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uitgebreide Samenvatting hoofdstuk 9 silverthorn human physiology
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Fleur sam. Ch. 9Neurons in the nervous system link together to form circuits that have specific functions.
Most complex circuits are in brain.
Computers cannot yet accurately model brain function bc computers lack plasticity, the
ability to change circuit connections and function in response to sensory input and past
experience.
Linking neurons together create affective behaviour, which are related to feeling and
emotion, and cognitive behaviours related to thinking.
All animals have ability to sense and respond to changes in their environment.
Every single-cell organism such as paramecium are able to carry out the basic tasks of life:
finding food, avoiding becoming food, finding a mate. But have no brain/integrating centre.
They use the resting membrane potential that exists in living cells and many of the same ion
channels are more complex animals to coordinate their daily activities.
Reflexes that do not require integration in the brain also occur in higher animals and are
called spinal reflexes in humans and other vertebrates.
In humans, the cerebrum is largest and most distinctive part of the brain, with deep grooves
and folds. The cerebrum is what makes us human. It allows reasoning and cognition.
The cerebellum, a region of the hindbrain devoted to coordinating movement and balance.
The cerebellum, like the cerebrum, is readily identifiable by grooves and folds.
In all vertebrates, the CNS consists of layers of neural tissue surrounding a fluid-filled central
cavity lined with epithelium.
In very early embryo, cells that will become nervous system lie in flattened region called
neural plate. As development proceeds (at day 20 of human development) neural plate cells
along the edge migrate toward the midline. By about day 23 of human development, the
neural plate cells have fused with each other, creating a neural tube.
Neural crest cells form the lateral edges of the neural plate now lie dorsal to the neural tube.
The lumen of neural tube will remain hollow and become the central cavity of the CNS.
Cells lining the neural tube will either differentiate into the epithelial ependyma or remain as
undifferentiated neural stem cells. The outer layers of the neural tube will become the
neurons and glia of the CNS.
Neural crest cells will become the sensory & motor neurons of the PNS.
By week 4 of human development, the anterior portion of the neural tube has begun to
specialize into the regions of the brain. 3 obvious divisions: forebrain, midbrain & hindbrain.
,Tube posterior to hindbrain will become the spinal cord. At this stage, region of forebrain
that will become cerebrum is not much larger than the other regions of the brain.
As development proceeds, the growth of the cerebrum begins to outspace that of other
regions.
By week 6, the CNS has formed the 7 major divisions present at birth, 6 regions of the brain:
1. Cerebrum
2. Diencephalon
3. Midbrain
4. Cerebellum
5. Pons
6. Medulla oblongata
7. Spinal cord.
Cerebrum & diencephalon develop from the forebrain.
Cerebellum, pons, medulla oblongata are divisions of the hindbrain.
By week 6, the central cavity (lumen) of the neural tube has begun to enlarge into the hollow
ventricles of the brain.
There are 2 lateral ventricles (1st & 2nd) & 2 descending ventricles (the 3rd & 4th).
The central cavity of the neural tube also becomes the central canal of the spinal cord.
By week 11, the cerebrum is noticeably enlarged. And at birth, the cerebrum is the largest
and most obvious structure seen in human brain.
The fully developed cerebrum surrounds the diencephalon, midbrain, pons, leaving only the
cerebellum and medulla oblongata visible below it.
Because of the flexion (bending) of the neural tube early in development, some directional
terms have different meaning when applied to the brain (fig. 9.2g).
The CNS, like the PNS is composed of neurons & supportive glial cells.
Interneurons are those neurons completely contained within the CNS. Sensory (afferent) &
efferent neurons link interneurons to peripheral receptors & effectors.
The tissues of the CNS are divided into gray & white matter.
- Gray matter consists of unmyelinated nerve cell bodies, dendrites, and axons. The
cell bodies are assembled in an organized fashion in both the brain & spinal cord.
They form layers in some parts of the brain and in other parts cluster into groups of
neurons that have similar functions. Clusters of cell bodies in the brain & spinal cord
are known as nuclei. Nuclei are identified by special names, bv. Lateral geniculate
nucleus, where visual information is processed.
- White matter is mostly myelinated axons and contains very few neuronal cell bodies.
Pale colour comes from the myelin sheaths that surround the axons. Bundles of
axons that connect different regions of the CNS are known as tracts. Tracts in the CNS
are equivalent to nerves in the PNS.
The consistency of the brain/spinal cord is soft and jellylike. Although individual
neurons/glial cells have highly organized internal cytoskeletons that maintain cell shape and
orientation, neural tissue has minimal ECM, and must rely on external support for protection
, from trauma. Support comes in the form of outer casing of bone, 3 layers of connective
tissue membrane & fluid between the membranes.
In vertebrates, the brain is encased in a bony skull/ cranium, and the spinal cord runs
through a canal in the vertebral column. The body segmentation that is characteristic of
many invertebrates can still be seen in the bony vertebrae, which are stacked on top of one
another and separated by disks of connective tissue.
Nerves of the PNS enter and leave the spinal cord by passing through notches between the
stacked vertebrae.
3 layers of membrane, collectively called the meninges, lie between the bones and tissue of
the CNS.
These membranes help stabilize the neural tissue and protect it from bruising against the
bones of the skeleton.
From bone towards neural tissue, the membranes are:
1. Dura mater, thickest one- associated with veins that drain blood from the brain
through vessels or cavities called sinuses.
2. Arachnoid, middle layer is loosely tied to inner membrane, leaving a subarachnoid
space between the 2 layers.
3. Pia mater, inner membrane, is thin membrane that adheres to surface of brain and
spinal cord. Arteries that supply blood to the brain are associated with this layer.
ECF is final protective component of CNS, which helps cushion the delicate neural tissue.
Cranium has internal volume of 1.4L, of which about 1L is occupied by cells. Remaining
volume is divided into 2 distinct extracellular compartments:
- Blood (100-150mL)
- Cerebrospinal fluid, found in the ventricles and in the space between pia mater and
arachnoid membrane.
- Interstitial fluid (250-300 mL), lies inside the pia mater.
Cerebrospinal fluid + interstitial fluid = extracellular environment for neurons.
The cerebrospinal & interstitial fluid compartments communicate with each other across the
leaky junctions of the pial membrane and ependymal cell layer lining the ventricles.
Cerebrospinal fluid (CSF) is salty solution that is continuously secreted by choroid plexus,
specialized region on walls of ventricles.
Choroid plexus is remarkably similar to kidney tissue & consists of capillaries & transporting
epithelium derived from the ependyma. Choroid plexus cells selectively pump sodium and
other solutes form plasma into the ventricles, creating osmotic gradient that draws water
along with the solutes.
From the ventricles, cerebrospinal fluid flows into the subarachnoid space between the pia
mater & the arachnoid membrane, surrounding the entire brain and spinal cord in fluid.
Cerebrospinal fluid flows around the neural tissue and is finally absorbed back into the blood
by special villi on the arachnoid membrane in the cranium.
Rate of fluid flow through the CNS is sufficient to replenish the entire volume of
cerebrospinal fluid 3 times a day.
Cerebrospinal fluid has 2 purposes:
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