Your symptoms are not suggestive of the motor neuron disease. Your muscle twitches are due to anxiety. Twitching is caused by minor muscle contractions in one particular area, or the persistent and uncontrolled twitching of a muscle group in a distribution of a single nerve fiber or filament. Usually they are benign and resolve on their own, but persistent flutter may need following investigations;
1) electromyography (EMG)
2) nerve conduction velocity (NCV)
3) blood electrolytes like calcium, magnesium, potassium, sodium, etc
4) thyroid and parathyroid function tests
5) blood chemistry and complete metabolic profile
The common causes can be;
1) Idiopathic; no cause is ascertained and majority falls in this group.
2) Increased caffeine intake
3) Increased alcohol intake
5) Deficiency in diet and nutrients / minerals
6) Some neurological diseases; uncommon.
Your shoulder pain and sciatica (and related symptoms; radiculopathy in lower limbs) are truly orthopedic issue in nature and unrelated to the neurological problem. Pinched nerves in the back can be due to;
1) Herniated/degenerated disc in back (as mentioned by you)
2) Lumbar spondylosis; bone spurs in the back vertebrae pressing on the nerves.
Following investigations would be required, if not yet done;
1) X-ray of the lower back
2) MRI of the spine
3) Nerve conduction velocity (NCV) study
4) Electromyography (EMG)
Following comprehensive measures would be helpful, if the symptoms are persisting;
1) Back care in the activities of daily living, which is an integral component of the treatment of back problems. Back education is one of the most important thing which teaches the basic body mechanics, like correct posture for standing, standing at a desk or drawing board, sitting, brushing teeth, washing the face, pushing and pulling a weight, lifting a weight, getting in and out of bed, sleeping, getting into and sitting in a car. The training for these routine activity helps in preventing the spasm of the muscles. One needs to consult an occupational therapist or physical therapist which can educate about the proper and improper behaviors when back is painful in case they have to sit, bend forward, lie down, walk, cough, or sneeze. Following more need to be done:
a) Avoid activities which increases the pain.
b) Rest intermittently
c) Avoid bending at 90 degrees
d) Pushing and pulling should be avoided till pain subsides
f) Avoid prolonged sitting and standing
g) Avoid sitting or sleeping on floor
2) Anti-inflammatory analgesics like ibuprofen, other analgesics and muscle relaxants
3) Local analgesic gels or sprays / ointment
4) Hot fomentation
5) Electrotherapy like trans cutaneous electrical nerve stimulation: done by physical therapist. Other thing are ice packs, heating pads, electrical stimulation, phonophoresis, iontophoresis, relaxation, and biofeedback.
6) Good supervised physiotherapy: The exercises consist of abdominal bracing, modified sit-ups, double-knee-to-chest or low back stretches, seat lifts, mountain and sag exercises, knee-to-elbow exercises, hamstring stretches, extension exercises, and extension flexibility exercises. Swimming exercises (pool exercises) are best for back pain. Initial stretching and later strengthening exercises are taught.
7) Lumbosacral corset or support or brace
8) If obese or over weight, reduce weight for long term benefit.
9) Epidural steroid shots
11) Chiropractor manipulation.
You can start the following exercises;
1) Straight leg raising: Lie on the bed with your back and remain in a relaxed position. Slowly raise one of your legs upward and keep it as straight as possible. Count up to ten, and slowly bring down the leg. Do the same with the other leg. Repeat this exercise ten times.
2) Curl ups; lie on the back with knees bent, fold arms across the chest, tilt the pelvis to flatten the back, and curl-up lifting the head and shoulders from the bed / couch. Hold for ten seconds, then slowly lower to starting position. As strength builds, aim to complete one sets of ten curls. The exercise should be done twice a day (both the sets).
This is a slide show for the exercises (you can pick up your own set, which suits you);
You can consult following specialists (apart from your GP);
b) MD in Physical medicine and rehabilitation.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.