Nerves Flashcards
Why does injury to the sciatic nerve lead to combination of motor and sensory deficits?
Injury to the sciatic nerve results in a combination of motor and sensory deficits because the sciatic nerve is a mixed nerve, meaning it contains both motor and sensory fibers that innervate different regions of the lower limb.
Motor Deficits:
• The sciatic nerve supplies the hamstring muscles (for knee flexion) and most of the muscles in the lower leg and foot through its branches (the tibial and common fibular nerves).
• Injury to the sciatic nerve affects hip extension, knee flexion, and foot movements, resulting in difficulty in walking, standing, or moving the foot.
Sensory Deficits:
• The sciatic nerve also provides sensory innervation to parts of the lower leg and foot, including the posterior thigh, leg, and sole of the foot.
• Damage to the sciatic nerve leads to numbness, tingling, or loss of sensation in these areas.
Combined Effects:
Since the sciatic nerve carries both motor and sensory fibers, injury to it leads to the loss of function (motor) and sensation (sensory) in the areas it supplies, resulting in a combination of motor and sensory deficits.
Explain why common fibular N. injury causes foot drop
Injury to the common fibular nerve (also called the common peroneal nerve) causes foot drop due to its role in controlling the muscles responsible for dorsiflexion of the foot.
Key Points:
1. Muscle Innervation: The common fibular nerve branches into the deep fibular nerve and superficial fibular nerve.
• The deep fibular nerve innervates the tibialis anterior, which is the primary muscle responsible for dorsiflexion (lifting the foot upwards).
• The superficial fibular nerve innervates the peroneal muscles, which are involved in eversion of the foot.
2. Effect of Injury:
• Injury to the common fibular nerve affects both branches, particularly the deep fibular nerve, impairing the function of the tibialis anterior.
• As a result, the dorsiflexion of the foot is lost, leading to foot drop — the inability to lift the front part of the foot, causing it to drag while walking.
3. Clinical Manifestation: The patient with a common fibular nerve injury may present with a characteristic high-stepping gait to compensate for foot drop, or the foot may hang in a plantarflexed position.
In summary, foot drop occurs because the tibialis anterior muscle, which is responsible for dorsiflexing the foot, is no longer innervated due to damage to the common fibular nerve.
Why does damage to tibial nerve lead to loss of plantar flexion?
Damage to the tibial nerve leads to the loss of plantarflexion because the tibial nerve innervates the muscles responsible for this movement.
Key Points:
1. Tibial Nerve Function:
• The tibial nerve innervates the posterior compartment of the lower leg, which includes the major plantarflexors of the foot, such as:
• Gastrocnemius
• Soleus
• Plantaris
• Flexor hallucis longus
• Flexor digitorum longus
• Tibialis posterior
2. Role in Plantarflexion:
• These muscles are primarily responsible for plantarflexion, which is the movement of pointing the toes downward, such as during walking, running, and standing on tiptoe.
3. Effect of Damage:
• When the tibial nerve is damaged, the function of these plantarflexor muscles is impaired, resulting in loss of plantarflexion.
• This leads to difficulty in walking, especially with pushing off the ground or maintaining balance during walking and standing.
In summary, damage to the tibial nerve impairs the function of the plantarflexor muscles (gastrocnemius, soleus, etc.), leading to the loss of plantarflexion.
Why does an injury to the obturator nerve cause difficulty in hip adduction?
An injury to the obturator nerve causes difficulty in hip adduction because the obturator nerve innervates the adductor muscles of the thigh, which are responsible for bringing the leg toward the midline.
Key Points:
1. Obturator Nerve Function:
• The obturator nerve primarily innervates the adductor group of muscles in the thigh, including:
• Adductor longus
• Adductor brevis
• Adductor magnus (partially)
• Gracilis
• Obturator externus
2. Role in Hip Adduction:
• These muscles are responsible for adducting the thigh (bringing the leg toward the body’s midline).
3. Effect of Injury:
• Injury to the obturator nerve results in weakness or paralysis of the adductor muscles.
• As a result, the person may have difficulty adducting the hip and may struggle to bring the leg inwards, affecting activities like crossing the legs or stabilizing the leg during walking.
In summary, an injury to the obturator nerve impairs the function of the adductor muscles, leading to difficulty in hip adduction.
How does the location of the femoral nerve make it susceptible to compression?
The femoral nerve is susceptible to compression due to its anatomical location and the areas it passes through. Here are the key reasons:
- Location in the Femoral Triangle:
• The femoral nerve passes through the femoral triangle, which is a relatively confined space in the upper thigh, bordered by the inguinal ligament, sartorius muscle, and adductor longus muscle.
• Any compression or swelling in this area, such as from hernias or trauma, can put pressure on the femoral nerve. - Course through the Inguinal Ligament:
• The femoral nerve runs beneath the inguinal ligament before entering the thigh. This ligament is a potential site of compression, especially in conditions like inguinal hernias or tight belts. - Proximity to the Pelvis and Hip Joint:
• The femoral nerve is in close proximity to the hip joint and pelvic structures, which can contribute to compression in certain positions, such as during prolonged sitting or from hip joint arthritis. - Compression from Enlarged Lymph Nodes:
• Lymphadenopathy or other pelvic masses (e.g., cancer or infection) can lead to the compression of the femoral nerve as it passes through the pelvis.
Effect of Compression:
• Compression of the femoral nerve can lead to weakness in the quadriceps (impairing knee extension) and sensory loss in the anterior thigh and medial leg.
In summary, the femoral nerve is vulnerable to compression due to its confined course through the femoral triangle, under the inguinal ligament, and its proximity to pelvic and hip structures.
Why does a sural nerve serve as a good candidate for nerve grafts?
The sural nerve is a good candidate for nerve grafts due to several key reasons:
- Sensory Only Function:
• The sural nerve is purely sensory, meaning it does not affect motor functions. This makes it less critical for daily activities and reduces the risk of functional impairment when used for grafting. - Superficial Location:
• The sural nerve is located near the skin surface along the posterior aspect of the leg and is easily accessible during surgery, making it convenient for harvesting. - Long Length:
• The sural nerve provides a relatively long length of nerve tissue, which is useful for grafting over larger defects or for repairing longer nerve injuries. - Minimal Functional Deficit:
• Donating the sural nerve for grafting typically causes minimal functional deficits because its sensory function is not essential for critical functions like motor control or proprioception in most people. - Reliable and Well-Defined Course:
• The sural nerve has a reliable and well-defined anatomical course, making it easier to identify and harvest without significant risk of injury to surrounding structures.
In summary, the sural nerve is ideal for nerve grafting because it is sensory, easily accessible, long, and causes minimal functional loss when harvested.
Explain the sensory and motor deficits in Meralgia Parasthetica.
Meralgia Paresthetica is a condition caused by compression of the lateral femoral cutaneous nerve (LFCN), which is a purely sensory nerve that supplies sensation to the outer part of the thigh. Here’s how it affects sensory and motor function:
Sensory Deficits:
• Numbness: The main symptom is numbness or tingling (paresthesia) in the anterolateral part of the thigh.
• Pain: Patients may experience a burning or sharp pain in the same area, often exacerbated by standing, walking, or wearing tight clothing or belts.
• No Loss of Proprioception: Since the LFCN is purely sensory and does not carry proprioceptive information, there is no loss of coordination or awareness of limb position.
Motor Deficits:
• No Motor Loss: The lateral femoral cutaneous nerve is a sensory nerve only, meaning there are no motor deficits associated with Meralgia Paresthetica. The motor function of the thigh muscles (such as those controlled by the femoral and sciatic nerves) remains intact.
Cause of Deficits:
• Compression: The LFCN is typically compressed as it passes under the inguinal ligament, especially in the groin area, or in certain situations like obesity, pregnancy, or wearing tight clothing (such as a tight belt or waistband).
• Reduced Sensory Input: The compression limits the nerve’s ability to send sensory signals, leading to the characteristic symptoms.
In summary, Meralgia Paresthetica results in sensory deficits (numbness, tingling, burning pain) over the anterolateral thigh, but does not cause motor deficits due to the sensory-only nature of the LFCN.
How does an injury to the lateral cutaneous nerve of the thigh cause localized numbness?
The lateral cutaneous nerve of the thigh (LFCN) is a purely sensory nerve that supplies sensation to the anterolateral part of the thigh.
Injury Effects:
• Localized Numbness or Paresthesia: Injury or compression of the LFCN leads to numbness, tingling, or burning pain in the outer thigh. This condition is known as Meralgia Paresthetica.
Common Causes:
• Compression: LFCN can be compressed as it passes under the inguinal ligament or in areas of increased pressure, such as from obesity, tight clothing, or pregnancy.
• Trauma: Direct injury to the nerve.
Motor Function:
• There are no motor deficits since the LFCN is purely sensory and does not control any muscles.
In summary, LFCN injury causes sensory loss (numbness/tingling) on the outer thigh but does not affect motor function.
Why does the saphenous vein provide sensory innervation only despite being a branch of the femoral nerve?
The saphenous vein provides sensory innervation only because it is a sensory branch of the femoral nerve, not a motor one. Here’s why:
• Anatomy: The saphenous nerve is the longest branch of the femoral nerve and provides sensory innervation to the medial aspect of the leg and foot.
• No Motor Function: Although it arises from the femoral nerve, which is a mixed nerve (sensory and motor), the saphenous nerve itself is purely sensory and does not carry motor fibers.
• Function: It transmits sensory signals from the skin, including pain, touch, and temperature sensations, from areas such as the medial side of the knee, lower leg, and foot.
Thus, the saphenous nerve provides only sensory innervation due to its specialized role, despite being a branch of the femoral nerve, which has motor functions.
What is the significance of the sciatic nerve’s bifurcation in the popliteal fossa?
The sciatic nerve bifurcates into the tibial nerve and common fibular (peroneal) nerve in the popliteal fossa, which is important for the following reasons:
1. Motor Function Distribution:
• Tibial nerve: Innervates the posterior muscles of the leg (e.g., gastrocnemius, soleus) and provides motor control for plantar flexion.
• Common fibular nerve: Divides into the deep fibular nerve (innervating the anterior compartment muscles for dorsiflexion) and the superficial fibular nerve (innervating muscles for eversion).
2. Vulnerable Site:
• The popliteal fossa is a shallow space behind the knee, and the bifurcation makes the nerves more vulnerable to injury due to trauma, compression, or surgical procedures.
3. Sensory Innervation:
• Both branches provide sensory innervation to parts of the lower leg and foot:
• The tibial nerve supplies sensation to the sole of the foot.
• The common fibular nerve supplies sensation to the dorsum of the foot and lateral side of the lower leg.
4. Clinical Relevance:
• Sciatic nerve injury at this bifurcation can lead to a combination of motor and sensory deficits, affecting both the lower leg muscles and sensation in the foot and leg.
In summary, the bifurcation of the sciatic nerve in the popliteal fossa is significant because it determines the motor and sensory innervation of the lower leg, and its location makes it prone to injury, which can cause both functional and sensory impairments in the leg and foot.
Why does the damage to deep fibular nerve specifically affect dorsiflexion of the foot?
The deep fibular (peroneal) nerve specifically innervates the anterior compartment muscles of the lower leg, which are responsible for dorsiflexion of the foot.
Anatomical Explanation:
• The deep fibular nerve innervates muscles like the tibialis anterior, extensor hallucis longus, and extensor digitorum longus, all of which are responsible for dorsiflexion (lifting the foot upwards).
Effect of Damage:
• Damage to the deep fibular nerve impairs the function of these muscles, leading to a condition known as foot drop. In this condition, the person is unable to dorsiflex the foot, resulting in difficulty in lifting the foot and dragging it while walking.
Why Dorsiflexion is Affected:
• Dorsiflexion depends on the tibialis anterior and other muscles innervated by the deep fibular nerve. Without proper nerve function, the foot cannot be lifted effectively, leading to inability to clear the ground during walking.
Thus, damage to the deep fibular nerve specifically impairs dorsiflexion of the foot by affecting the muscles that control this movement.
