The Nervous System
The nervous system controls and coordinated functions throughout the body
and responds to internal and external stimuli
The messages carried by the nervous system are electrical signals called impulses
The cells that transmit these impulses are called neurons
Neurons can be classified into three types according to the direction in which an
impulse travels
 Sensory neurons carry impulses from the sense organs to the spinal cord and brain
 Motor neurons carry impulses from the brain and the spinal cord to muscles and
 Inter neurons connect sensory and motor neurons and carry impulses between
The largest part of a typical neuron is the cell body
 It contains nucleus and much of the cytoplasm
 Most of the metabolic activity of the cell takes place in here
Spreading out from the cell body are short, branched extensions called dendrites
 It carries impulses from the environment or from other neurons toward the cell
The long fiver that carries impulses away from the cell body is called the axon
 It ends in a series of small swellings called axon terminals, located some distance
from the cell body.
In most animals, axons and dendrites are clustered into fibers called nerves
In some neurons, the axon is surrounded by an insulating membrane known as the myelin
The myelin sheath that surrounds a single long axon leaves many gaps, called nodes,
where the axon membrane is exposed
As an impulse moves along the axon, it jumps from one node to the next, which
increases the speed at which the impulse can travel
Nerve impulses are electrical
A neuron has an electrical potential or voltage across its cell membrane of 70mv
At rest, the inside of the neuron’s membrane has a negative charge
The potential is produced by the actions of a cell membrane pump, powered by the
energy of ATP
This membrane protein forces sodium ions out of the cell and pumps potassium ions in
As a result of the active transport, the cytoplasm of the neuron contains more
potassium ions and fewer sodium ions than the surrounding medium
Potassium ions leak out across the cell membrane more easily than sodium ions leak in
So, potassium ions leak out of the cell to produce a negative charge on the inside of the
This charge difference is known as the resting potential of the neuron
A nerve impulse causes a movement of ions across the cell membrane
An impulse begins when a neuron is stimulated by another neuron or by the environment
 A nerve impulse is self-propagating
Once it begins, the impulse travels rapidly down the axon away from the cell body and
toward the axon terminals
An impulse is a sudden reversal of the membrane potential
The cell membrane of a neuron contains thousands of protein channels, or gates, that
allow ions to pass through
 Generally, they are closed
 At the leading edge of an impulse, the sodium gates open, allowing positively
charged sodium ions to flow inside the cell membrane
 The inside of membrane then, temporarily becomes more positive than he outside,
reversing the resting potential
 This reversal of charges is called an action potential
 As the impulse passes, the potassium gates open, allowing positively charged
potassium ions to flow out
 This restores the resting potential so that neuron is once again negatively charged
on the inside of cell membrane and positively charged on the outside
The minimum level of a stimulus that is required to activate a neuron is called the
Any stimulus that is stronger than the threshold will produce an impulse
A nerve impulse follows the all-or-none principle
 Either the stimulus will produce an impulse, or it won’t produce an impulse.
Polarized refers to the state of the membrane at rest, negative on the inside and
positive on the outside
Depolarization occurs when the membrane potential moves in the positive direction
Repolarization occurs when the membrane potential returns to its resting value
If a neuron is polarized and at the resting potential (-70mV), and depolarizes slightly to
the threshold potential (-50mV), voltage-gated sodium and voltage-gated potassium
channels will open
The voltage-gated sodium channels open first, allowing sodium to enter the cell
according to its concentration gradient
The entering sodium ions depolarize the cell further, allowing it to reach a maximum of
+35 mV before the voltage-gated sodium channels close
Then the voltage-gated potassium channels open, allowing potassium to exit the cell
according to its concentration gradient
The existing potassium ions repolarize the cell, actually bypassing the resting
membrane potential, to a minimum of -90mV before the voltage-gated potassium
channels close
Finally, the Na+/K+ATPase and the potassium leak channels return the membrane to its
resting polarized state
At the end of the neuron, the impulse reaches an axon terminal
Usually the neuron makes contact with another cell at this location
The neuron may pass the impulses along to the second cell
 Motor neurons, for example, pass their impulses to muscle cells
The location at which a neuron can transfer an impulse to another cell is called a
 A small cleft, or gap, separates the axon terminal from the dendrites of the
adjacent cell, in this case a neuron
The terminals contain tiny sacs, or vesicles, filled with the neurotransmitters
Neurotransmitters are chemicals used by a neuron to transmit an impulse across a
synapse to another cell
When an action potential arrives at an axon terminal, the sacs release the
neurotransmitters into the small gap between the two cells
The neurotransmitter molecules diffuse across the gap and attach themselves to
receptors on the membrane of the neighboring cell
 This causes positive sodium ions to rush across the cell membrane, stimulating the
second cell
 If the stimulation exceeds the cell’s threshold, a new impulse begins
Divisions of the Nervous system
The central nervous system relays messages, processes information, and
analyzes information
 The peripheral nervous system receives information from the environment and
relays commands from the central nervous system to organs and glands.
The central nervous system consists of the brain, and the spinal cord
Both the brain and spinal cord are wrapped in three layers of connective tissue known
as meninges
It is the main switching unit of the central nervous system
The largest and most prominent region of the human brain is the cerebrum
It is responsible for the voluntary, or conscious, activities of the body
Site of intelligence, learning, and judgment
The cerebrum consists of two surfaces
The outer surface of the cerebrum is called the cerebral cortex and consists of gray
 Gray matter consists mainly of densely packed nerved cell bodies
 The cerebral cortex processes information from the sense organs and controls
body movements
The second largest region of the brain is the cerebellum
It is located at the back of the skull
It coordinates and balances the actions of the muscles so that the body can move
gracefully and efficiently
The brain stem connects the brain and spinal cord
It includes two regions known as the pons and the medulla oblongata
Each of these regions regulates the flow of information between the brain and the rest
of the body.
Some of the body’s most important functions are controlled in the brain stem
 Blood pressure
 Heart rate
 Breathing
 Swallowing
The thalamus receives messages from the sense organs
The hypothalamus is the control center of recognition and analysis of hunger, thirst,
fatigue, anger, and body temperature
Certain kinds of information, such as reflexes, are processed directly in the spinal cord
The spinal cord is the main communications link between the brain and the rest of body
A reflex is a quick, automatic response to a stimulus
 Sneezing and blinking are examples of reflexes
A reflex allows body to respond to danger immediately, without spending time thinking
about a response
The peripheral nervous system lies outside of the central nervous system
It consists of all of the nerves and associated cells that are not part of the brain and
the spinal cord
The sensory division of the peripheral nervous system transmits impulses from sense
organs to the central nervous system. The motor division transmits impulses from the
central nervous system to the muscles or glands
The motor division is further divided into the somatic nervous system and the
autonomic nervous system
The peripheral nervous system transmits impulses from sense organs to the central
nervous system and back to muscles of glands
The somatic nervous system regulates activities that are under conscious control, such
as the movement of the skeletal muscles
Some somatic nerves are also involved with reflexes and can act with or without
conscious control
The rapid response is possible for humans because receptors in skin stimulate sensory
neurons, which carry the impulse to spinal cord
Even before the information is relayed to the brain, group of neurons in spinal cord
automatically activates the appropriate motor neurons
These motor neurons cause the muscles in legs to contract
The pathway that an impulse travels from foot back to leg is known as a reflex arc
A reflex arc includes a sensory receptor, sensory neuron, motor neuron, and effector
In some other reflex arcs, a sensory neuron communicates directly with a motor neuron
The autonomic nervous system regulates activities that are autonomic, or involuntary
The nerves of the autonomic nervous system control functions of the body that are not
under conscious control
 It regulates the heartbeat and controls the contraction of smooth muscles in the
digestive system and in blood vessels
 The autonomic nervous system can be divided as two part that have opposite effects on
the organs they control
Sympathetic nervous system
 High blood pressure
 High pulse rate
 Increasing breathing rate
 Fight or Flight
Parasympathetic nervous system
 Low blood pressure
 Low pulse rate
 Decreasing breathing rate
 Resting and digesting
Heart rate is increased by the sympathetic nervous system, but decreased by the
parasympathetic nervous system
Because there are two different sets of neurons, the autonomic nervous system can
quickly speed up the activities of major organs in response to a stimulus
The Endocrine System
The endocrine system is made up of glands that release their products into the
bloodstream and these products broadcast messages throughout the body
The chemicals that “broadcast: messages from the endocrine system are called
Hormones are chemical that travel through the bloodstream and affect the activities
of other cells
Hormones do this by binding to specific chemical receptors on those cells
Cells that have receptors for a particular hormone are called the target cells
If a cell does not have receptors or the receptors do not respond to a particular
hormone, the hormone has no effect on it
In general, the body’s response to hormone re slower and longer-lasting than the
responses to nerve impulses
A gland is an organ that produces and released a substance, or a secretion
Exocrine glands release their secretions through tube like structures called ducts
 Exocrine glands include those that release sweat, tears, and digestive juices
Endocrine glands release their secretions (hormones) directly into the bloodstream
 Hypothalamus gland makes hormones that control the pituitary gland and makes
hormones that are stored in the pituitary gland
 Pituitary gland produces hormones that regulate many of the other endocrine
 The four parathyroid glands release parathyroid hormone, which regulates the
level of calcium in the blood
 During childhood, the thymus gland releases thymosin, which stimulates T cell
 The adrenal glands release epinephrine, which help the body deal with stress
 The pineal gland releases melatonin, which is involved in rhythmic activities, such
as daily sleep-wake cycles
 The thyroid gland produces thyroxin, which regulates metabolism
 The pancreas produces insulin and glucagon, which regulate the level of glucose in
the blood
 Ovaries produce estrogen and progesterone. Estrogen is required for the
development of female secondary sex characteristics and for the development of
eggs. Progesterone prepares the uterus for a fertilized egg
 The testes produce testosterone, which is responsible for sperm production and
the development of male secondary sex characteristics
Hormones fall into two general groups
 Steroid hormones are produced from a lipid known as cholesterol
 Nonsteroid hormones include proteins, small peptides, and modified amino acids
Because they are lipids, steroid hormones can cross cell membranes easily, passing
directly into the cytoplasm and even into the nuclei of target cells
 A steroid hormone enters a cell by passing directly across its cell membrane
 Once inside, it binds to a steroid receptor protein (found only in its target cells) to
form a hormone-receptor complex
 The hormone-receptor complex enters the nucleus of the cell, where it binds to a
DNA control sequence
 This binding initiates the transcription of specific genes to messenger RNA
 The messenger RNA moves into the cytoplasm and directs protein synthesis
Because steroid hormones affect gene expression directly, they can produce dramatic
changes in cell and organism activity
Nonsteroid hormones generally cannot pass through the cell membrane of their target
 A nonsteroid hormone binds to receptors on the cell membrane
 The binding of the hormone activates an enzyme on the inner surface of the cell
 This enzyme activates secondary messengers that carry the message of the
hormones inside the cell
 Calcium ions, cAMP (cyclic adenosine monophosphate), nucleotides, and even fatty
acids can serve as second messengers
 Once released, these second messengers can activate of inhibit a wide range of
other cell activities
Even though the endocrine system is one of the master regulators of the body, it too
must be controlled
Like the most systems of the body, the endocrine system is regulated by feedback
mechanisms that function to maintain homeostasis
 A feedback mechanism is a self-regulating mechanism that increases of decreases
an action of the level of a particular substance
Thyroxin, the thyroid glands principal hormone, affects the activity of cells throughout
the body
Increased levels of thyroxin in the blood stimulate cells to become more active
The activity of the thyroid gland is controlled by the hypothalamus and the anterior
pituitary gland
When the hypothalamus senses that the thyroxin level in the blood is low, it secretes a
hormone known as thyroid-releasing hormone, which stimulates the anterior pituitary to
secrete thyroid-stimulating hormone
Thyroid-stimulating hormone stimulates the release of thyroxin by the thyroid gland,
which sets up the feedback loop
 This feedback mechanism keeps the level of thyroxin in the blood relatively
Generally speaking, one hormone negates the other, or makes it inefficient in a
negative feedback
sensed by the
which secretes
tells anterior
decreased level of
pituitary gland to
thyroid gland
releases thyroxine
high levels
of cortison
The hormones released from the endocrine glands travel throughout the body,
reaching almost every cell
It is a bean-sized structure that dangles on a slender stalk of tissue at the base of the
It is divided into two parts
 Anterior pituitary
 Posterior pituitary
The pituitary gland secretes nine hormones that directly regulate many body functions
and controls the actions of several other endocrine glands
The hypothalamus is the part of the brain above and attached to the posterior
The hypothalamus controls the secretions of the pituitary gland
The activity of the hypothalamus is influenced by the levels of hormone in the blood
and by sensory information collected by other parts of the central nervous system
Interactions between the nervous system and the endocrine system take place at the
The close connection between the hypothalamus and the pituitary gland means that the
nervous and endocrine systems can act together to help coordinate body activities
The thyroid gland is located at the base of the neck and wraps around the upper part
of the trachea
The thyroid gland has the major role in regulating the body’s metabolism
The four parathyroid glands are found on the back surface of the thyroid gland
Hormones from the thyroid gland and the parathyroid glands maintain homeostasis in
blood calcium levels
Some hormones are important for promoting proper nerve and muscle function and bone
The adrenal glands are two pyramid-shaped structures that sit on top of the kidneys,
one gland on each kidney
The adrenal glands help the body prepare for and deal with stress
An adrenal gland has an outer part called the adrenal cortex and an inner part called
the adrenal medulla
About 80% or adrenal gland is adrenal cortex
It produces corticosteroids
Some hormones regulate the reabsorption of sodium ions and the excretion of
potassium ions by the kidneys
It is regulated by the sympathetic nervous system
Produces “fight or flight” responses to stress
Nerve impulses from the sympathetic nervous system stimulate cells of the adrenal
It is an unusual organ
It is located along the right side of and behind the stomach
The hormone-producing portion of the pancreas consists of clusters of cells that
resemble islands
 These clusters of cells are called islets of Langerhans
Each islet include…
 BETA cells which secrete a hormone called insulin
 ALPHA cells which secrete a hormone called glucagon
Insulin and glucagon released from the pancreas help to keep the level of glucose in the
blood stable
Insulin stimulates cells in the liver and muscles to remove sugar from the blood and
store it as glycogen or fat
Glucagon stimulates the liver to break down glycogen and release glucose back into the
The gonads are the body’s reproductive glands
The gonads serve two important functions: the production of gametes and secretion of
sex hormones
The female gonads, or the ovaries produce eggs
The male gonads, or the testes produce sperm
The gonads also produce sex hormones