·
Central nervous system (CNS).
·
Peripheral nervous system
(PNS).
Recruitment and the ‘all or none law’
According to the ‘all or none law’ the impulse sent down the
neuron can’t gauge the intensity of the contraction it will stimulate. It will
either activate a contraction or not.
To vary the intensity of a muscle contraction, different numbers
of motor units must be stimulated at varying frequencies.
Notes
Motor unit
recruitment is the process of stimulating all the muscle fibres connected to
the motor neurons that the central nervous system wants to activate. Different
numbers of motor units need to be recruited to achieve different levels of
force.
Muscle fibres
innervated by motor neurons contract with full force; the impulse sent down the
neuron can’t gauge the intensity of the contraction it will stimulate – only
whether or not it will activate a contraction. This is referred to as the ‘all
or none law’.
So changing the amount of force or the duration
you would like your muscles to contract depends on the number and frequency of
motor units recruited or activated. As more units are recruited, and as the
frequency of stimulation increases, muscle tension increases.
Many motor units can be stimulated at the same
time to achieve a short, sharp, strong contraction or perhaps in an alternating
sequence over a longer period of time for a less intense, longer contraction.
Which of the following would increase the
intensity or duration of a muscular contraction?
Consider the options and select all that apply.
Recruit fewer motor unitsStimulate the motor neuron more frequentlyDecrease the frequency of motor neuron stimulationRecruit more motor units
Notes
Activation of more motor units produces a greater
force. Larger motor units will contract along with the small motor units until
all muscle fibres in a single muscle are activated and the maximum muscle force
is produced.
Consecutive stimuli on the motor unit from the
motor neuron produce a greater force than a single contraction. Decreasing the
interval between the stimuli allows the muscle to produce a larger force with
the same amount of motor units.
Proprioceptors
Act as sensors in the muscles and connective tissue of the limbs.
Provide feedback on the impact of the immediate environment on our
musculoskeletal system.
Notes
Proprioceptors act as sensors in the muscles and
connective tissue of the limbs. They provide feedback on the impact of the
immediate environment on our musculoskeletal system, such as joint angle,
muscle length and muscle tension. This sensory input is integrated to give
information about the position of the limbs.
With this information we can respond to our
environment with the most appropriate kind of muscular reaction, such as
recruiting more motor units to move in a new direction or with a different
amount of force.
Stretch reflex
Golgi tendon organs: tell us about muscle tension.
Muscle spindles: tell us about muscle length.
Notes
There are different types of proprioceptor that
provide different sensory information for the nervous system to respond to.
We have Golgi tendon organs which tell us about
muscle tension and we have muscle spindles which tell us about muscle length.
Muscle spindles sense changes in muscle length
and respond by causing muscular contractions. If a muscle lengthens and
activates the muscle spindle proprioceptor the nervous response is to contract
that same muscle to shorten it back up. This response is called the ‘stretch
reflex’.
When we move, we often take advantage of the
stretch reflex by lengthening and reactively shortening our muscles in
repetitive patterns; walking and running are perfect examples of this – where
our muscles lengthen and shorten repetitively for extended periods of time.
Reciprocal inhibition
When a muscle contraction is activated the antagonist muscle group
must be inhibited to allow for the contraction.
Notes
Opposing muscles groups work in synergy with each
other. When a muscle contraction is activated, the antagonist muscle group must
be inhibited to allow for the contraction to happen. This process is called
reciprocal inhibition.
Neuromuscular
adaptations to exercise
Cardiovascular training causes adaptations in the
neuromuscular system in relation to type 1 muscle fibres. These changes
include: increased size and number of mitochondria, increased oxygen delivery
to muscle fibres, increased aerobic enzymes in the muscle tissue, a greater
supply of glycogen and triglycerides for energy, and improved tissue tolerance
and resistance to fatigue.
Notes
There are a number of changes that occur within
the neuromuscular system as a result of exercise and training. Adaptations in
the neuromuscular system can have dramatic effects on the body and can be seen
in improvements in speed and acceleration, coordination, strength, endurance
and most aspects of fitness. Most types of training affect the nervous system
and, as it’s the controlling communication network of the body, if you can
develop your nervous system your muscular system will follow suit and overall
performance will improve.
The endocrine system
Hormones: The chemicals released into the
bloodstream to help control and manage the internal environment of the body.
Endocrine system: The technical name given to our hormonal controls.
Hormones are chemicals that are released into the
bloodstream to help control and manage the internal environment of the body.
The technical name given to our hormonal controls is the ‘endocrine system’.
Hormones are released from various glands around the body known as the
endocrine glands.
To make sense of this complex system to we need
to look at how a hormone functions, look at the major endocrine glands and
where they’re located, and then finally explore the key hormones that play a
role in health and exercise.
How hormones work
Endocrine function:
1.
Endocrine gland receives
stimulus.
2.
Gland releases chemical
hormone.
3.
Hormone travels in bloodstream.
4.
Hormones received by target
cell receptors.
5.
Cell stimulated to cause
desired response.
6.
Feedback to the originating
gland.
Notes
Endocrine function:
An endocrine gland receives a stimulus that
initiates a specialised chemical called a hormone into the surrounding
bloodstream.
The hormone is transported around the body within
the bloodstream seeking out target cells.
Once the hormone reaches a target cell it docks
within the cells receptor site initiating the desired response.
Off-piste
It is not uncommon to find that there may be multiple hormones
involved before the final cellular or glandular response is achieved. For
example, the hypothalamus in the brain releases a hormone that stimulates the
pituitary gland, which in turn releases a hormone that stimulates the adrenal
gland, which then releases cortisol (a hormone that stimulates the body in many
different ways to manage stress).
Feedback loop between the targeted tissue or
organ and the originating endocrine gland help to reduce or stop hormone
production.
Endocrine glands
The majority of the endocrine system and its
related functions are governed by a number of specialised glands that are
scattered throughout the body from the brain to the reproductive organs. These
glands produce the hormones that help maintain the delicate internal
environment of the body.
Pituitary gland: this is often considered the master gland, as
many of the early endocrine outputs originate from here.
Thyroid gland: this is considered to be the master regulator of
metabolism.
Adrenal glands: these are named after their location on top of
the renals (or kidneys). These small glands help primarily control and manage
the stress response in the body.
Pancreas: this sits below the stomach and helps in the
control of carbohydrate metabolism.
Ovaries: these are located in the lower female abdomen on
either side of the uterus. They are responsible for numerous functions of
female sexuality.
Testes: these are located in the scrotum and are
responsible for numerous functions of male sexuality.
The pancreas
Insulin control:
1.
Food ingested leads to rise in
blood glucose.
2.
Pancreas releases insulin which
drives glucose into the cells.
Glucagon control:
1.
Long periods between meals or
prolonged exercise may lower blood glucose levels.
2.
Pancreas releases glucagon
which causes liver to break down glycogen.
3.
Glucose from liver glycogen
restores blood levels.
Notes
The pancreas produces two hormones called insulin
and glucagon. They both help to regulate blood glucose.
Insulin: Rising blood glucose, as a result of
food intake, signals to the body that the pancreas needs to release insulin.
Insulin signals for cellular channels to open and allow the glucose to move out
of the blood and into the cells for use. This results in a reduction of blood.
Glucagon: When an individual has gone for a long
period without food or has engaged in prolonged activity, glucose levels may
drop. The pancreas releases glucagon which stimulates the liver to break down
glycogen into glucose. This release helps to restore flagging blood glucose
back to more stable levels.
Off-piste
Insulin is responsible for driving glucose into adipose tissue and
so plays a role in contributing to fat cell storage. Insulin is also
responsible for driving proteins into cells. So protein-rich foods also
stimulate an insulin response; though not as strongly as carbohydrate-rich
foods.
Sex hormones
Ovaries: release oestrogen, which
influences fat storage around the hips, buttocks and thighs.
Testes: release testosterone, which
plays a part in stimulating the growth of muscle tissue.
Notes
The sexual glands in males and females release
important hormones that bring about the physical changes that are typically
witnessed during puberty. In females the ovaries release oestrogen and in males
the testes release testosterone. These hormones also support health during
adulthood. They govern vital functions related to growth, repair and storage in
the body.
Testosterone in both males and females plays a
part in stimulating the growth of muscle tissue, whereas oestrogen influences
fat storage around the hips, buttocks and thighs. Both hormones also play a
role in directing bone growth during puberty and in adulthood. The correct
balance of sex hormones is necessary for health, as a teenager and as an adult.
Adrenal hormones
The adrenals are well located on top of the
kidneys, as they have a direct link to the major blood vessels of the body.
This allows their valuable hormones to get out into circulation and into effect
very quickly.
Adrenaline is the most well known hormone in the
catecholamine category. Adrenaline and noradrenaline are released from the
medulla when life necessitates a typical ‘fight-or-flight’ response; they are
the hormones of action as they drive heart rate, blood flow, and breathing and
alertness levels upwards preparing the body very rapidly for activity.
The primary corticosteroid released from the
adrenal cortex is called cortisol. Cortisol helps provide needed reserves in
the body to manage stress, whether mental, physical or emotional. Cortisol is
primarily catabolic and helps break down carbohydrates and fats to provide
energy for the body during stressful periods. It offers support during
short-term bouts of stress. Long-term chronic stress and the resulting excess
cortisol can lead to deterioration in health by unbalancing the endocrine
system.
Aldosterone is another of the corticosteroids
released from the adrenal cortex. It is important in maintaining the water
balance in the bloodstream, as it helps regulate the sodium/potassium balance
in the blood. These minerals play a major role in water balance.
Growth and thyroid hormones
Growth hormone:
·
Released directly from the
pituitary.
·
Stimulates bone growth, protein
synthesis and fat burning.
Thyroid hormones:
·
Released from the thyroid gland
after stimulus from the pituitary.
·
Controls cellular energy
production, body temperature and metabolic rate.
Notes
The remaining two hormones we’re going to look at are primarily driven by the pituitary gland. They are the ‘growth hormone’ and the ‘thyroid hormones’.
Growth hormone is released directly from the pituitary gland and stimulates growth in all areas of the body. In particular, growth hormone drives bone growth during puberty, but it also stimulates protein synthesis in muscle tissue and helps break down and release fat tissue from storage sites around the body for oxidation.
The thyroid hormones are not released directly from the pituitary, but from the thyroid gland located in the upper chest. But, it is secretions from the pituitary that stimulate the thyroid gland to release important thyroid hormones. They help to regulate several processes; including the use of oxygen in producing cellular energy, the maintenance of body temperature and overall metabolic rate.
Both growth and thyroid hormones have a very broad influence on the way the body operates and maintains good health. So it’s vital that they are kept at the correct levels in the body.
Glands and hormones
Pancreas: The pancreas releases glucagon when blood sugar
(glucose) levels fall too low.
Adrenal cortex: Cortisol increases blood sugar and aids fat,
protein and carbohydrate metabolism.
Pituitary gland: Growth hormone stimulates growth in all areas of
the body.
Adrenal medulla: Adrenaline regulates heart rate and is crucial
for the fight-or-flight response.
FYI : Cortisol helps
break down and release glucose and stored fats for use under stressful
conditions such as exercise. Glucagon helps release stored glucose from the
liver if blood glucose drops too low. Low blood glucose can often occur during
exercise, especially if adequate food has not been ingested prior to the
activity. The other hormones may all justifiably have some involvement around
exercise, but they are not directly responsible for the release of stored
energy.
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