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uitgebreide Samenvatting hoofdstuk 8 silverthorn human physiology
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Fleur sam ch 8Chapter 8.
The brain and spinal cord are the integrating
center for movement, homeostasis, and many
other body functions. They are the control center
of the nervous system, which is a network of a
billion of nerve cells linked together in a highly
organized manner to form the rapid control
system of the body.
Neurons carry electrical signal rapidly and over
long distances, they have long, thin extensions
called processes that can extend to a meter in
length. In most pathways. Neurons release chemical signals, neurotransmitters,
into the ECF to communicate with neighbouring cells, but they can also be linked by
gap junctions, allowing electrical signals to pass directly from cell to cell.
Using electrical signals to release chemicals from a cell is not unique to neurons. Bv.
Pancreatic beta cells generate an electrical signal to initiate exocytosis of insulin-
containing storage vesicles. Single-celled protozoa and plants also employ similar
electrical signal mechanisms.
Although electrical signalling is universal, sophisticated neural networks are unique to
animal nervous systems. Reflex pathways in the nervous system do not follow a
straight line from 1 neuron to the next, 1 neuron may influence multiple neurons, or
many neurons may affect the function of a single neuron.
The intricacy of neural networks and their neuronal components underlie the
emergent properties of the nervous system. Emergent properties= complex
processes, like consciousness, intelligence, emotion that cannot be predicted from
what we know about properties of individual nerve cells and their specific
connections.
8.1 organization of the nervous system
The nervous system can be divided into two parts:
Central nervous system (CNS): brain and the spinal cord
Peripheral nervous system (PNS): consists of the sensory (afferent) and efferent
neurons.
Information through the nervous system follows in a basic pattern of a reflex:
Stimulus sensor input signal integrating center output signal target
response.
If a response is needed, the CNS sends output signals that travel through efferent
neurons to their targets, mostly muscles and glands.
Efferent neurons are subdivided into two parts:
the somatic division: which controls skeletal muscles
autonomic division: which controls smooth and cardiac muscles, exocrine glands
and some types of adipose tissue.
o Sympathetic (stress)
o Parasympathetic (relax)
,Many internal organs receive innervation from both types of autonomic neurons,
often they have antagonistic control over a single target.
‘’motor neuron’’ can be referring to all efferent neurons, but clinically its used to
describe somatic motor neurons that control skeletal muscles.
Autonomic division of the PNS is called the visceral nervous system, bc it controls
contraction and secretion in various internal organs.
In recent years, a third division received attention: the enteric nervous system, the
network of neurons in the walls of the digestive tract. It is controlled by the autonomic
part of the nervous system, but also is able to function autonomously as its own
integrating center.
CNS can initiate activity without sensory input, bv. When you decide to do something.
The CNS does not need to create any measurable output to the efferent division. Bv.
Thinking/dreaming are complex higher brain functions that can take place totally
within the CNS.
8.2 cells of the nervous system
The nervous system is composed primarily of 2 cell types:
- Neurons; basic signaling units of the nervous system.
- Glial cells: support cells
The neuron is the functional unit (is the smallest structure that can carry out the
functions of a system) of the nervous system.
Neurons are uniquely shaped cells with long processes that extend outward from the
nerve cell body. These processes are usually classified as either dendrites (receive
incoming signals) or axons (carry outgoing information).
Shape, number, length of axons and dendrites vary from 1 neuron to the next, but
these structures are an essential feature that allows neurons to communicate with
one another and with other cells.
Neurons may be classified either structurally or functionally:
- Structurally, neurons are classified by the number of processes that originate from the
cell body (See image). Neuron is often shown as multipolar, with many dendrites and
branched axons.
o Multipolar neurons in the CNS look different from multipolar efferent neurons.
o In other structural neuron types, the axons and dendrites may be
missing/modified.
o Pseudo unipolar neurons have the cell body located off one side of a single
long process that is called the axon (during development the dendrites fuse
and became part of axon).
o Bipolar neurons have a single axon and single dendrite coming off the cell
body.
o Anaxonic neurons lack an identifiable axon but have numerous branched
dendrites.
- Functionally classified:
, o Sensory (afferent) neurons: carry info on temp, pressure, light, other stimuli
from sensory receptors to the CNS. Peripheral sensory neurons are
pseudounipolar, with cell bodies located close to the CNS and long processes
that extend out to receptors in the limbs and internal organs. In these sensory
neurons, the cell body is out of the direct path of signals passing along the
axon. Sensory neurons in the nose and eye are much smaller bipolar neurons.
Signals that begin at the dendrites travel through the cell body to the axon.
o Interneurons (interconnecting neurons): neurons that lie entirely within the
CNS. Come in variety of forms but often have quite complex branching
processes that allow them to communicate with many other neurons. Can be
small compared to model neuron.
o Efferent (somatic motor and autonomic) neurons, axons may divide several
times into branches called collaterals. Efferent neurons have enlarged endings
called axon terminals. Many autonomic neurons also have enlarged regions
called varicosities. Both axon terminals and varicosities can store and
release neurotransmitters.
The axons of both afferent and efferent peripheral neurons are bundled together in
nerves that extend from the CNS to the targets of the component neurons.
Nerves that carry only afferent signals are called sensory nerves, efferent are called
motor nerves.
Nerves that can go in both directions are called mixed
nerves.
Many nerves are large enough to be seen with naked eye
bv. Phrenic nerve runs from spinal cord to muscle of
diaphragm.
The cell body of a neuron is like a typical cell, with a
nucleus and all organelles needed to direct cellular
activity.
An extensive cytoskeleton extends outwards to the
dendrites and axons.
The position of the cell body varies in different types of
neurons, but in most neurons the cell body is small,
making up 1/10th or less of the total cell volume. Despite
small size, cell body with its nucleus is essential to the
cell bc it contains DNA that’s template for protein
synthesis.
Dendrites are thin, branched processes that receive the incoming information from
neighboring cells. They increase the surface area of the neuron, allowing it to receive
communication from multiple other neurons. Simplest neurons have only 1 single
dendrite. Neurons in brain have multiple dendrites with incredibly complex branching.
Dendrite’s surface area can be expanded even more by the presence of dendritic
spines that vary from thin spikes to mushroom-shaped knobs.
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