BIO 012 Lecture Notes - Lecture 9: Axon Terminal, Neuroglia, Sensory Neuron
12 Jun 2018
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Chapter 45: Neurons, Glia, and Nervous Systems
45.1: What Cells are Unique to the Nervous System?
Nervous systems - encode, process, and store a wide variety of info from the external and internal
environments, and that info is used to control and regulate the physiological processes and behavioral
actions of the organism.
- Nervous systems are able to carry out these functions b/c of the properties of 2 types of cells:
nerve cells, or neurons, and glial cells, or glia.
o All neurons are excitable – can generate and transmit electric signals which are known
as action potential
o Glia modulate the activities of neurons and support them.
The structure of neurons reflects their functions. Neurons have a basic structure of 4 regions
- A cell body – nucleus and organelles
- Dendrites – extend from the cell body – bring info from other neurons or sensory cells to the
cell body.
- Axon – carry info in the form of action potentials away from the originating cell body (the
presynaptic cell – where the message hits the terminal end before the synapse) to the
receiving target cell (the postsynaptic cell – where the message crosses synapse to dendrites)
- At the postsynaptic cell, the axon divides into a spray of fine nerve endings. At the tip of each
of these tiny nerve endings is a swelling called the axon terminal. The axon terminal comes
close to the membrane of the target cell forming a synapse (a space) at which the info
conveyed by the action potential is communicated from the presynaptic cell to the
postsynaptic cell.
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Glia Cells – 4 types
- Glial cells make myelin. Myelin is the covering produced by oligodendrocytes and Schwann
cells. Not all axons are myelinated but those that are can conduct action potentials more
rapidly than the non-myelinated ones.
1. Oligodendrocytes – (in the central nervous system – CNS) - wrap around the axons of neurons,
covering them with concentric layers of insulating plasma membrane.
2. Outside the brain and spinal cord, glia called Schwann cells wrap axons.
3. Microglia (type of glial cell in the brain only) – come from stem cells in bone marrow and move to
the nervous system – act as macrophages and mediators of inflammatory responses – giving the
nervous system immune defenses (CNS immunity).
- The blood-brain barrier protects the brain (CNS) from any toxic chemicals in the blood.
Regulates substance entry to the brain. Capillaries have no pores.
4. Astrocytes regulate the environment of the nervous system. They also surround the most
permeable blood vessels in the brain (6 functions)
- BBB – blood brain barrier
- Nourishes neurons with glycogen
- Signaling (release neurotransmitters)
o Tripartite synapses – the intimate contact of the astrocytes with the neural
components of a synapse that inspired the ideas that a synapse includes connections
from astrocytes and not just the pre- and postsynaptic neurons
- Re-uptake of neurotransmitters – the neurotransmitters move towards the post synaptic
neuron to activate it but if too many are let out then it over activates the post synaptic neuron.
The astrocyte would be activated to reuptake the neurotransmitters and stay in balance.
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- Repair and regenerate neurons
- Communicates changes in blood
- Regulation of potassium in extra cellular brain
45.4: How Are Neurons and Glia Organized into Information – Processing Systems?
Nervous systems can process info because their neurons are organized into neural networks. These
networks include 3 functional categories of neurons which are input, output, and integration.
- Sensory neurons (afferent neurons) carry sensory info into the nervous system.
- Motor neurons (efferent neurons) carry commands out of the nervous system to muscles and
glands
- Interneurons integrate and store info and communicate between afferent neurons and
efferent neurons.
Nervous systems range in complexity. With increase in complexity, nerve nets →ganglia →brain
- The idaria’s nerve net is most developed around the tentacles and the oral opening, where
it facilitates detection of food or danger and causes tentacles to extend or retract. (ex: sea
anemone)
- Ganglia – collection of neurons where info can be processed (ex: in worms)
- As animals increase in complexity, some ganglia may become enlarged or fused together at
the anterior end, forming a brain.
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Document Summary
Nervous systems - encode, process, and store a wide variety of info from the external and internal environments, and that info is used to control and regulate the physiological processes and behavioral actions of the organism. Neurons have a basic structure of 4 regions. A cell body nucleus and organelles. Dendrites extend from the cell body bring info from other neurons or sensory cells to the cell body. At the postsynaptic cell, the axon divides into a spray of fine nerve endings. At the tip of each of these tiny nerve endings is a swelling called the axon terminal. The axon terminal comes close to the membrane of the target cell forming a synapse (a space) at which the info conveyed by the action potential is communicated from the presynaptic cell to the postsynaptic cell. Myelin is the covering produced by oligodendrocytes and schwann cells.
