Transcript The Nervous System

The Nervous System
By.Brett Bennell
And
Lisa Matzelle
Neuron Structure
• Three classes of neurons:sensory neurons, interneurons, and
motor neurons.
• Sensory neuron takes messages from sensory receptors to
the CNS.
• Interneuron lies in the CNS.
• Motor neuron takes messages away from the CNS.
• Neurons all vary in appearance, but have three of the same
parts which are:a cell body, dendrites, and an axon.
• The cell body contains the nucleus, as well as other
organelles.
• An axon conducts nerve impulses away from the cell body
toward other neurons or target structures.
• Dendrites are extensions from a cell body that receive signals
from other neurons and send them on to the cell body.
Myelin Sheath
• Myelin Sheath is formed by a type of neuroglia called
Schwann cells, which contain the lipid substance myelin in
their plasma membranes.
• A Myelin sheath develops when Schwann cells wrap
themselves around an axon many times.
• Gaps where there are no myelin sheath are called nodes of
Ranvier.
• Myelin gives fibres their white, glistening appearance and
serves as an excellent insulator.
• It also helps accelerate nerve impulses.
• Only long axons tend to have a myelin sheath.
• These axons carry messages from one part of the nervous
system to another.
Resting Potential
• Resting potential is the polarity across the plasma
membrane of a resting neuron due to and unequal
distribution of ions.
• The unequal distribution of these ions is due to the action of
the sodium-potassium pump.
• The pump is always working because the membrane is
somewhat permeable to these ions, and they tend to diffuse
toward their lesser concentration.
• The sodium-potassium pump, makes a greater concentration
of Na+ outside an axon and a greater concentration of K+
inside an axon.
Action Potential
• An action potential is a rapid change in polarity across an
axomembrane as the nerve impulse occurs.
• Action potential is an all-or-none phenomenon.
• If a stimulus causes the axomembrane to depolarize to a
certain level, which is called a threshold, an action potential
occurs.
• The action potential requires two types of gated channel
proteins in the membrane.
• One gated channel protein opens to allow Na+ to pass
through the membrane, and another opens to allow K+ to
pass through the membrane.
Synaptic Integration
• A single neuron has many dendrites plus the cell body, and
both can have synapses with many other neurons.
• Integration is the summing up of inhibitory and excitatory
signals received by a postsynaptic neuron.
• If a neuron receives both inhibitory and excitatory signals,
the summing up of these signals may prohibit the axon from
firing.
• Excitatory signals have a depolarizing effect, and inhibitory
signals have a hyperpolarizing effect.
Neurotransmitter Molecules
• There have been at least 25 different neurotransmitters that
have been identified.
• Two very well-known ones are acetylcholine (Ach) and
norepinephrine (NE).
• Once a neurotransmitter has been released into a synaptic
cleft and has initiated a response, it is removed from the
cleft.
• The short existence of neurotransmitters at a synapse
prevents continuous stimulation (or inhibition) of
postsynaptic membranes.
• Many drugs that affect the nervous system act by interfering
with or potentiating (enhancing) the action of
neurotransmitters.
• Drugs can enhance or block the release of a
neurotransmitter.
Central Nervous System
• The spinal cord and the brain make up the central nervous
system (CNS), where sensory information is received and
motor control is initiated.
• Bone, and cerebrospinal fluid is used to protect both the
spinal cord and the brain.
• The CNS is composed of two types of nervous tissue—gray
matter and white matter.
• The gray matter is gray because it contains cell bodies and
short, nonmyelinated fibers.
• The white matter is white because it contains myelinated
axons that run together in bundles called tracts.
Spinal Cord
• The spinal cord extends from the case of the brain through a
large opening in the skull called the foramen magnum and
into the vertebral canal formed by openings in the vertebral.
• The spinal cord serves as a means of communication
between the brain and much of the body.
• The spinal cord is also a center for reflex actions.
• Spinal nerves project form the cord between the vertebrae
that make up the vertebrae column.
• A cross section of the spinal cord shows a central canal, gray
matter, and white matter.
• Gray matter is centrally located and shaped like the letter H.
The Brain
• The human brain has been called the last great frontier of
biology.
• Modern neurosciences modern goal is to understand the
structure and function of the brain’s various parts so well
that it will be possible to prevent or correct the thousands of
mental disorders that rob humans beings of a normal life.
• The brain has four ventricles called, in turn, the two lateral
ventricles, the third ventricle, and the fourth ventricle.
• The cerebrum is associated with the two lateral ventricles,
the diencephalons with the third ventricle, and the brain
stem and the cerebellum with the fourth ventricle.
Diagrams of the Brain
The Cerebrum
• The cerebrum is also known as the telencephalon, is the
largest portion of the brain in humans.
• The cerebrum is the last center to receive sensory input and
carry out integration before commanding voluntary motor
responses.
• Cerebrum has two halves.
• The cerebrum carries out higher thought processes required
for learning and memory and for language and speech.
• Shallow grooves called sulci divide each hemisphere into
lobes.
The Cerebral Cortex
• The cerebral cortex is a thin but highly convoluted outer
layer of gray matter that covers the cerebral hemisphere.
• It contains over one billion cell bodies and is the region of
the brain that accounts for sensation, voluntary movement,
and all the thought processes we associate with
consciousness.
• Voluntary commands to skeletal muscles begin in the
primary motor area.
• The premotor area organizes motor functions for skilled
motor activities.
• Broca’s area is a motor speech area in the lledt frontal lobe.
The Diencephalon
• The hypothalamus and the thalamus are in the
diencephalon.
• The hypothalamus is an integrating center that helps
maintain homeostasis by regulating hunger, sleep, thirst,
body temperature, and water balance.
• The hypothalamus serves as a link between the nervous and
endocrine system.
• The thalamus is on the receiving end for all sensory input
except smell, visual, auditory, and somatosensory
information arrives at the thalamus via the cranial nerves
and tracts form the spinal cord.
• The pineal gland, which secretes the hormone melatonin is
also located in the diencephalon.
The Cerebellum
• The cerebellum is separated from the brain stem.
• It has two portions that are joined by a narrow median
portion.
• The cerebellum receives sensory input from the eyes, ears,
joints, and muscles about the present position of body parts,
and it also receives motor output form the cerebral cortex
about where these parts are located.
• It also insures that all of the muscles work together to
produce smooth, coordinating voluntary movements.
• The cerebellum also assists with the learning of a new skill.
The Brain Stem
• The brain stem contains the midbrain, the pons, and the
medulla oblongata.
• The midbrain acts as a relay station for tracts passing
between the cerebrum and the spinal cord or cerebellum.
• It also has a reflex centers for visual, auditory, and tactile
responses.
• The pons functions with the medulla oblongata to regulate
breathing rate and has reflex centers concerned with head
movements in response to visual and auditory stimuli.
• The medulla oblongata contains a number of reflex centers
for regulation heartbeat, breathing, and vasconstriction,
vomiting, coughing, sneezing, hiccupping, and swallowing.
• The medulla oblongata lies just superior of the spinal cord.
Limbic System
• The limbic system is a complex network of tracts and nuclei
that incorporates medial portions of the cerebral lobes, the
basal nuclei, and the diencephalon.
• It blends primitive emotions an higher mental functions into
a united whole.
• Two significant structures within the limbic system are the
hippocampus and the amygdala, which are essential for
learning and memory.
• The hippocampus is well situated in the brain to make the
prefrontal area aware of past experiences stored in
association areas.
• The amygdala , in particular, can cause these experiences to
have emotional overtones.
• The prefrontal area consults the hippocampus in order to
use memories to modify our behavior.
Memory and Learning
• Memory is the ability to holds a thought in mind or to recall
events from the past, ranging from a word we learned only
yesterday to an early emotional experience that has shaped
our lives.
• Learning takes place when we retain and utilize past
memories.
• Short term memory lies just dorsal to our forehead.
• Long term memory is like remembering lots of phone
numbers.
• Long term memory can be a mixture of semantic memory
(numbers, words, etc) and episodic memory (persons, events,
etc).
Language and Speech
• Language is dependent upon memory; special areas in the
left hemisphere help account for our ability to comprehend
and use speech.
• People with speech disorders have been known for some
time that damage to a motor speech area called Broca’s area
results in an inability to speak.
• The right hemisphere is more global in its approach to
things as in the left hemisphere it is more specific.
The Peripheral Nervous System
• The peripheral nervous system lies outside the central
nervous system, and is composed of nerves and ganglia.
• Sensory fibers carry information to the CNS, and motor
fibers carry information away from the CNS.
• Ganglia are swellings associated with nerves that contain
collections of cell bodies.
• Humans of 12 pairs of cranial nerves attached to the brain.
• The vagus nerve has branches not onluy to the pharynx and
larynx, but also to most of the internal organs.
• The spinal nerves of humans emerge in 31 pairs form either
side of the spinal cord.
• In the PNS, cranial nerves take impulses to and from the
brain, and spinal nerves take impulses to and from the
spinal cord.
Somatic System
• The PNS is divided into the somatic system and the
autonomic system.
• The somatic system serves the skin, skeletal muscles, and
tendons.
• Some actions in the somatic system are due to reflexes,
autonomic responses to a stimulus.
• A reflex occurs, quickly, without out even having to think
about it.
• Some other actions that are voluntary, originate in the
cerebral cortex, as when we decide to move a limb.
Autonomic System
• The autonomic system of the PNS regulates the activity of
cardiac and smooth muscle and glands.
• The system is divided into the sympathetic and
parasympathetic divisions.
• These two divisions have several features in common:1)they
function automatically and usually in an involuntary
manner; 2)they innervate all internal organs; and 3)they
utilize two neurons and one ganglion for each impulse.
• Reflex actions, such as those that regulate blood pressure
and breathing rate, are especially important to the
maintenance of homeostasis.
Sympathetic Division
• Most preganglionic fibers of the sympathetic division arise
form the middle, or thoracic-lumbar, portion of the spinal
cord and almost immediately terminate in ganglia that lie
near the spinal cord.
• The sympathetic division is the most important during
emergency situations.
• The sympathetic division brings about those repsonses we
associate with “fight or flight”.
Parasympathetic Division
• The parasympathetic division includes a few cranial nerves
as well as fibers that arise form the sacral (bottom) portion
of the spinal cord.
• The preganglionic fiber is long, and the postganglionic fiber
is short because the ganglia lie near or within the organ.
• This division promotes all the internal responses we
associate with a relaxed state.
• The neurotransmitter utilized by the parasympathetic
division is acetylcholine (Ach).
Drug Abuse
• A wide variety of drugs affect nervous system and cal alter
the mood and /or emotional state.
• Such drugs have two general effects:1)they impact the
limbic system, and 2)they either promote or decrease the
action of a particular neurotransmitter.
• Drug abuse is apparent when a person takes a drug at a dose
level and under circumstances that increase the potential for
a harmful effect.
• Taking drugs that affect the nervous system lead to physical
dependence and withdrawal symptoms.
A Small List of Some Harmful
Drugs
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Alcohol
Nicotine
Cocaine
Heroin
Marijuana
All these drugs listed above have two things in common they
are very addictive, and they can be very damaging to the
brain if there is constant use with them.
THE END
By. Brett Bennell and Lisa Matzelle