Nervous System - Anatomy

The ankle jerk (Achilles) reflex tests the nerve root S1, S2. Biceps reflex tests roots C5, C6. Brachioradialis tests C5, C6, C7. Triceps reflex tests C6, C7, C8 nerve roots. Patellar (knee jerk) reflex tests the L2, L3, L4 nerve roots. The hamstring reflex tests nerve roots L5 and S1.

The ankle jerk (Achilles) reflex tests the nerve root S1, S2. Biceps reflex tests roots C5, C6. Brachioradialis reflex tests C5, C6, C7. Triceps reflex tests C6, C7, C8 nerve roots. Patellar (knee jerk) reflex tests the L2, L3, L4 nerve roots.

This man is suffering from acute inflammatory demyelinating polyradiculoneuropathy (AIDP), also known as Guillain-Barre Syndrome. As suggested by the name, the pathology is due to demyelination.

AIDP is a symmetric ascending paralysis that begins in the feet and ascends to the trunk and upper extremities. The paralysis will often impair breathing and frequently affects the facial nerve as well. This disease commonly follows an illness, especially infectious diarrhea caused by Campylobacter jejuni. The immune response to foreign antigens mistakenly targets host nerve tissues, specifically the myelin sheaths.

Inhibition of ACh release into synaptic cleft can also result in weakness, but describes the effects of botulinum toxin in botulism. The toxin is made by the bacteria Clostridium botulinum as a foodborne illness. Patients experience weakness, trouble seeing, fatigue, and possible speech difficulties. Muscular weakness may follow. Diagnosis is made by identifying the toxin in food or feces.

Blockade of action potentials is seen in conduction blockages as a result of nerve injury, such as neuropraxia, axonotmesis, or neurotmesis.

Blockade of presynaptic voltage-gated calcium channels describes Lambert-Eaton Syndrome, a presynaptic neuromuscular junction disorder characterized by muscular weakness of the limbs. It is an autoimmune condition that attacks presynaptic voltage-gated calcium channels. Proximal muscles are affected first, so patients have difficulty climbing stairs or standing up from sitting. They also may have difficulty reaching over their head or combing their hair. Diagnosis is made by EMG, which shows normal latency and conduction velocities, but small amplitudes. Rapid bursts of stimuli or exercising the muscle will lead to greatly increased amplitudes due to an influx of calcium.

Blockade of postsynaptic ACh receptors is seen in myasthenia gravis, a postsynaptic neuromuscular junction disorder very similar to Lambert-Eaton Syndrome. It is an autoimmune disorder directed against postsynaptic acetylcholine receptors and is characterized by easy fatigue. Muscles become progressively weaker with activity, so patients tend to have symptoms at the end of the day. Earliest signs include difficulty keeping the eyes open, or difficulty with chewing, speaking, or swallowing. EMG studies will show progressively decreasing amplitudes with repeated stimuli.

The affected dermatome is supplied by the spinal root C3.

This is a matter of simple memorization of dermatomes in the body. Each dermatome is innervated by a single spinal nerve, which is responsible for sensation in the area. The neck is supplied mostly by C3, with C4 and C5 supplying parts of the posterior neck.

C2 supplies the back of the skull. C6 supplies the lateral aspect of the arm and forearm. T2 supplies the upper chest and medial aspect of the arm. T4 is a common landmark dermatome as it supplies the nipples. Another landmark is T10, which supplies the umbilical cord.

You suspect that the trigeminal nerve (CN V) is compromised and would expect there to be sensory deficits in the face.

To approach this problem, we need to think about a few things:

1. If the blink reflex is absent, what cranial nerves are involved?

The afferent part of the blink reflex is supplied by the trigeminal nerve (V), while the efferent action is from the facial nerve (VII).

2. The patient is able to make different facial expressions and shut her eyes tightly. Which cranial nerve is involved in facial expressions?

The facial nerve innervates muscles of facial expressions. The patient's ability perform these actions tells us the facial nerve is intact; thus, the trigeminal nerve must be the part of the blink reflex that is deficient.

3. What other function does the trigeminal nerve have?

The trigeminal nerve provides sensation to the face and scalp, and also innervates the muscles of mastication.

Absent gag reflex can be due to deficits in the glossopharyngeal nerve (IX, afferent portion) or vagus nerve (X, efferent portion). Lack of eye movement can be due to occulomotor (III), trochlear (IV), and/or abducens (VI) cranial nerves. Inability to turn the head is a result of sternocleidomastoid deficiency, which is innervated by the accessory nerve (XI). An impaired sense of smell suggests olfactory nerve (I) deficit.

"Saturday night" palsy is a nickname for radial nerve paralysis. Because the wrist extensors are innervated by the radial nerve, this type of paralysis would prevent a person from being able to perform wrist extension. While the supinator is innervated by the radial nerve, the biceps brachii is also a supinator, so although supination may be weaker with radial nerve paralysis, it would not be absent.

The biceps brachii is innervated by C5 and C6 nerve roots. So, in the presence of a C5 nerve root irritation, biceps tendon reflexes may be diminished. the triceps brachii is innervated by C7 and C8 nerve roots, and the lower extremity muscles are innervated by L2 through S2 nerve roots, so testing these muscles would not tell us anything about a C5 nerve irritation.

The Romberg test involves maintaining an upright stance with feet together, which tests the vestibular portion of the vestibulocochlear nerve. The accessory nerve controls shoulder girdle and neck movement, the olfactory nerve provides the sense of smell, the hypoglossal nerve controls tongue movement, the facial nerve controls the muscles of facial expression (amon other functions) and the trigeminal nerve controls the muscles of mastication (among other functions). Thus, the Romberg test would not test any of these functions

The triceps brachii deep tendon reflex tests the C7 and C8 nerve roots, so testing it would not provide any information about a lumbosacral nerve root pathology. The cremasteric reflex tests the T12 and L1 nerve roots, the Achilles deep tendon reflex tests the S1 and S2 nerve roots, the anal wink reflex tests the S2, S3, and S4 nerve roots, and the patellar deep tendon reflex tests the L3 and L4 nerve roots.

Cranial nerve (CN) III (oculomotor nerve) is responsible for certain eye movements. It innervates the following muscles: superior rectus, inferior rectus, medial rectus, and inferior oblique. It also functions in pupillary constriction, accommodation, and eyelid opening (via levator palpebrae). CN III is a motor nerve along with parasympathetic function. If there is injury to CN III these functions will be impaired leading to the presentation of the observations in the question stem.

The ulnar nerves serves the dorsal and palmar surfaces of the hand from the midline of the ring finger over to the little finger (dorsal and palmar).

The sciatic nerve is a large nerve that runs through the buttocks and down the lower limb. The sciatic nerve innervates skin of the leg, and muscles in the posterior thigh, leg, and foot. The sciatic nerve is derived from spinal nerves L4-S3. The L4-S3 nerve root also supplies the tibial nerve.

L4-S1 is the nerve root for the superior gluteal nerve. Common fibular nerve is supplied by spinal nerves L4-S2. The obturator and femoral nerves are derived from L2-L4.

The femoral nerve is a nerve in the thigh that innervates the skin on the upper thigh and inner leg and shin. It also innervates the muscles that extend the knee (quadriceps). It is the largest branch of the lumbar plexus and derives from spinal nerves L2-L4. The obturator nerve is also supplied by spinal nerves L2-L4.

The sciatic nerve is derived from spinal nerves L4-S3. The L4-S3 nerve root also supplies the tibial nerve. L4-S1 is the nerve root for the superior gluteal nerve. Common fibular nerve is supplied by spinal nerves L4-S2.

The axillary nerve supplies three muscles, the deltoid, teres minor, and the long head of the triceps. The deltoid is a muscle of the shoulder and teres minor is a muscle involved in the rotator cuff. The axillary nerve also supplies sensory information from the shoulder joint, and to the skin below the deltoid. The axillary nerve may be injured during an anterior dislocation of the shoulder joint or a fracture of the surgical neck of the humerus.

The radial nerve supplies the posterior portion of the upper limb. The radial nerve innervates the medial and lateral head of the triceps brachii muscle (the long head is supplied by the axillary nerve) and anconeus. The radial nerve originates from the brachial plexus and consists of fibers of nerves C5, C6, C7, C8, and T1.

The pupillary light reflex and ocular accommodation reflex involves cranial nerve II for the sensory aspect and cranial nerve III for the motor application. The jaw jerk reflex uses cranial nerve V for both sensory and motor. The corneal (blink reflex) utilizes cranial nerve V for sensory and VII for motor. The gag reflex utilizes cranial nerve IX for sensory and X for motor.

The gag reflex utilizes cranial nerve IX for sensory and X for motor. The pupillary light reflex and ocular accommodation involves cranial nerve II for the sensory aspect and cranial nerve three for the motor application. The jaw jerk reflex uses cranial nerve V for both sensory and motor. The corneal (blink reflex) utilizes cranial nerve V for sensory and VII for motor.

Hemiballism describes a sudden, uncontrolled flinging movement; this occurs if there is damage/destruction of subthalamic nuclei. Chorea is an abnormal, involuntary movement that can be seen in Huntington's disease; it occurs after destruction of the caudate nucleus. Dystonias are abnormal movements and are most often referred to as resting tremors of Parkinson's disease. Parkinsons disease results from a loss of dopamine from the substantia nigra.

A dermatome is an area of skin supplied by a single spinal nerve. Along the thorax and abdomen the dermatomes are stacked like coins/discs, each layer supplied by a different spinal nerve. Along the arms and legs the pattern is different. Symptoms that follow a dermatome may indicate a pathology that involves the nerve root. There are a number of important dermatomes one being the inguinal region innervated by the L1 nerve root.

A dermatome is an area of skin supplied by a single spinal nerve. Along the thorax and abdomen the dermatomes are stacked like coins/discs, each layer supplied by a different spinal nerve. Along the arms and legs the pattern is different. Symptoms that follow a dermatome may indicate a pathology that involves the nerve root. There are a number of important dermatomes one being the layer of the umbilicus innervated by the T10 nerve root.