Chapter 3: What Are the Nervous System’s Functional Units?
Describing the nervous system cell structure and function.
Cells of the Nervous System
Brain cells give the nervous system its structure, and mediate its moment-to-moment
activity. This activity underlies our behavior. These brain cells consist of neurons and
glial cells.
There are two major theories about neurons
Golgi’s neuron theory:
The neuron system is an interconnected network of fibers. Information flows
around this ‘nerve net’ like water running through pipes, and produced behavior.
Cajal’s neuron theory:
The neuron system is made up of discrete cells, which begin life as simple
structures, and get more complex as a person ages. Neurons come in many
shapes and sizes, and can be distinguished from the glial cells that make up the
other half of the brain tissue.
The structure of a neuron:
The core region is called the cell body (Soma).
The branching extensions are called the
dendrites. Each dendrite can have one or more
dendritic spines, which extend the dendrite.
These collect information from other cells.
The main root of the neuron is called the axon,
starting at the Axon Hillock. The axon carries
messages to other neurons. The axon can split
into multiple axon collaterals. These collaterals
enable the neuron to send information to many
other neurons at a time.
The axon collaterals may divide into multiple
terminal buttons, or end feet. These sit close to,
but don’t touch, another neuron. This near
connection is called the synapse, which
includes the surfaces of the end foot and
dendritic spine, and the space in between these
two.
,Sensory neurons:
These neurons are the simplest of the three types, and transmit incoming sensory
information to the neurons that carry information.
- The bipolar neuron transmits this information from the retina to the brain, and has a
simple structure of only a single short dendrite and a single short axon on the other
end of the cell body.
- The somatosensory neuron carries information from the body to the spinal cord. This
longer distance asks for a more complex structure where the dendrite connects
directly to its axon, and the cell body sits more to the side.
Interneurons:
These neurons link up sensory and motor neurons. They are small and have a lot
(sometimes so many the axon is hard to see) of dendrites.
- The pyramidal cell has a longer axon and a pyramid-shaped cell body, and two sets of
dendrites. These carry information from the cortex to the rest of the brain and spinal
cord.
- The Purkinje cell is a interneuron with extremely branched dendrites that form a fan
shape. It carries information from the cerebellum to the rest of the brain and spinal
cord.
Motor neurons:
These neurons collect information from many sources. They have extensive dendritic
networks, large cell bodies and long axons. Motor neurons are found in the lower
brainstem and spinal cord.
→ By exciting or inhibiting one another, members of the neuronal network can
detect sensory information and decide what kind of motor response to make
that information.
Type Appearance Features and function
Glial cells (glial→glue)
Ependymal cell Small ovoid Secretes cerebrospinal fluid (CSF)
are support cells that
Astrocyte Star-shaped Contributes to neuronal support,
don’t transmit nutrition and repair. Forms the
information, but help blood-brain barrier and heals
neurons to carry out scarring after injury.
this task. Microglial cell Small Derived from blood. It has a
defensive function to remove dead
tissue.
Oligodendroglial Forms myelina round central-
cell nervous-system axons in the brain
and spinal cord.
Schwann cell Wraps around the peripheral
nerves to form myelin around the
axon.
,Cell Membrane: Barrier and Gatekeeper
The membrane separates the intracellular fluid from the extracellular fluid, allowing the
cell to be an independent unit. It regulates the differing concentration of salts and other
chemicals, and does not let these pass the membrane. Smaller particles like oxygen,
carbon dioxide, and glucose can pass through the cell membrane.
The Nucleus and Protein Synthesis
The nucleus is like the office in a factory. Here lie blueprints for making proteins and
RNA. These blueprints are genes, which consist of segments of DNA that encode the
synthesis of particular proteins. Within these genes are chromosomes which hold an
organism’s DNA library. Each chromosome contains thousands of gene blueprints. A
human has 23 pairs of chromosomes.
, The Endoplasmic Reticulum: Protein Manufacture
DNA → mRNA → protein
When DNA is transcripted into messenger RNA, it carries the protein code out of the
nucleus. There, in the endoplasmic reticulum, the mRNA comes in contact with
ribosomes. As this ribosome moves along the mRNA, it translates the bases into a
specific amino acid chain (polypeptide chain), which forms the protein.
Proteins and RNA: The Cell’s Products
A protein is a folded-up polypeptide chain. The longer the chain, the more complex
shapes it can form. This shape is important to the function that the protein serves.
Misfolded proteins can result in more than 100 nervous system diseases.
RNA is produced from genes in neurons. A neuron contains as many as 20.000 genes
that produce RNAs, which get translated into proteins.
Golgi bodies and Microtubules: Protein Packaging and Shipment
The wrapping of a protein in a membrane and labelling takes place in the Golgi bodies.
They are then loaded onto motor molecules that move along microtubules radiating
through the cell.
If a protein is meant to stay in the cell, it is released into the intracellular fluid. If it is to
be in the membrane, it is carried there. If it is to be exported from the cell, it is usually
transferred out through exocytosis, where the membrane in which the protein is
wrapped fuses with the cell membrane, causing the protein to be excreted into the
extracellular fluid.
Crossing the Cell Membrane: Channels, Gates, and Pumps
Some membrane proteins form channels, through which substances can pass. These
channels are designed by size to accommodate only certain substances.
Other membrane proteins form gates that regulate the passage of substances. Gates
change their chape in response to some trigger. These can open and close to regulate
substances entering or leaving the cell.
Changes in a protein’s shape can allow it to act as a pump, that can actively carry
substances across a cell’s membrane.
Describing the nervous system cell structure and function.
Cells of the Nervous System
Brain cells give the nervous system its structure, and mediate its moment-to-moment
activity. This activity underlies our behavior. These brain cells consist of neurons and
glial cells.
There are two major theories about neurons
Golgi’s neuron theory:
The neuron system is an interconnected network of fibers. Information flows
around this ‘nerve net’ like water running through pipes, and produced behavior.
Cajal’s neuron theory:
The neuron system is made up of discrete cells, which begin life as simple
structures, and get more complex as a person ages. Neurons come in many
shapes and sizes, and can be distinguished from the glial cells that make up the
other half of the brain tissue.
The structure of a neuron:
The core region is called the cell body (Soma).
The branching extensions are called the
dendrites. Each dendrite can have one or more
dendritic spines, which extend the dendrite.
These collect information from other cells.
The main root of the neuron is called the axon,
starting at the Axon Hillock. The axon carries
messages to other neurons. The axon can split
into multiple axon collaterals. These collaterals
enable the neuron to send information to many
other neurons at a time.
The axon collaterals may divide into multiple
terminal buttons, or end feet. These sit close to,
but don’t touch, another neuron. This near
connection is called the synapse, which
includes the surfaces of the end foot and
dendritic spine, and the space in between these
two.
,Sensory neurons:
These neurons are the simplest of the three types, and transmit incoming sensory
information to the neurons that carry information.
- The bipolar neuron transmits this information from the retina to the brain, and has a
simple structure of only a single short dendrite and a single short axon on the other
end of the cell body.
- The somatosensory neuron carries information from the body to the spinal cord. This
longer distance asks for a more complex structure where the dendrite connects
directly to its axon, and the cell body sits more to the side.
Interneurons:
These neurons link up sensory and motor neurons. They are small and have a lot
(sometimes so many the axon is hard to see) of dendrites.
- The pyramidal cell has a longer axon and a pyramid-shaped cell body, and two sets of
dendrites. These carry information from the cortex to the rest of the brain and spinal
cord.
- The Purkinje cell is a interneuron with extremely branched dendrites that form a fan
shape. It carries information from the cerebellum to the rest of the brain and spinal
cord.
Motor neurons:
These neurons collect information from many sources. They have extensive dendritic
networks, large cell bodies and long axons. Motor neurons are found in the lower
brainstem and spinal cord.
→ By exciting or inhibiting one another, members of the neuronal network can
detect sensory information and decide what kind of motor response to make
that information.
Type Appearance Features and function
Glial cells (glial→glue)
Ependymal cell Small ovoid Secretes cerebrospinal fluid (CSF)
are support cells that
Astrocyte Star-shaped Contributes to neuronal support,
don’t transmit nutrition and repair. Forms the
information, but help blood-brain barrier and heals
neurons to carry out scarring after injury.
this task. Microglial cell Small Derived from blood. It has a
defensive function to remove dead
tissue.
Oligodendroglial Forms myelina round central-
cell nervous-system axons in the brain
and spinal cord.
Schwann cell Wraps around the peripheral
nerves to form myelin around the
axon.
,Cell Membrane: Barrier and Gatekeeper
The membrane separates the intracellular fluid from the extracellular fluid, allowing the
cell to be an independent unit. It regulates the differing concentration of salts and other
chemicals, and does not let these pass the membrane. Smaller particles like oxygen,
carbon dioxide, and glucose can pass through the cell membrane.
The Nucleus and Protein Synthesis
The nucleus is like the office in a factory. Here lie blueprints for making proteins and
RNA. These blueprints are genes, which consist of segments of DNA that encode the
synthesis of particular proteins. Within these genes are chromosomes which hold an
organism’s DNA library. Each chromosome contains thousands of gene blueprints. A
human has 23 pairs of chromosomes.
, The Endoplasmic Reticulum: Protein Manufacture
DNA → mRNA → protein
When DNA is transcripted into messenger RNA, it carries the protein code out of the
nucleus. There, in the endoplasmic reticulum, the mRNA comes in contact with
ribosomes. As this ribosome moves along the mRNA, it translates the bases into a
specific amino acid chain (polypeptide chain), which forms the protein.
Proteins and RNA: The Cell’s Products
A protein is a folded-up polypeptide chain. The longer the chain, the more complex
shapes it can form. This shape is important to the function that the protein serves.
Misfolded proteins can result in more than 100 nervous system diseases.
RNA is produced from genes in neurons. A neuron contains as many as 20.000 genes
that produce RNAs, which get translated into proteins.
Golgi bodies and Microtubules: Protein Packaging and Shipment
The wrapping of a protein in a membrane and labelling takes place in the Golgi bodies.
They are then loaded onto motor molecules that move along microtubules radiating
through the cell.
If a protein is meant to stay in the cell, it is released into the intracellular fluid. If it is to
be in the membrane, it is carried there. If it is to be exported from the cell, it is usually
transferred out through exocytosis, where the membrane in which the protein is
wrapped fuses with the cell membrane, causing the protein to be excreted into the
extracellular fluid.
Crossing the Cell Membrane: Channels, Gates, and Pumps
Some membrane proteins form channels, through which substances can pass. These
channels are designed by size to accommodate only certain substances.
Other membrane proteins form gates that regulate the passage of substances. Gates
change their chape in response to some trigger. These can open and close to regulate
substances entering or leaving the cell.
Changes in a protein’s shape can allow it to act as a pump, that can actively carry
substances across a cell’s membrane.
