Motor System: Arms

Published on 09/04/2015 by admin

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Last modified 09/04/2015

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Motor System

Arms

BACKGROUND

Upper motor neurone or pyramidal weakness predominantly affects finger extension, elbow extension and shoulder abduction. N.B. Elbow flexion and grip are relatively preserved.

Muscles are usually innervated by more than one nerve root. The exact distribution varies between individuals. The main root innervations and reflexes are shown in simplified form in Table 17.1. More detailed root distribution is given below.

Table 17.1

Nerve roots: simplified root innervations and main reflexes

Root	Movements	Reflex
C5	Shoulder abduction, elbow flexion	Biceps
C6	Elbow flexion (semi-pronated)	Supinator
C7	Finger extension, elbow extension	Triceps
C8	Finger flexors	Finger
T1	Small muscles of the hand	No reflex

The three nerves of greatest clinical importance in the arm are the radial, ulnar and median nerves.

• The radial nerve and its branches supply all extensors in the arm.

• The ulnar nerve supplies all intrinsic hand muscles except ‘LOAF’ (see below).

• The median nerve supplies:

– L lateral two lumbricals

– O opponens pollicis

– A abductor pollicis brevis

– F flexor pollicis brevis.

N.B. All intrinsic hand muscles are supplied by T1.

WHAT TO DO

Look at the arms

Note wasting and fasciculations, especially in the shoulder girdle, deltoid and small muscles of the hands (the first dorsal interossei and abductor pollicis brevis).

Test tone (see Chapter 15).

PRONATOR TEST

Ask the patient to hold his arms out in front with his palms facing upwards and to close his eyes tightly (demonstrate).

Watch the position of the arms.

What you find and what it means:

• One arm pronates and drifts downwards: indicates weakness on that side.

• Both arms drift downwards: indicates bilateral weakness.

• Arm rises: suggests cerebellar disease.

• Fingers continuously move up and down—pseudoathetosis—indicates deficit of joint position sense.

Basic screening examination

A simple screening procedure is outlined below. Some further muscle power tests are given afterwards. Perform each test on one side, then compare to the other side.