4. Somatic motor system

Somatic motor neurons

Spinal and cranial nerves

Spinal nerves of the somatic motor system

ventral horn of the spinal cord

Cranial nerves of the somatic motor system

motor nuclei of the brainstem 

Motor control centers

Motor area of cerebral cortex, basal nuclei, cerebellum, brainstem centers, spinal cord.

Pathways connecting control centers to one another and to motor neurons

Corticospinal and corticobulbar tracts
Medial pathways and lateral pathways

Motor neuron cell bodies

anterior gray horn of spinal cord

Motor neuron axons 

ventral root of every spinal nerve

Motor neuron axons are considered 

 Type A fibers – large diameter, myelinate

Somatic motor neurons of the spinal cord innervate

skeletal muscles of the arms, legs, body trunk

Motor neuron cell body in brainstem 

cranial nerve motor nuclei in medulla, pons, and midbrain

Motor neuron axons in the brain stem

in some cranial nerves

Eye muscles

CN III, IV, VI

Muscles of mastication 

CN V (Trigeminal Nerve)

Muscle of facial expression

CNVII

Muscles of neck and pharynx

CN IX, XI

Muscles of tongue

CN XII (Hypoglossal Nerve)

Reflex conrol of muscles

direct relay of sensory information to motor neurons

Voluntary control of muscles

cerebral cortex plans and initiates voluntary
muscle movements

Subconscious control of skeletal muscle 

various brainstem centers help control basic movements

Role of cerebellum and basal nuclei in control of skeletal muscle

both modulate ongoing muscle activity via input to subconscious and voluntary centers

Reflex

rapid, automatic, predicatble response to stimuli

Neural reflex pathways 

sensory input to CNS and motor neuron output to effectors but can be modified by input from higher brain centers

Step 1 of reflex pathway

activaton of sesnory receptor by stimulus causing AP which propagates into CNS

Step 2 of reflex pathway

Neurotransmitter releases onto all neuron on which sesnory neurons synpases (divergence)

Step 3 of relfex pathway

Information processing occurs at many levels of nervous system.

Step 4 of reflex pathway

Motor neuron receives multiple inputs that determine AP
frequency (convergence)

Strength and duration of skeletal muscle contraction is determined by the

motor neuron action potential frequency

Monosynaptic pathway 

Sensory neuron synapses directly on a motor neuron.

Polysynaptic pathway

At least one interneuron between sensory afferent and motor efferent. longer delay and more complex responses

Postural reflex

helps maintain uprigh position and balance of the body.

Stretch reflex 

a monosynaptic reflex that causes a muscle to contract in response to its own stretch.

The stretch reflex is a reflex that
A. occurs only when the tendon is stretched by
tapping with a reflex hammer during a
neurological exam
B. is constantly functioning to maintain proper
length of body postural muscles
C. will not be affected by damage to
descending motor control pathways
D. All of the above

B. is constantly functioning to maintain proper
length of body postural muscles

Flexor and Crossed Extensor reflexes

withdraws a limb from pain (flexor response) while extending the opposite limb to maintain balance.

Reinforcment of reflex 

facilitation that enhances spinal nerve and
cranial nerve reflexes

inhibition of reflexes

suppression that reduces spinal and cranial reflexes

development of refelx control

appers early as babies have reflex to withdrawn from pain1

Precentral gyrus 

primary motor cortex located in the frontal lobe.

Area of cortex

devoted to specific region of body is
proportional to number of motor units, not to body area
size

Premotor cortex

somatic motor association area

Premotor cortex function 

Coordinates incoming data and integrates learned motor responses

Corticospinal tracts

Provides voluntary skeletal muscle control via rapid
direct pathwa

Corticobublar tracts

Synapse onto motor neurons in cranial nerve nuclei to control head and neck muscles

Corticospinal tracts 

Synapse on motor neurons in the anterior gray horn of the spinal cord. facilitating voluntary movement of limbs and trunk.

Medial portion of motor homonculus (motor cortex)

controls lower limbs and foot movements.

lateral portion of motor homonculus (motor cortex)

controls the face and upper limb

When would damage affects be ipsilateral 

after decussation (spinal cord)

When would damage affects be contralateral

before decussation (cortex/brainstem)

Medial pathway

primarily control of muscle tone of the neck, trunk,
and proximal limb muscle

Lateral pathway

muscle tone of the distal parts of the limbs
• Can assist with precise movements of distal limb

Vestibular nuclei in brain stem 

Sensory input from receptors for balance and equilibrium (located in the inner ear).

Tectum of mesenchepalon of brainstem

Superior colliculi is visual
Inferior colliculi is auditory

Reticular formation of brainstem. 

Input from many sensory & motor centers

Basal nuclei are foound where in brain

found in the cerebral hemispheres, primarily involved in motor control and coordination. They play a crucial role in regulating voluntary movement.

Basal nuceli ARE NOT 

in direct pathway to motor neurons. they provide background patterns or rythmic cycles of movement 

Cerebellum 

monitors proprioceptive information,
visual information and vestibular sensation to make smooth movement with least amount of motor commands