to produce multi-segmented peripheral nerves.

The main exception is the cervical region. There are 8 cervical spinal nerves and only 7 cervical vertebrae. Cervical spinal nerves exit the vertebral column superior to the same-named vertebra. For example, the C1 spinal nerve exits the vertebral column between the skull and C1, C2 spinal nerve exits between C1 & C2 vertebrae, and C8 spinal nerve exits between C7 & T1 vertebrae.

Herniations of the IV disc tend to occur posterolaterally.

Example: herniation of IV disc between L3 & L4 would less typically affect L3 spinal nerve as it exits too superiorly in the IV foramen, but it MAY affect the L4 spinal nerve.

2. Identify the hip joint and associated structures. Explain what types of movement occur at these joints and other pertinent clinical information.

The hip joint is synovial, ball & socket joint, and is the second most mobile joint in the body (second to the glenohumeral joint - both ball & socket joints). The joint is formed by the acetabulum of the coxal bone and the head of the femur.

Accessory Joint Structures, Ligaments, & Tendons

• Acetabular labrum is similar in structure to the glenoid labrum of the glenohumeral joint, and allows for a better fit between the acetabulum and head of femur

• Iliofemoral ligament is often considered to be the strongest ligament in the body

  • Most prominent ligament, more anteriorly-placed, and provides a strong role in prevention of hyperextension of the hip joint

• Medial & lateral rotator muscles of the hip play an important role in support and structure of the hip joint

Movements: The hip is the second most mobile joint of the body but considerably more stable than the glenohumeral joint. Its movements include:

• Flexion/extension

• Abduction/Adduction

• Medial (internal) rotation/Lateral (external) rotation

• Circumduction

Clinical Considerations: fractures of proximal femur (‘fractured hip’)

• There are many different types of femoral fractures that are often called a ‘fractured hip’

• A fracture to the neck or proximal femur may endanger the dominant blood supply of the head of the femur 

  • Due to relatively poor, retrograde blood supply, may lead to avascular necrosis of head of femur

3. Identify the major muscles, attachments, innervation, and actions of the gluteal/posterior hip region with an emphasis on these muscles’ roles in locomotion and stance.

Gluteal muscles

The gluteal muscles are arranged in three layers from superficial to deep (and largest to smallest): gluteus maximus m., gluteus medius m., & gluteus minimus m.

Attachments

• Gluteus maximus has numerous attachments but an important distal attachment is the iliotibial (IT) tract (particularly dense portion of the deep fascia (fascia lata) of the lateral thigh)

• Gluteus medius & minimus mm. have a distal attachment on the greater trochanter of the femur.

Actions

• Gluteus maximus m.: powerful extender of the hip joint. 

  • Especially true in extension, occurring during climbing stairs or inclines and for rising from a sitting or squatting position.

• Gluteus medius & minimus mm.: typically work as a pair and abduct the hip joint.

  • Keeps the pelvis from dropping on the unsupported side when only one leg is supporting the body.

    • Hip abductors contract on one side to keep the pelvis from falling or dropping on the other side when the leg is not supporting it. (e.g. during the normal walking cycle)

Innervation

• Gluteus maximus m.: inferior gluteal n.

• Gluteus medius & minimus mm.: superior gluteal n.

• Injury to the superior gluteal n. could present as a positive Trendelenburg sign/gluteal gait

  • With injury to the superior gluteal n., when asked to stand on one leg, the pelvis dips on the unsupported side (because the gluteus medius & minimum are not abducting the hip on the opposite side).

Lateral Rotator Muscles of Hip

• Deep to the gluteus maximus m. and inferior to the gluteus medius & minimus mm.

  • Five small muscles that predominantly laterally rotate the hip joint. 

• Most have a distal attachment on the greater trochanter of the femur.

• Piriformis m.

  • Largest and most superior 

  • Originates inside the pelvic cavity (on the anterior surface of the sacrum) and exits through the greater sciatic foramen of the pelvis (having a close relationship with the sciatic n.).

  • A hypertrophied piriformis m. may compress the sciatic n. in piriformis syndrome.

4. Diagram how neurovasculature exits the pelvis in relation to the piriformis muscle and greater sciatic foramen.

Many of the nerves that innervate the lower limb are branches of the sacral plexus, and exit the internal pelvic region via the greater sciatic foramen. The blood supply for the gluteal region branches from the internal iliac a. and also traverses the greater sciatic foramen.

The piriformis muscle has a similar route for its distal tendon to attach on the greater trochanter of the femur.

The piriformis m. is the ‘key’ to the hip due to its important position in regards to neurovasculature in the posterior hip region.

The superior gluteal n., a., & v. enter the gluteal region SUPERIOR to the piriformis muscle, while many neurovasculare components enter the gluteal region INFERIOR to the piriformis m. - most notably, the sciatic n., inferior gluteal n., a., & v., and pudendal n. and internal pudendal a. & v. (before the pudendal neurovasculature re-enters the pelvis).

5. Identify the major muscles, innervation, attachments, actions, & clinical considerations of the anterior thigh.

The most dominant/largest muscle in the anterior compartment of the thigh is the quadriceps femoris m.; however, it is not the only muscle in this compartment - some of which are deep (and less visible) in this region but powerful (in particular, iliopsoas m.).

The muscles in this compartment include:

• Sartorius m.: most superficial; longest muscle in the body 

• Iliopsoas m.: deep in superolateral anterior thigh

• Pectineus m., &

• Quadriceps femoris m.

  • Rectus femoris m.: runs straight down middle of quadriceps region

  • Vastus lateralis m. 

  • Vastus medialis m.

  • Vastus intermedius m.: deep to rectus femoris m.

The primary innervation to the anterior compartment of the thigh is the femoral n. - so not a sciatic nerve branch, which makes this compartment (with the medial compartment of the thigh and gluteal region) unique for the lower limb.

Joints crossed:

• Hip joint (iliopsoas m., rectus femoris m., sartorius m., pectineus m.)

• Knee joint (quadriceps femoris m., sartorius m.)

Notable Distal Attachments

• Iliopsoas m. has a distal attachment on the lesser trochanter of the femur.

All components of the quadriceps femoris m. share a common quadriceps tendon. 

• The patella (‘knee cap’) forms within the quadriceps tendon. It serves to withstand compression and provide protection during movements of the knee. 

• The tendon continues from the patella to the tibial tuberosity of the tibia - this portion is typically referred to as the patellar ligament/tendon.

Dominant Actions:

• Flexion of hip joint (iliopsoas m. is dominant flexor; rectus femoris, pectineus, & sartorius typically play synergistic roles)

• Extension of knee joint (quadriceps femoris m.)

Clinical Considerations

• The patellar tendon (ligament) reflex is a myotatic (deep tendon) reflex test commonly performed during physical exams to test the integrity of the L2-L4 spinal cord segments & the femoral n.

  • The patellar ligament is ‘tapped’ with a reflex hammer, and if normal, the leg should extend (usually in a jerk-like manner) and quadriceps femoris m. can be felt to contract.

6. Identify the major muscles, innervation, attachments and actions of the medial thigh.

The muscles of the medial thigh compartment are often referred to as the adductor compartment of the thigh.

There are 5 muscles in this compartment:

• Adductor longus m.: anteriorly situated

• Adductor brevis m.: deep to adductor longus

• Adductor magnus m.

  • Two parts: 

    • adductor part & 

    • hamstring part (more closely related to the posterior hamstring muscles)

• Gracilis m.: most medial muscle in this compartment

• Obturator externus m.

Innervation

The dominant innervation for this compartment is the obturator n., a branch of the lumbar plexus. The exception is the ‘hamstring part’ of the adductor magnus, which is innervated by the tibial part of the sciatic n.

Attachments

Most of the adductor muscles have a distal attachment to the linea aspera of the posterior femur.

Actions

The dominant action of the muscles in this compartment is adduction of the hip joint.

The hamstring part of the adductor magnus will work with the hamstring muscles of the posterior thigh to extend the hip joint.

7. Identify the major muscles, innervation, attachments, and actions of the posterior thigh.

The muscles of the posterior compartment of the thigh are often referred to as hamstring muscles.

There are 3 major muscles in this compartment:

• Semitendinosus m.: medial and more superficial (appears to sit ‘on top’ of semimembranosus m.)

  • Tendinous for half its length

• Semimembranosus m.: medial and deeper, but typically still visible due to the tendinous nature of semitendinosus m.

  • Tendons tend to be flattened - more ‘membranous’

• Biceps femoris m.: lateral

  • Long head

  • Short head

Innervation

The dominant innervation of this compartment is the tibial division of the sciatic n. The tibial n. is one of the terminal branches of the sciatic nerve. The short head of the biceps femoris m. is innervated by the other terminal branch of the sciatic nerve - the common fibular (peroneal) division. In the posterior thigh, typically, the sciatic n. still appears as one very large nerve, but the muscular branches to the hamstring muscles come from fibers of the specific divisions (tibial or common fibular) of the sciatic nerve.

Attachments

The proximal attachment of all of the hamstring muscles is on the ischial tuberosity, except for the short head of the biceps femoris m.

The distal attachments are on the tibia and fibula.

Actions

The hamstring muscles are two joint muscles, meaning they cross and can affect actions at two joints: the hip & knee joints.

• The primary action proximally is extension of the hip joint.

• The primary action distally is flexion of the knee joint.