The Neuromuscular System and Exercise

The Neuromuscular System

l    Consists of:

l   Nervous System

l   Central Nervous System – Brain and Spinal Cord

l   Peripheral Nervous System

l   Somatic system – excites muscle activation
l   Autonomic system – excites or inhibits muscle activation
l   Sympathetic – excites
l   Parasympathetic – inhibit or slow down

l   Muscular System

l   Skeletal

l   Cardiac

l   Smooth

The Neuromuscular Connection

l   Motor Unit

l   Defined as the nerve + the muscle fibers that it innervates

l  The ratio of muscle fibers to nerve relates to function

l   Delicate, precise work requires a lower muscle fiber number to nerve ratio, e.g. 10 muscle fibers to one nerve (eye muscles)
l   Less complex movements require a higher muscle fiber number to nerve ratio, e.g. 3000 muscle fibers to one nerve (leg muscles

The Neuromuscular Junction

l   Gap between the nerve ending and muscle fiber

Neuromuscular Fatigue

l   Fatigue may represent a disruption in the normal function of one of these components:

l   Central nervous system (brain and spinal cord)

l   Peripheral nervous system (nerve itself)

l   Neuromuscular junction

l   Muscle fiber

Central Nervous System Fatigue

l   Influenced by peripheral fatigue factors such as lactic acid accumulation in muscle

l   Influenced by ion disturbances in the blood


  * in these cases, the brain and spinal cord send inhibitory signals back to origins of fatigue

Nerve fatigue

l   Not likely that the nerve fatigues but is possible since it requires nutrients to survive.  If lacking nutrients, it could slow in its function.

Neuromuscular Junction Fatigue

l   It is possible that levels of acetylcholine could be depleted resulting in fatigue.

Muscle Fiber Fatigue

l   Muscle fiber is most likely site of fatigue due to:

l   Accumulation of lactic acid

l  Prevents binding of calcium to troponin

l   Makes it difficult for myosin to bind to actin fiber

l   Depletion of ATP & PC stores

l   Depletion of muscle glycogen

l   Lack of O2 getting to the muscle

l  Insufficient blood flow

Muscle Types

l   Skeletal

l   Function is to create movement

l   Cardiac

l   Function is to pump blood for movement to occur

l   Smooth

l   Function is to constrict blood vessels to allow for appropriate blood distribution, and move contents of internal organs

Skeletal Muscle

l   Structure

l   Held together by connective tissue

l  Epimysium completely surrounds muscle

l  Perimysium surrounds bundles of muscle fibers (~150 fibers); bundles known as fasciculi

l  Endomysium surround each muscle fiber

l   Composition

l   Contains ~ 75% water, 20% protein, 5% other substances (minerals, fuel, enzymes, etc.)

Types of Skeletal Muscle

l   Fiber Types:

l   Type I = slow-contracting

l   Type II = fast-contracting

l  Type IIA – has both anaerobic and aerobic abilities

l  Type IIB – has primarily anaerobic ability


Comparison of Fiber Types

Fiber Type Composition in Elite and Non-athletes

Can we change fiber types with training?

l   Genetic code largely determines a person’s predominant fiber type.

l   However, with specific training, one can improve either their aerobic or anaerobic capacities to a certain level

l  For example, a person with primarily fast-twitch fiber muscles can improve their endurance to a certain level if they do long distance running but will most likely not attain elite status as a marathon runner.