22
HUMAN ANATOMY
The nervous system is a control network
that connects all body systems. It is split
into the central and peripheral nervous
systems (PNS). The PNS is comprised of the
somatic and autonomic nervous systems.
SYSTEM OVERVIEW
The somatic nervous system consists of nerves
carrying sensory and motor signals to and
from the spinal cord and brain. The
autonomic nervous system (ANS) is
divided into two functional systems:
the sympathetic nervous system and
parasympathetic nervous system, which
accounts for many of yogas benefits.
NERVOUS
SYSTEM
Brain
Responsible for control
and cognition
Spinal cord
Your body’s primary
communication highway
Brachial plexus
A collection of nerves
around your armpit
Sciatic nerve
Largest nerve
in your body
Cranial nerves
12 pairs of peripheral nerves
Ulnar nerve
On the little finger
side, causes tingling
when hitting “the
funny bone
Femoral nerve
Supplies sensation
around thigh and leg
Tibial nerve
Branches o
sciatic nerve
Digital branches
of bular nerve
Supply sensation to foot
Spinal cord
In this superior, or bird’s-eye, view of
a vertebra, you can see how your spinal cord
is protected by the bony encasement of the
spinal column. Spinal nerves project o to
the side, in between the vertebrae.
Spinal cord
connects the
brain and body
Spinal nerve carries
messages to and
from your central
nervous system
Vertebra
protects
spinal cord
Median nerve
Can be pressed on
in carpal tunnel
Lumbar plexus
A collection of nerves
around lower back
Sacral plexus
A collection of nerves
around your sacrum
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23
NERVE STRUCTURE
Neurons are the main cells of your nervous
system. Axons are bundled together in your
PNS to make nerves. Nerves are like highly
conductive electrical wires sending signals
throughout your body. Some are wrapped
with a fatty substance called myelin, making
their signals travel faster.
Cell body
Contains nucleus
Dendrite
Smaller projections that
receive signals from
neighboring neurons
Myelin
Covering that insulates
and speeds up signal
Epineurium
Connective tissue
around nerve
Perineurium
Connective tissue
around bundle
of neurons
Endoneurium
Connective tissue
around neuron
Axon
Largest projection
that transmits signal
to next cell
NERVE SIGNAL
Nerve signals are pulses of electricity
along the cell membrane due to the
movement of electrically charged
particles called ions.
NERVE NEURONS
THE AUTONOMIC
NERVOUS SYSTEM
The autonomic nervous system (ANS)
can be thought of as your body’s
autopilot. Its functions are automatic
and they include processes such as
your heart rate, breathing, digestion,
and excretion, which happen without
you having to consciously think about
them. The ANS is further divided
into two systems of control that
complement each other: the
sympathetic nervous system (SNS)
and the parasympathetic nevous
system (PSNS).
Stimulates proper
digestion
Stimulates glucose
storage as glycogen
Slows heartbeat
Constricts airways
Constricts pupils
Stimulates pancreatic
enzyme release
Stimulates peristalsis
Contracts bladder
Constricts blood
vessels
Slows urine
output
Accelerates
heartbeat
Inhibits proper
digestion
Inhibits peristalsis
(see p.39)
Relaxes bladder
PARASYMPATHETIC NERVOUS SYSTEM
The PSNS is known as “rest and digest”
or the “relaxation response” because it
creates a restful state of optimal function.
SYMPATHETIC NERVOUS SYSTEM
The SNS is known as “fight or flight”
or the “stress response” because it
helps you deal with stressful situations.
Dilates
pupils
Nerve signal
Brain and spinal
cord monitor internal
conditions of body
Dilates airways
Stimulates glucose
production
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24
CEREBRAL CORTEX
Compared to other mammals, our brains are massive for
our bodies, with a particularly developed cerebral cortex.
Most of the cortex is on the outside of the brain, except
the insula. It is composed of gray matter, which is filled
with synapses or connection points between neurons.
Your cortex has five lobes and many functional areas.
INSIDE THE BRAIN
The brain contains many dierent structures
and scientists are still figuring out what their
functions are. Some of these structures
monitor conditions inside your body and
relay information. The limbic system is the
emotional center of your brain.
LOBES OF THE BRAIN
The brain is separated into five main divisions,
called lobes, including the insula which is
inside the brain (not seen here).
INTERNAL STRUCTURES
This image shows the brain as if it were cut
in half down the middle (a midsagittal section)
to reveal structures inside the cerebrum.
Parietal lobe
Processes body
sensation
Occipital lobe
Back area
of the cortex
processes vision
Frontal lobe
Responsible for
decision-making and
motor functions
Temporal lobe
Involved in
smell, hearing,
and memory
Corpus callosum
Connects two sides
of brain
Hypothalamus
Controls much of
neuroendocrine
function
Thalamus
Relay center for
information
LATERAL VIEW
MIDSAGGITAL
SECTION
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25
How yoga aects your brain
This chart looks at the neuroscience that may
explain the vast mental and physical benets
of yoga. Modern science shows us that the
brain maintains its ability to adapt across a
lifetime, making it possible to break bad habits
and negative patterns. It can also create the
key chemicals that pharmaceutical companies
synthesize in a lab. Research is uncovering the
huge potential of yoga therapy to help people
on a global scale. These eects stem from
yoga’s multidimensional approach, reected in
its 8-limb structure (see p.198), which includes
guidelines on self-control and self-regulation.
Cerebellum
Involved in bodily
movement,
muscle control,
and balance
Hippocampus
Memory center
that allows
neurogenesis
(see pp.26–27)
Brainstem
Regulates
autonomic
functions like
breathing and
heart rate
Olfactory bulb
Detects scents
and triggers
memories
Amygdala
Fear center
Pons
Communication
center on brainstem
Cingulate gyrus
Regulates emotions
and behavior
Caudate nucleus
Involved in learning
and processing
memories
Fornix
Plays a role in
memory processing
Putamen
Involved in
movement and
learning
Pineal gland
Regulates
sleep–wake cycle
LIMBIC SYSTEM
Brain alpha wave activity increased
Alpha waves are associated with relaxation.
GABA increased Gamma-aminobutyric
acid counteracts anxiety and stress
symptoms, leading to more relaxation.
Serotonin increased Serotonin helps
regulate your mood. Low levels of usable
serotonin are associated with depression.
BDNF increased Brain-derived
neurotrophic factor is a protein responsible
for neuron health and neuroplasticity. Yoga
can boost levels of BDNF, which may help
people with chronic pain or depression.
Cortisol reduced Cortisol is a stress
hormone. When your baseline increases
and levels are too high for too long, it can
lead to inammation and weight gain.
Norepinephrine reduced A decrease
in norepinephrine, or adrenaline, means
fewer stress hormones in your system.
Dopamine regulated Dopamine
acts as your body’s reward system and
dysfunction is associated with addiction.
Research suggests that meditation results
in improved self-regulation.
HUMAN ANATOMY Nervous system
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26
NEURAL PATHWAYS
The brain develops neural connections—and
eventually becomes conditioned—based on your
choices and experiences. It is said that neurons that
fire together, wire together. The more you practice
an activity—or a mindset—the more networks are
created. With approximately 100 billion neurons,
the brains possible connections are vast. Yoga
practices facilitate this process.
CHANGING BRAIN
Neuroplasticity is the ability of your brain to be
molded. Not long ago, scientists thought the brain
couldnt change after childhood and degraded with
age. Now we know that nervous tissue adapts. Like
exercise aects your muscles, your brain tissue either
develops or atrophies based on stimulation.
BUILDING A NETWORK
Repetition strengthens
and continues to build
a neural network.
BUILDING
CONNECTIONS
A new experience
triggers neurons
to form new
connections.
More cells
join network
Nerve signal
Nerve signal
New
connection
Brain cell
New connection
Synapse
CYCLIC NATURE OF HABITS
UNSTIMULATED BRAIN
Without stimulation, fewer
connections are made. The
brain tissue looks like a dying
tree with sparse branches.
STIMULATED BRAIN
With stimulation, more
connections form. The brain
tissue looks like a thriving
tree with dense branches.
VRITTI
(THOUGHT
PATTERN)
KARMA
(ACTION)
How yoga boosts your brain
There is no neuroplasticity pill. The
most eective way to shape your brain
is through behavioral changes.
Although any yoga practice should
encourage neuroplasticity, try the tips
here for improved results.
Samskara
Yogis perhaps conceptualized
neuroplasticity with samskaras:
impressions due to past thoughts
and actions. Yoga can help beat bad
habits or conditioned responses
by aecting neural pathways and
samskaras. This occurs at a
synaptic level each time you
consciously change your
thoughts and actions through
awareness and practice. The
more you travel that new path,
the stronger the connection
between the neurons gets.
Increase the intensity
Moderate to vigorous
physical activity, like from
sun salutations, is one of
the most eective ways of
increasing brain-derived
neurotrophic factor. This is
a nerve growth factor, which
is like a glue that helps to
wire in neural connections.
Change your routine
Purposefully and consciously
changing your yoga practice
routine benets your mind
and your body.
Join a class
The act of moving with
a group and following
the teacher activates
mirror neurons. The
mirror neuron system is
a recently discovered
network of nerves involved
in emulation of movement
and developing compassion.
Meditate
Research shows that
meditation builds gray matter
in your cerebral cortex.
SAMSKARA
(IMPRESSION)
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