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Nervous system
and sense organs
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© Zanichelli editore 2016
Functions of the nervous system
The nervous system is composed of a network of cells
that work extremely fast to:
•collect information from the environment;
•process the information;
•activate appropriate responses to the stimuli.
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© Zanichelli editore 2016
Structure of the nervous system
The central nervous system (CNS) includes:
• the brain;
• the spinal cord.
The peripheral nervous system (PNS) is formed by:
• ganglia;
• nerves.
Neurons are the cellular units of the nervous system. They
are electrically excitable cells, specialized in generating and
transmitting electrical signals.
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© Zanichelli editore 2016
Three types of neurons
Neurons have a cell body, many dendrites and an axon
terminating in a synapse.
There are three types of neurons:
• sensory neurons collect information from the
environment;
• motor neurons generate responses to stimuli, sending
signals to muscles or glands;
• interneurons are located in the brain and in the spinal
cord, they receive information from other neurons.
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Glial cells
Glial cells are not involved in the transmission of nervous
signals, but they have an important role in supporting,
protecting and providing nutrients to neurons.
Oligodendrocytes and
Schwann cells form myelin
sheath around axons of
neurons in the central nervous
system.
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The generation of the nervous signal
Across the membrane of cells there is a differential distribution
of electrical charges due to a different concentration of ions
inside and outside the cell. This difference creates the
membrane potential.
Sensory stimuli alter the
membrane potential.
If the sum of signals arriving to
dendrites is higher than a certain
++++– – – – ++++ ++++
threshold value, an action
– – – –++++ – – – – – – – –
potential is generated and
transmitted along the axon.
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Propagation of the nervous signal
Propagation of the nervous signal can be continuous, in
axons not surrounded by myelin, or saltatory, in myelinated
axons.
Saltatory propagation is much faster.
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Synapses
Synapses are the structures that connect the axon terminal
of a neuron with the proceeding cell which can be another
neuron, a muscle cell or a gland.
There are electrical and
chemical synapses.
In chemical synapses, the
action potential releases
neurotransmitters that bind to
receptors on the post-synaptic
membrane.
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Neuromuscular junction
and muscle contration
The contraction of skeletal muscles is controlled by motor
neurons that form synapses with muscle fibers called a
neuromuscular junction.
Muscle fibers are composed of myofibrils that contain many
repeating sarcomeres.
Inside the sarcomere there are two types of filaments, formed
by actine or myosine.
The movements of the filaments cause the sarcomere to
shorten, causing the muscle to contract.
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© Zanichelli editore 2016
Central nervous system
The central nervous system (CNS) consists of the brain
and spinal cord. It is responsible for processing information
collected from the environment and the production of
responses.
It is formed by white matter and grey matter.
The brain is divided into two hemispheres and is
responsible for superior cognitive functions (memory,
abstract thinking, learning…).
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© Zanichelli editore 2016
Peripheral nervous system
The peripheral nervous system (PNS) consists of ganglia
and nerves.
In the human body there are 12
pairs of cranial nerves and 31
pairs of spinal nerves.
They transmit nervous signals
from the environment to the CNS
and from the CNS to the effector
organs.
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Somatic and autonomic peripheral
nervous system
The peripheral nervous system contains different parts:
• the somatic nervous system;
• the autonomic nervous system.
The autonomic nervous system is divided into:
• sympathetic system;
• parasympathetic system.
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Sensory receptors and sense organs
Sensory receptors are usually formed by the modified
dendrite of a sensory neuron. It responds to a specific
stimulus coming from the environment.
Receptors can be grouped into sense organs, such as eyes
and ears, or dispersed throughout the body as single cells or
small aggregates.
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Taste and smell
Taste receptors are grouped in taste buds, located around
small structures in the tongue, called papillae. They respond
to chemicals in foods and they are responsible for the
perception of taste.
Olfactory receptors are located in the epithelium of the nasal
cavities. They respond to volatile chemical substances and are
responsible for the perception of smell.
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Vision
Photoreceptors (cones and rods) are neurons that respond to
light stimulation. They are located in the retina, inside the eye.
When stimulated, they transmit signals to the brain that
processes them as images.
rods
retina
pupil
iris
lens
Cones are responsible for
the perception of colors.
Rods are very sensitive to
light and allow vision in
conditions of poor lighting.
optic
nerve
cones
© Zanichelli editore 2016
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Hearing and balance
Hair cells are receptors located in the inner ear that respond
to vibrations caused by sound waves. They send signals to the
brain which processes them as sounds.
The ear is also responsible for the sense balance (vestibular
system). Otoliths and the semicircular canal system provide
information about the movements and orientation of the head.
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Touch
Touch consists of sensations generated by
mechanoreceptors, thermoreceptors and pain receptors
distributed in the body, mainly in the skin.
They provide information about pressure, vibration,
temperature and pain.
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© Zanichelli editore 2016