Somatic Motor System

(Somatic) Motor system

Voluntary control of skeletal muscle and movement of skeletal system
+
Fine tuning of movement in face of external inputs -gravity, changing forces and loads to limbs and body
Control of balance and posture
Gait

Motor cortex - initiates signals for voluntary movement
Pyramidal tracts - conduct signals down spinal cord
Ventral motor spinal roots - exit spinal cord (combining with dorsal sensory root) into spinal nerves
Spinal nerve - supply areas to scalp or trunk (or combine in plexuses to supply upper and lower limb)
Brachial and lumbar plexus - (Upper and lower limb only, not head/trunk) distribute peripheral nerves to limbs
Peripheral nerves - terminate at muscle at neuromuscular junction releasing Acetyl Choline neurotransmitter to activite muscle
Neuromuscular junction - contain nicotinic receptors (M subtype - Nm) that trigger action potential
Muscle - contacts in response to neuromuscular junction activation

see Control of movement

Pyramidal - gross movements
Extra-pyramidal - automatic relaxation / elongation of antagonist muscle groups with contraction / shortening of agonist muscle groups
Cerebellar - assist in switching of opposing muscle groups

Upper motor neuron (passes through internal capsule and enters brainstem crossing at pyramidal decussation and descends down spinal cord)
Lower motor neuron - Ventral (anterior) horn of spinal cord and exits ventral (anterior) nerve root

UMN pyramidal lesion - weakness later developing into spasticity (e.g. stroke)
LMN pyramidal lesion - weakness (e.g. brachial plexus trauma)
Extra-pyramidal lesion - abnormal spontaneous movements and control of voluntary movements (e.g. Parkinson's disease)
Cerebellar lesion - problems with co-ordination and balance (e.g. alcoholic cerebellar disease)