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2. Nervous system and sense organs
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Nervous system and sense organs 2
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3. Functions of the nervous system
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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. 3
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4. Structure of the nervous system
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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. 4
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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. 5
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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. 6
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7. The generation of the nervous signal
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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. + + + + – – – – + + + + + + + + – – – – + + + + – – – – – – – – 7
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8. Propagation of the nervous signal
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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. 8
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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. 9
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10. Neuromuscular junction and muscle contration
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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. 10
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11. Central nervous system
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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…). 11
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12. Peripheral nervous system
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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. 12
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13. Somatic and autonomic peripheral nervous system
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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. 13
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14. Sensory receptors and sense organs
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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. 14
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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. 15
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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.
Cones are responsible for the perception of colors.
Rods are very sensitive to light and allow vision in conditions of poor lighting.
rods
retina
pupil
iris
lens
optic nerve
cones 16
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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. 17
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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. 18
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