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Brain

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Brain
Your brain is an essential organ that controls many body functions. Your brain
receives and interprets all the sensory information you encounter, like sights,
sounds, smells and tastes. Your brain has many complex parts that work together to
help you function.
What are the main parts of the brain?
Your brain’s structure is complex. It has three main sections:

Cerebrum: Your cerebrum interprets sights, sounds and touches. It also
regulates emotions, reasoning and learning. Your cerebrum makes up about
80% of your brain.

Cerebellum: Your cerebellum maintains your balance, posture, coordination
and fine motor skills. It's located in the back of your brain.

Brainstem: Your brainstem regulates many automatic body functions. You
don’t consciously control these functions, like your heart rate, breathing,
sleep and wake cycles, and swallowing. Your brainstem is in the lower part of
your brain. It connects the rest of your brain to your spinal cord.
What are the lobes that make up your brain?
Each side of your brain has different lobes (sections). While all the lobes work
together to ensure normal functioning, each lobe plays an important role in some
specific brain and body functions:

Frontal lobes: The frontal lobes are in the front part of your brain, right
behind your forehead. This is the largest lobe and it controls voluntary
movement, speech and intellect. The parts of your frontal lobes that control
movement are called the primary motor cortex or precentral gyrus. The parts
of your brain that play an important role in memory, intelligence and
personality include your prefrontal cortex as well as many other regions of
your brain.

Occipital lobes: These lobes in the back of your brain allow you to notice and
interpret visual information. Your occipital lobes control how you process
shapes, colors and movement.

Parietal lobes: The parietal lobes are near the center of your brain. They
receive and interpret signals from other parts of your brain. This part of your
brain integrates many sensory inputs so that you can understand your
environment and the state of your body. This part of your brain helps give
meaning to what's going on in your environment.

Temporal lobes: These parts of the brain are near your ears on each side of
your brain. The temporal lobes are important in being able to recall words or
places that you've been. It also helps you recognize people, understand
language and interpret other people’s emotions.

Limbic lobes: The limbic lobe sits deep in the middle portions of your brain.
The limbic lobe is a part of your temporal, parietal and frontal lobes.
Important parts of your limbic system include your amygdala (best known for
regulating your “fight or flight” response) and your hippocampus (where you
store short-term memories).

Insular lobes: The insular lobes sit deep in the temporal, parietal and frontal
lobes. The insular lobe is involved in the processing of many sensory inputs
including sensory and motor inputs, autonomic inputs, pain perception,
perceiving what is heard and overall body perception (the perception of your
environment).
What other parts of the brain send and receive signals?
Although most brain cells reside on the surface of your brain (called gray matter)
and the cabling (white matter) is deep and connects various parts of your brain,
there are some nuclei (collection of brain cells) that reside deep in your brain. They
include:
Thalamus: Your thalamus is a structure residing deep in your cerebrum and above
your brainstem. This structure is sometimes referred to as the switchboard of the
central nervous system. It relays various sensory information, like sight, sound or
touch, to your cerebral cortex from the rest of your body.
Hypothalamus: Your hypothalamus sits below your thalamus. It's important in
regulating various hormonal functions, autonomic function, hunger, thirst and sleep.
Your hypothalamus and pituitary gland are important structures involved in the
control of your hormonal system.
Pituitary gland: Your pituitary gland sends out hormones to different organs in your
body.
Basal ganglia: Your basal ganglia are a group of nuclei deep in your cerebrum that is
important in the control of your movement, including motor learning and planning.
Brainstem nuclei: There are a number of nuclei situated in your brainstem involved
in a variety of different functions including cells that give rise to a number of
important cranial nerves, normal sleep function, autonomic functions (breathing and
heart rate) and pain.
Reticular formation: Your reticular formation is a part of your brainstem and
thalamic nuclei. These are a part of your reticular activating system (nuclei plus the
white matter connecting these nuclei), which lies in your brainstem, hypothalamus
and thalamus. The reticular activating system (RAS) mediates your level of
awareness, consciousness and focus. They also help control your sleep-wake
transitions and autonomic function.
Parts of a Neuron and Their Function
Neurons or nerve cells are the basic building blocks or units of the nervous system.
Nearly 86 billion neurons work co-ordinately within the nervous system to keep the
body organized. They are highly specialized cells that act as information processing
and transmitting units of the brain. A group of neurons forms a nerve.
Although they have a characteristic elongated shape, they vary widely in size and
properties based on their location and type of functions they perform.
While they have the common features of a typical cell, they are structurally and
functionally unique from other cells in many ways. All neurons have three main
parts: 1) dendrites, 2) cell body or soma, and 3) axons. Besides the three major
parts, there is the presence of axon terminal and synapse at the end of the neuron.
Dendrites
They are specialized extensions that resemble the branch of a tree. Dendrites help
to increase the surface area available for connections with the adjacent neurons and
thus in receiving incoming signals from them. Some neurons have very small and short
dendrites, while in others, they are very long. Most neurons have numerous
dendrites, while a few others have only one of them.
Functions

Acquiring chemical impulse from other cells and neurons

Converting the chemical signals into electrical impulses

Carrying electrical impulses towards the next part of the neuron, the cell body
Cell Body or Soma
It is the core of the neuron, similar to a cell that contains the nucleus and all other
cellular organelles. The cell body is also the largest part of a neuron enclosed by
a cell membrane that protects the cell from its immediate surroundings and allows
its interaction with the outside environment. They attach to all the dendrites and
thus integrate all the signals. All metabolic activities of the cell take place in the
cell body, which also contains DNA, the genetic material of the neuron.
Functions

Supporting and organizing the functions of the whole neuron

Joining the signals received by the dendrites and passing them to the axons,
the next part of the neuron
Axons
They are fine, elongated fiber-like extensions of the nerve cell membrane. Axons
run from the cell body of one neuron until the terminal of the next neuron. They are
the longest part of the neuron that varies widely in length, with some are as tiny as
0.1 millimeters, while some others can be over 3 feet. The larger the diameter of
the axon, the faster is the rate of transmission of nerve signals.
Sometimes, a single axon is highly branched to allow better communication with
multiple target neurons at the same time.
Parts of an Axon
a) Axon hillock – The part of the axon which remains attached to the cell body or
soma.
b) Myelin sheath – The layer of fatty acid produced from specialized cells called
Schwann cells that are wrapped around the axon.
c) Nodes of Ranvier – The gaps between the discontinuous myelin sheath that is
running along the axon.
Functions

Axons help to receive signals from other neurons and transmit the outflow of
the message to the adjacent connected neurons and also to other muscles and
glands by changing the electrical potential of the cell membrane called
the action potential.

Myelin sheath insulates the axon and thus prevents shock similar to an
insulated electric wire

Myelin sheath also increases the speed of the flow of signals through the axon

Nodes of Ranvier allows diffusion of ions in and out of the neuron and thus
maintains the electrical potential of the neuron
Apart from all the major parts described so far in a neuron, there are other
structures (that are not basic parts) found at the junctions between the neurons
that help to establish functional links or connections between them. They are
described below.
Connecting Parts of a Neuron: Axon Terminal and Synapse
Axon terminal also called synaptic bouton or terminal button is the terminal branches
of the axon located at the very end of the neuron. They are farthest from the soma
and contain chemical messengers called neurotransmitters in specialized structures
called synaptic vesicles.
Synapse or synaptic cleft is the small space or gap between two adjacent neurons.
It is formed between the axon terminal of one neuron and the dendrites of the next
neurons.
Functions

Releasing of neurotransmitters through specific transport vesicles, called
synaptic vesicles from one neuron to the adjacent connected neurons
called exocytosis

Synaptic vesicles of one neuron for conducting nerve impulse to the adjacent
connected neuron through exocytosis

Sending neuronal information from one nerve cell to another and also to other
cells of the muscle or gland with the help of neurotransmitters

Re-up taking of excessive neurotransmitters from the synaptic cleft
References:
Overview of neuron structure and function – Khanacademy.org
Neurons
–
Courses.lumenlearning.com/
https://www.khanacademy.org/science/biology/human-biology/neuron-nervoussystem/v/anatomy-of-a-neuron
An Overview of the Different Parts of a Neuron – Verywellmind.com
What is a neuron? – Qbi.uq.edu.au
Spinal Cord
The spinal cord is a long, tube-like band of tissue. It connects your brain to your
lower back. Your spinal cord carries nerve signals from your brain to your body and
vice versa. These nerve signals help you feel sensations and move your body. Any
damage to your spinal cord can affect your movement or function.
The spinal cord is a continuation of the brainstem. It extends from the foramen
magnum at the base of the skull to the L1/L2 vertebra where it terminates as
the conus medullaris (medullary cone). A thin thread called filum terminale extends
from the tip of the conus medullaris all the way to the 1st coccygeal vertebra (Co1)
and anchors the spinal cord in place.
You can easy remember the extent of the spinal cord with a mnemonic 'SCULL',
which stands for ' Spinal Cord Until L2 (LL)'.
Throughout its length, the spinal cord shows two well defined enlargements to
accommodate for innervation of the upper and lower limbs: one at the cervical level
(upper limbs), and one at the lumbosacral level (lower limbs).
Like the vertebral column, the spinal cord is divided into segments: cervical, thoracic,
lumbar, sacral, and coccygeal. Each segment of the spinal cord provides several pairs
of spinal nerves, which exit from vertebral canal through the intervertebral
foramina. There are 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1
coccygeal pair of spinal nerves (a total of 31 pairs).
Cross section
The spinal cord is made of gray and white matter just like other parts of the CNS.
It shows four surfaces: anterior, posterior, and two lateral. They feature fissures
(anterior) and sulci (anterolateral, posterolateral, and posterior).
Spinal meninges
The spinal cord and spinal nerve roots are wrapped within three layers
called meninges. The outermost is the dura mater, underneath it is the arachnoid
mater, and the deepest is the pia mater. Dura mater has two layers (periosteal and
meningeal), between which is the epidural space. Between the arachnoid and pia
mater is the subarachnoid space, it is filled with cerebrospinal fluid.
Blood supply
The spinal cord is supplied by branches of the vertebral and segmental arteries.
The vertebral artery gives rise to anterior and posterior spinal arteries. Segmental
arteries, such as the deep cervical, ascending cervical, and posterior intercostal give
rise to 31 pairs of radicular arterial branches which supply the roots of spinal nerves.
Similar named veins accompany the arteries. Anterior and posterior spinal veins drain
into radicular veins, which then empty into the (internal and external) vertebral
venous plexus. This network eventually empties into the vertebral (neck) and
segmental (trunk) veins. Blood supply is always an inevitable part of any anatomy
study unit.
Spinal cord tracts
Spinal cord neural pathways are found within the spinal cord white matter. On each
side, the white matter is divided into three funiculi: anterior, lateral, and posterior.
Ascending tracts convey information from the periphery to the brain. On the other
hand, the descending tracts carry information from the brain to the periphery. The
spinal cord is more than just a conduit, as it also modifies and integrates the
information that pass through it.
Spinal nerves
Spinal nerves are grouped as cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5),
sacral (S1-S5), and coccygeal (Co1), depending from which segment of the spinal cord
they extend.
Segmentation of the spinal cord corresponds to the intrauterine period in which the
spinal cord occupies the entire vertebral canal. For this reason in adulthood, where
the vertebral column is longer than the cord, each spinal cord segment is located
higher than its corresponding vertebra. These differences become more obvious
distally towards the lumbar and sacral segments of the spinal cord–for example
spinal cord segment L5 is at the level of the L1 vertebra.
Spinal nerves, however, exit the vertebral column at their correspondly numbered
vertebrae. Cervical spinal nerves exit through the intervertebral foramina directly
above their corresponding vertebrae, whilst thoracic, lumbar and sacral spinal nerves
exit directly below.
In order for the more distal spinal nerves to exit they must first descend through
the vertebral canal. Since the lumbar and sacral spinal nerves are the farthest from
their intervertebral foramina, they are the longest. While descending towards their
corresponding intervertebral foramina, lumbosacral spinal nerves form a bundle
called the cauda equina (meaning horse’s tail).
Each spinal nerve has an anterior and posterior root. Anterior roots transmit motor
information, and they originate from the anterior horns of the gray matter and exit
the spinal cord through the anterolateral sulcus. The posterior roots transmit
sensory information and have sensory ganglion attached to them. They originate
from the posterior horns of gray matter and exit through the posterolateral sulcus
of the spinal cord.
The anterior and posterior roots merge just before the intervertebral foramen, and
form the trunk of the spinal nerve. The trunk is very short, and soon after exiting
the vertebral column, it divides into four branches: anterior ramus, posterior ramus,
communicating ramus, and meningeal ramus.
Reflex arc
A huge part of spinal cord function is under the influence of the brain, as it functions
to relay information to and from the periphery. But there are many reflexes that
are generated in the spinal cord independently from the brain. Spinal reflexes are
either monosynaptic or polysynaptic.
Monosynaptic reflexes play out with only two neurons participating in the reflex arc,
one sensory and one motor. The first-order neuron (sensory) is in the spinal ganglion,
while the second-order neuron (motor) is in the anterior horn of the spinal cord).
The sensory neuron gathers impulses from the muscle and sends this information to
the motor neuron which innervates the same muscle. The motor neuron then causes
contraction of the innervated muscle. An example of a monosynaptic reflex is the
stretch reflex.
Polysynaptic reflexes on the other hand have multiple neurons participating. Besides
one sensory and one motor neuron, there are also one or more interneurons between
them making this communication indirect. They are more complex than monosynaptic
reflexes as they involve muscle groups instead of a single muscle. An example is the
withdrawal reflex.
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