MSC IV_Forensic medicine - Mechanical injuries.pdf
20200829190340_PPT1 Chemical and Biology
1. Week 1
Introduction: Understanding human consciousness:
consciousness:
A physiological approach and Structure and function of cells of
function of cells of the nervous system
SCIE6057 –Chemistry & Biology
3. Introduction of Human Brain
• Introduction: Understanding human consciousness: A
physiological approach
-. The phenomenon of consciousness:
> Blindsight
> Split brains
> Unilateral neglect
4. Human Brain
• The human brain is a wrinkled, walnut-shaped hunk of tissue
weighing about 1.3 kilograms.
• The human brain is an amazingly intricate network of neurons
(cells that receive and transmit electrochemical signals). There are
100 billion neurons in complex array, the estimated 100 trillion
connections among them, and the almost infinite number of paths
that neural signals can follow through this morass
• The largest part of the brain consists of two symmetrical
parts, called the cerebral hemispheres, which receives
sensory information from the opposite sides.
• The corpus callosum permits the two hemisphere to share
information so that each side knows what the other side is
perceiving and doing.
5.
6. Human Consciousness
• The term consciousness can be used to refer to a variety of
concepts, including simple wakefulness. Thus, a researcher may
write about an experiment using “conscious rats”, referring to the
fact that the rats were awake and not anesthetized. However, the
word consciousness to refer to the fact that humans are aware
of−and can tell others about−our thoughts, perceptions, memories,
and feelings.
• We know that consciousness can be altered by changes in the
structure or chemistry of the brain; therefore we may hypothesize
that consciousness is a physiological function, just like behavior.
• Verbal communication makes cooperation possible and permits us
to establish customs and law of behavior. perhaps the evolution of
this ability is what has given rise to the phenomenon of
consciousness.
7. The Phenomenon Of Consciousness
• Blindsight is the ability of a person who cannot see objects in his
or her blind field to accurately reach for them while remaining
unconscious of perceiving them; caused by damage to the
“mammalian” visual system of the brain.
8. The Phenomenon Of Consciousness
• Split brains
− Studies of humans who have undergone a particular
surgical procedure demonstrate dramatically how
disconnecting parts of the brain involved with perceptions
from parts that are involved with verbal behavior also
disconnects them from consciousness. This results suggest
that the parts of the brain involved in verbal behavior may
be the ones responsible for consciousness.
9. The split brain operation. A “window” has been opened in the side of the brain, the corpus
callosum being cut at the midline of the brain
10. -. The effects of cutting the corpus callosum (the split-brain operation)
reinforce the conclusion that we become conscious of something only
if information about it is able to reach the part of the brain responsible
for verbal communication, which are located in the left hemisphere. If
the information does not reach these parts of brain, then that
information does not reach consciousness.
11. Identification of an object in response to an olfactory stimulus by a person with a split brain
12. • Unilateral Neglect
− Unilateral (one-sided) neglect is a syndrome in which people
ignore objects located toward their left and the left sides of
objects located anywhere; most often caused by damage to the
right parietal lobe.
− The parietal lobe receives information directly from the skin, the
muscles, the joints, the internal organs, and the part of the inner
ear that is concerned position. But that is not all, the parietal
cortex receives auditory and visual information as well.
14. • Information, in the form of light, sound waves, odors, tastes, or contact with
objects, is gathered from the environment by specialized cells called sensory
neurons. Movement are accomplished by the contraction of muscles, which are
controlled by motor neurons.
• In between sensory neurons and motor neurons come the interneurons ─
neuron that lie entirely within the central nervous system.
15. Three types of neurons. The brain and spinal cord form the central nervous
system (CNS) of vertebrates, and sensory and motor neurons form the peripheral
nervous system (PNS). Sensory neurons of the peripheral nervous system carry
information about the environment to the CNS. Interneurons in the CNS provide
links between sensory and motor neurons. Motor neurons of the PNS system carry
impulses or “commands” to muscles and glands (effectors).
16.
17. Overview Of The Nervous System
The Major Divisions of the Nervous System
• The central nervous system (CNS) consists of the parts that
encased by bones of the skull and spinal column: the brain and the
spinal cord.
• The peripheral nervous system (PNS) is found outside these bones
and consists of the nerves and most of the sensory organs.
18. Neurons
• The neuron (nerve cell) is the information-
processing and information-transmitting element
system.
• Four structures or regions of neurons:
1. Cell body or soma
2. Dendrites
3. Axon
4. Terminal buttons.
See the interactive CD
19. • Soma. The soma (cell body) contains the nucleus
and much of the machinery that provides for the
cell.
20. • Dendrites. Dendron is the Greek word for tree,
and the dendrites of the neuron look very much
• Neurons “converse” with one another, and the
important recipient of this messages.
• The messages that pass from neuron to neuron
across the synapse, a junction between the
sending cell and a portion of the somatic and
of the receiving cell.
• Axon. The axon is long, slender tube, often
covered by a myelin sheath.
• The axon carries information from the cell body
to terminal buttons.
21. • Action potential is a brief electrical / chemical event that starts at
the end of the axon next to the cell body and travels toward the
terminal buttons.
• The action potential is like a brief pulse; in a given axon the action
potential is always of the same size and duration. When it reaches
a point where the axon branches, it split but does not diminish in
size. Each branch receives a full-strength action potential.
• Terminal buttons. The bud at the end of a branch of an axon;
forms synapses with another neuron; sends information to that
neuron.
22. • Three principal types of neurons are classified according
to the way in which their axons and dendrites leave the soma:
Multipolar neuron
Bipolar neuron
Unipolar neuron
23. Supporting Cells
• Neurons constitute only about half the volume of the Central
Nervous System / CNS.
• The rest consists of a variety of supporting cells
• Glia. The most important supporting cells of the CNS are the
neuroglia, or “nerve glue.”
• Glia (also called glial cells) do indeed glue the CNS together, but
do much more than that.
• Neurons lead a very sheltered existence; they are buffered
physically and chemically from the rest of the body by the glial
cells.
24. • Glial cells surround neurons and hold them in place, controlling
their supply of nutrients and some of the chemicals they need to
exchange messages with other neurons; they insulate neurons
from one another so that neural messages do not get scrambled.
• Glial cells also act as housekeepers, destroying and removing the
carcass of neurons that are killed by disease or injury.
• The three most important types of glial cells are:
1. Astrocytes
2. Oligodendrocytes
3. Microglia
25. Astrocytes
• Astrocytes provide physical support to neurons and clean up debris
within the brain.
• They produce some chemicals that neurons need to fulfill their
function.
• Astrocytes help to control the chemical composition of the fluid
surrounding neurons by actively taking up or releasing substances
whose concentration must be kept within critical levels.
• Astrocytes are involved in providing nourishment to neurons.
26. Structure and Location of Astrocytes
The processes of astrocytes surround capillaries and neurons of the
• Besides transporting chemicals to the neurons, astrocytes
serves as the matrix that holds neurons in place−the “nerve
glue.”
• These cells also surround and isolate synapse, limiting the
dispersion of neurotransmitter that are released by the
terminal buttons.
27. • Besides transporting chemicals to the neurons, astrocytes serves
as the matrix that holds neurons in place−the “nerve glue.”
• These cells also surround and isolate synapse, limiting the
dispersion of neurotransmitter that are released by the terminal
buttons.
• Astrocytes phagocyte the dead neurons or debris.
28. Oligodendrocytes
• Oligodendrocytes provide support to axon and produce the
myelin sheath, which insulates most axon from another.
• Myelin, 80 % lipid, 20 % protein, is produced by
oligodendrocytes in the form of a tube surrounding axon.
• The bare portion of myelinated axon is called a node of
Ranvier.
• In the central nervous system, the oligodendrocytes support
axons and produce myelin. In peripheral nervous system, the
Schwann cells perform the same functions.
29. Microglia
• Microglia are the smallest of the glial cells.
• Like some of astrocytes, the act as phagocytes, engulfing and
breaking down dead and dying neurons.
• In addition, the serve as one the representatives of immune system
in brain, protecting the brain from invading microorganism.
• They are primarily responsible for the inflammatory reaction in
response to brain damage.
30. Blood-Brain Barrier
• Blood Brain Barrier : a semipermeable barrier between the blood and the brain
produced by the cells in the walls of the brain’s capillaries.
• The brain receives a copious supply of blood and chemically guarded by the blood-
brain barrier.
• The blood receives approximately 20 percent of the blood flow from the heart and it
receives continuously.
The blood-brain barrier. (a) The cells that
form the walls of the capillaries in the body
outside the brain have gaps that permit the
free passage of substances into and out of
the bloods. (b) the cell that form the walls
of the capillaries in the brain are tightly
joined.
31. • Other parts of the body, such as skeletal muscles and digestive
system, receive varying quantities of blood, depending on their
needs, relative to those other regions.
• The brain always receives its share. The brain cannot store it fuel
(primarily glucose), nor can it temporarily extract energy without
oxygen, as the muscles can.
• A region of the medulla where the blood-brain barrier is weak;
poisons can be detected and can initiate vomiting area postrema.
33. • Structure and function of cells of nervous system 2:
-. Communication within a neuron:
> Resting membrane potential
> The Conduction of action potential
-. Communication between neurons:
> The structure of synapse
> The release of neurotransmitter
34. • The giant axon of the squid can be 100 to 1000 times larger than a
mammalian axon. The giant axon innervates the squid's mantle
muscle. These muscles are used to propel the squid through the
water.
• Much of what we know about how neurons work comes from
experiments on the giant axon of the squid.
• How giant is this axon? It can be up to 1 mm in diameter - easy to
see with the naked eye.
35. Communication within a neuron
Resting Membrane Potential
•The membrane potential is the difference in
the inside and the outside of a cell.
•When both electrode tips are in the extracellular
difference between them is zero. However, when the
intracellular electrode is inserted into a neuron,
about 70mV is recorded. This steady membrane
mV is called the neuron’s resting potential.
36. Resting potential
• When a neuron is not sending a signal, it is "at rest.“ When a neuron is at
rest, the inside of the neuron is negative relative to the outside. At rest,
potassium ions (K+) can cross through the membrane easily. Also at rest,
chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing.
The negatively charged protein molecules (A-) inside the neuron cannot cross
the membrane.
37. Action Potential
• The resting potential tells about what happens when a neuron is at
rest.
• An action potential occurs when a neuron sends information down
an axon, away from the cell body.
• The action potential is an explosion of electrical activity that is
created by a depolarizing current.
(Animation 2.2, The Action Potential)
38. • This means that some event (a stimulus) causes the resting
potential to move toward 0 mV. When the depolarization reaches
about -55 mV a neuron will fire an action potential.This is the
threshold.
• If the neuron does not reach this critical threshold level, then no
action potential will fire.
• When the threshold level is reached, an action potential of a fixed
sized will always fire, for any given neuron, the size of the action
potential is always the same size.
39. • The action potential is a rapid change in the membrane potential of
a muscle cells or nerve cells. Where the action potential is
characterized by the sudden change from normal resting
membrane potential (resting potential) becomes positive
membrane potential (depolarization) and then ended with nearly
the same speed back to the negative membrane potential
(repolarization). Changes in the electrical potential caused by
changes in electrolyte concentration inside and outside the cell.
The main electrolytes which contribute to the potential difference
between the inside with the outside of the excitable cell membrane
is sodium (Na +), potassium (K +) and Chloride (Cl).
40. Conduction of the Action Potential
• Action potentials are conducted without decreasing in amplitude,
so the last action potential at the end of an axon is just as large as
the first action potential.
• How are action potentials produced and how are they conducted
along the axon? The answer to both questions is basically the
same: though the action of voltage-activated ion channels-ion
channels that open or close in response to changes in the level of
the membrane potential.
41. • There is a brief period of about 1 to 2 milliseconds after the initiation
of an action potential during which it is impossible to elicit a second
one. This period is called absolute refractory period. The absolute
refractory period is followed by the relative refractory period, i.e. the
period during which it is possible to fire the neuron again, but only
by applying higher-than-normal levels of stimulation.
• The experiment of conduction of the action potential establishes a
basic law of axonal conduction; the all-or-none law.
• This law states that an action potential either occurs or does not
occur; and once triggered, it is transmitted down the axon to its
end. An action potential always remains the same size, without
growing or diminishing.
42. • The action potential is an all-or-none events, the action potential is
not the basic element of information; rather, variable information is
represented by an axon’s rate of firing.
• A high rate of firing causes a strong muscular contraction, and a
strong stimulus causes a high rate of firing in axon that serve the
eyes. Thus, the all-or-none law is supplemented by the rate law.
• Action potentials are not the only kind of electrical signals that
occur in neurons.
• Conduction of an action potential in a myelinated axon is
somewhat different from conduction in an unmyelinated axon.
• In the myelinated area there can be no inward flow of Na+ when
the sodium channels open, because there is no extracellular
43. • Myelinated axons conduct impulses more rapidly than
unmyelinated axons because the axon potentials in myelinated
axons are only produced at the nodes of Ranvier.
• One action potential still serves as the depolarization stimulus for
the next, but the depolarization at one node spreads quickly
beneath the insulating myelin to trigger opening of voltage-gated
channels at the next node in a process called saltatory conduction.
44. Communication between neurons
• The primary means of communication between neurons is synaptic
transmission – the transmission of message from one neuron to
another through a synapse.
• The messages are carried out by neurotransmitters, released by
terminal buttons.
45. Synapse Structure
• Synapses can occur in three places: on dendrites, on the soma, and
on other axons. These synapses are referred to as axodendritic,
axosomatic, and axoaxonic.
Axodendritic synapses can occur on the smooth surface of dendrite
or on dendritic spines-small protrusions that stud the dendrites of
several types of large neurons in the brain.
46. Types of synapses. Axodendritic synases can accur on the smooth
surface of a dendrite (a) or on dendrite spines (b) Axosomatic synapses
occur on somatic membrane (c) Axoaxonic synapses consist of
synapses between two terminal buttons (d).
48. Release of Neurotransmitter
• How does an action potential cause synaptic vesicles to release
the neurotransmitter? The process begins when a population of
synaptic vesicles become “docked” against the presynaptic
membrane, ready to release their neurotransmitter into the synaptic
cleft.
• Docking is accomplished when clusters of protein molecules attach
to other protein molecules located in the presynaptic membrane.
(Animation 2.3, Synapses)
49. Release of Neurotransmitter
An action potential opens calcium channels, which enter and bind with the protein
embedded in the membrane of synaptic vesicles docked at the release zone. The
fusion pores open, and the neurotransmitter is released into the synaptic cleft.
