Jasmine's Biology Blog : Nervous System

Nervous system is complex network of nervous tissue that sends electrical and chemical signals

includes central nervous system (CNS) and peripheral nervous system (PNS) together
CNS made of brain and spinal cord
PNS made of nervous tissue that lies outside of CNS

nerves in the legs, arms, hands, feet, and organs of the body

nervous system mediates communication between different parts of body

Nerve cells

two main types of nerve cells in nervous tissue
neuron

"conducting" cell that transmits electrical signals, and structural unit of nervous system

glial cell

provides a support system for neurons and are involved in synapse formation
astrocytes: type of glial cell in brain, important for maturation of neurons and involved in repairing damaged nervous tissue

neurons and glial cells make up most of the brain, spinal cord, and nerves that branch out to every part of the body
both neurons and glial cells are referred to as nerve cells

Structure of a neuron

neuron has a special shape

allows it to pass electrical signal to another neuron and other cells

electric signals move rapidly along neurons so that they can pass "messages" from one part of the body to another

called nerve impulses

neurons made of cell body (soma), dendrites, and axon
cell body contains nucleus and other organelles
dendrites extend from cell body and receive nerve impulses from another cell
cell body collects information from dendrites and passes it to an axon
axon is a long, membrane-bound extension of the cell body

passes nerve impulse onto next cell

end of axon is called axon terminal

point where the neuron communicates with the next cell

summary

dendrites receive information
cell body gathers it
axons pass information to another cell

myelin sheath

axons of man neurons are covered with this insulating phospholipid layer
speeds up transmission of a nerve impulse
outgrowth of glial cells

Schwann cells

type of glial cell
flat and thin, containing a nucleus and other organelles

oligodendrocytes

supply myelin to those of the brain or spinal cord
myelinated neurons are white, and makes up "white matter" in brain

myelin is not continuous along axon
regularly spaced gaps are called Nodes of Ranvier

only points at which ions can move across axon membrane through ion channels
nodes act to strengthen nerve impulse by concentrating the flow of ions at nodes of Ranvier along axon

neurons are specialized for passing of cell signals
many different shapes and sizes
synapse: specialized junction at which neurons communicate with each other
a neuron can have one or many axons

longest axon of human motor neuron can be over a meter long, reaching rom base of spine to toes

sensory neurons have axons that run over 1.5 meters in adults

Ion channels and nerve impulses

ion transport proteins have special role in nervous systems

voltage-gated ion channels and ion pumps are essential for forming nerve impulse

uses energy to build and maintain a concentration gradient between extracellular fluid and cell's cytosol

concentration gradient results in net negative charge on inside of membrane and a positive charge on the outside.

ion channels and ion pumps only allow certain ions through the cell membrane

potassium channels will only allow potassium ions through
sodium-potassium pumps acts only on sodium and potassium ions

all cells have electrical charge, due to concentration gradient of ions that exist across membrane
number of positively charged ions outside of membrane is greater that number of positively charged ions in cytosol
this charge difference causes voltage difference across membrane
voltage: electrical potential energy caused by separation of opposite charges

across the membrane in this case

voltage across membrane is called membrane potential

basis of conduction of nerve impulses along cell membrane of neurons

ions that are important in formation of nerve impulse includes sodium (Na+) and potassium (K+)

Resting potential

when a neuron is not conducting nerve impulse, it is at rest
resting potential: resting state of neuron, during which neuron has an overall negative charge
in neurons resting potential is approximately -70 mV
reasons for overall negative charge:

sodium potassium pump removes Na+ ions from cell by active transport
net negative charge inside cell is because of higher concentration of Na+ ions outside of cell than inside of cell
most cells have potassium-selective ion channel proteins that remain open all the time
K+ ions move down concentration gradient (passive) through potassium channels and out of cell
results in a build-up of excess positive charge outside the cell
number of large, negatively charged molecules (proteins) inside the cell

Action potential

electric charge that travels along membrane of a neuron
generated when neuron's membrane potential is changed by chemical signals from nearby cell
cell membrane potential changes quickly from negative to positive as sodium ions flow in and potassium ions flow out of cell through ion channels
cell becomes depolarized
action potential works on all or nothing basis
membrane potential has to reach a certain level of depolarization (threshold), or an action potential will not start

threshold varies (~15 mV)

if membrane depolarization does not reach threshold, then an action potential will not happen
first channels to open are sodium-ion channels, allowing sodium ions to enter the cell
increases positive charge in cell
starts up action potential
potassium-ion channels close, and sodium-potassium pump restores resting potential of -70 mV
action potential will move down axon towards synapse like a wave
in myelinated neuron, ion flows occur only at nodes of Ranvier
action potential signal "jumps" along axon membrane, from node to node, rather than all along the membrane
due to clustering of Na+ and K+ ion channels at Nodes of Ranvier

Types of neurons

neurons are specialized for processing and transmission of cellular signals
can be classified by structure of function
strucural classification is based on number of dendrites and axons that a cell has
functional classification is based on direction in which nerve impulse is moving in relation to CNS
sensory neurons

carry signals from tissues and organs to CNS
are sometimes called afferent neurons
typically have long dendrite and short axon
found in reflex arcs
involved in several forms of involuntary behavior, like pain avoidance (reflex)

motor neurons

carry signals from CNs to muscles and glands
are sometimes called efferent neurons
long axon and short dendrites

interneurons

connect sensory and motor neurons in pathways that go through CNS
called association or relay neurons
found only in CNS where they connect neuron to neuron

Communication between neurons

communicate with each other at specialized junctions

called synapses

also found at junctions between neurons and other cells
two types of synapses
chemical synapses: uses chemical signaling molecules as messengers
electrical synapses: uses ions as messengers
synaptic cleft

gap between axon terminal and receiving cell

transmitting cell is called presynaptic neuron
receiving cell called postsynaptic cell

or, if it is another neuron, postsynaptic neuron

brain has a large amount of synapses
approx. one trillion neurons (including glial cells) have an average of 7,000 synaptic connections to other neurons
brain of a three year old child has about 10 quadrillion synapses

number declines with age
adult has between 1-5 quadrillion synapses

Neurotransmitter release

when action potential reaches axon terminal, it causes neurotransmitter vesicles to fuse with terminal membrane
neurotransmitter is released into synaptic cleft
neurotransmitter

chemical message used to relay electrical signals between neuron and another cell
neurotransmitter molecules are made inside of presynaptic neuron and stores in vesicles at axon terminal
some neurons only make one type of neurotransmitter, but most neurons make two or more kinds of neurotransmitters

when action potential reaches axon terminal, neurotransmitter vesicles are caused to fuse with terminal membrane
neurotransmitter is released into synaptic cleft
then diffuse across synaptic cleft and bind to receptor proteins on membrane of postsynaptic cell

Neurotransmitter action

neurotransmitters can have excitatory or inhibitory effect on postsynaptic cell
excitatory neurotransmitter

initiates action potential

inhibitory neurotransmitter

prevents an action potential from starting

glutamate is more common excitatory transmitter in body
GABA and glycine are inhibitory neurotransmitters
release of excitatory neurotransmitter causes inflow of positively charged sodium ions into postsynaptic neuron
inflow of positive charge causes depolarization of membrane
depolarization spreads to rest of postsynaptic neuron
effect of neurotransmitter also depends on receptor it binds to

a single neurotransmitter may be excitatory to receiving neuron, or it may inhibit an impulse by causing a change in membrane potential

synapses can also be excitatory or inhibitory and will either increase or decrease activity in target neuron

depends on opening or closing of ion channels

neurotransmitter receptors can be gated ion channels that open or close through neurotransmitter binding
can also be protein-linked receptors

not ion channels
cause a signal transduction that involves enzymes and other molecules in postsynaptic cell

Neurotransmitter reuptake

reuptake

removal of neurotransmitter from synapse by pre-synaptic neuron
happens after neurotransmitter has transmitted nerve impulse
without reuptake, neurotransmitter molecules might continue to inhibit or stimulate and action potential

reuptake is carried out by transporter proteins that bind to released transmitter and actively transports it across plasma membrane into pre-synaptic neuron
reuptake is target of some types of medicine
is a form of recycling because neuron takes back released neurotransmitter for later use
another way neurotransmitter is removed from a synapse: digestion of enzyme

Neurotransmitters and diseases

Parkinson's disease

deficiency of neurotransmitter dopamine
progressive dead of brain cells give this deficit, causing tremors and a stiff, unstable posture
L-dopa is given as a medicine that eases symptoms of Parkinson's disease
acts as a substitute neurotransmitter, but cannot reverse disease

tetanus

Clostridium tetani produces neurotoxin
bacteria usually gets into body through injury cause by object contaminated with C. tetani spores
blocks release of neurotransmitter GABA

GABA causes skeletal muscles to relax after contraction

when release of GABA is blocked, muscle tissue does not relax and remains contracted
can be fatal when it affects muscles used in breathing
tetanus is treatable and can be prevented with vaccination

botulism

caused by Clostridium botulinum
produces toxin that is occasionally found in preserved foods that were improperly sterilized
butolin toxin blocks release of excitatory neurotransmitter acetylcholine
blockage of acetylcholine causes progressive relaxation of muscles because they are unable to contract
paralysis of muscles used for breathing can be fatal unless treated with a respirator

Central nervous system

includes the brain and the spinal cord
largest part of the nervous system
brain is central control of nervous system
spinal cord carries nerve impulse from brain to body and from body to brain
together with PNS, it controls every activity in the body
brain is protected by skull
spinal cord protected by vertebrae

The brain

most complex organ in the body
contains about 100 billion neurons

can be connected to tens of thousands of other neurons in the brain

source of what makes us human: conscious mind
mind is set of cognitive processes related to perception, interpretation, imagination, memories, and language
regulates processes related to homeostasis (respiration and heartbeat, etc)
average human adult brain weights 1-1.5 kg
brain uses about 20-25% of total energy used by adult body

in infants, uses about 60% of total energy

cerebrum

controls conscious functions

problem-solving and speech

midbrain and stem are more involved with unconscious (automatic) functions

breathing, heartbeat, and temperature regulation

cerebrum involved in coordination and control of body movement

Cerebrum (in detail)

what most people would think of as the "brain"
lies on top of the brainstem
made of two cerebral hemispheres

connected to each other at corpus callosum
callosum is a wide, flat bundle of axons found deep inside the brain
mammals have the largest and most well-developed cerebrum of all species

each hemisphere can be divided into four parts (lobes)
frontal lobe, parietal lobe, temporal lobe, and occipital lobe
both hemispheres look identical

functional differences between them

each cerebral hemisphere receives sensory information

also controls muscle movements of opposite side of body

cerebral cortex

highly-folded outer layer of cerebrum
2-4mm thick
lobes that make up cerebral cortex are named after skull bones that cover those areas of brain
many folds in cortex allows large surface area of brain to fit into skull
controls higher functions: consciousness, reasoning, emotions, and language
also controls sensory functions: touch, taste, smell, and responses to external stimuli

white matter

in the cerebrum and found below the cerebral cortex
made up of myelinated axons that act as "cables" to link up certain parts of the right and left hemispheres

Diencephalon

region of brain that includes structures such as thalamus, hypothalamus, and portion of pituitary gland
thalamus believed to "translate" sensory signals for cerebral cortex

also plays important role in regulating states of sleep and wakefulness

hypothalamus controls certain metabolic processes and other autonomic activities

body temperature, hunger, thirst, and circadian cycles
makes and releases neurotransmitters that control action of pituitary gland

thalamus, hypothalamus, and hippocampus together are called limbic system

"emotional center" of brain

Brain stem

sometimes called the "lower brain"
lower part of brain that joined to the spinal cord
three parts: midbrain, pons, and medulla oblongata
midbrain

more involved with unconscious, autonomic functions
deals with several types of sensory information (sound and sight)
"translates" sensory information to be sent to forebrain
helps coordinate large body movements like walking and running

pons

relays messages to different parts of the brain (cerebrum and cerebellum)
helps regulate breathing
might have role in dreaming

medulla oblongata

also called medulla
shares some function of pons
controls several homeostasis functions such as breathing, heart and blood vessel activity, swallowing, and digestion

brain stem is information highway
all information coming from body to brain and information from cerebrum to body go through brain stem
sensory pathways for pain, temperature, touch, and pressure sensation go upward to cerebrum
motor pathways for movement and other body processes go downward to spinal cord
most axons in motor pathway cross from one side of CNS to the other, passing through medulla oblongata
right side of brain controls movement of left side of body
left side of brain controls movement of right side of body

Cerebellum

found just below occipital lobe of cerebrum
plays important role in coordination and control of body movements
many nerve pathways link cerebellum with motor neurons

neurons that send information to muscles, causing them to move
group of nerves that provides information on the position of body in space

cerebellum processes information from both pathways
uses feedback on body position to fine tune body movements

hand-eye coordination is a fine-tuned body movement

if cerebellum is damaged, there will be no paralysis, but fine movement of the body will be negatively affected

Spinal cord

thin, tubular bundle of nervous tissue
extends from medulla oblongata and continues to lower back
ends in group of fibrous extensions
protected by spinal vertebrae
main function of spinal cord is an information superhighway that links sensory messages from body to brain
outer cortex of cord contains white matter
central region, grey matter, is made up of un-myelinated neurons

Peripheral nervous system

consists of nervous tissue that lies outside the central nervous system
nervous tissue of PNS serves limbs and organs
CNS interacts with peripheral nervous system through twelve pairs of cranial nerves
connects brain to areas of head and neck and
31 pairs of spinal nerves connect the spinal cord to rest of the body (such as internal organs, arms, and legs
nerve: an enclosed, cable-like bundle of axons
peripheral nervous system is not protected by bone, making it more vulnerable to toxins and injuries
spinal nerves originate from spinal cord
control functions of the rest of the body
each spinal nerve has a dorsal root and ventral root
dorsal root

nerve highway that carries sensory information from sensory receptors in body to CNS

ventral root

contains axons of motor neurons which carry information away from CNS to muscles and glands of body

dorsal and ventral root "highways" are parts of two subdivisions of PNS
sensory division

carries sensory information from sensory receptors in body to CNS
keeps CNS constantly updated on events happening inside and outside the body

motor division

also called efferent division
carries nerve impulses from CNS to muscles, glands, and organs of the body
nerve impulses causes muscles to contract and glands to secrete chemical signals

Somatic and autonomic nervous systems

motor division of PNS is divided into somatic nervous system and autonomic nervous system
somatic nervous system

part of PNS that is associated with conscious control of body through movement of skeletal muscles
also through perception of external stimuli through senses
includes all neurons connected with muscles, skin, and sense organs
made up of sensory nerves that receive sensory information from external environment
motor nerves responsible for muscle contraction
sensory, interneurons, and motor neurons are found in reflex arc
reflex: automatic (involuntary) action caused by defined stimulus and carried out through reflex arc
ex: person stepping on sharp object would start reflex action through creation of stimulus

stimulus would be passed along sensory neurons to spinal cord
usually process by interneuron to create immediate response by initiating a motor response by pulling the food away from the object
reflexive action would occur as pain sensation is arriving in brain

autonomic nervous system (ANS)

part of PNS that maintains homeostasis in body
body carries out these activities without conscious control
sometimes also called involuntary nervous system
ANS made up of sensory and motor neurons that send messages to and from internal organs
neurons form reflex arcs that pass through medulla oblongata
low level brain functions will continue to function, called vegetative state

ANS subdivisions
sympathetic division

stimulates body systems during emergency situations
gets body ready for "fight or flight"

parasympathetic division

controls non-emergency functions like digestion

Sense organs and sensory perception

senses are body's means of making sense of information your nervous system receives
enables you to adapt to change in environment and survive
sensory division is organized into developed sense organs

groups of tissues that work together in responding to a specific kind of physical stimulus

sense organs correspond to defined region within the brain where nerve signals are received and interpreted
sense organs include: eyes, ears, nose, mouth, and skin
all have sensory receptors that are specific for certain stimuli
chemoreceptors respond to stimuli
mechanoreceptors respond to mechanical stress or strain
thermoreceptors respond to temperature changes
photoreceptors respond to variations in light
baroreceptors respond to pressure
specific areas of brain interpret information from each sense organ

generally agreed that humans have at least seven different senses
sight, sound, taste, smell, touch, balance, and body awareness

Sight

sight (vision) describes ability of brain and eye to detect wavelengths of electromagnetic radiation

interprets image as "sight"

different receptors are responsible for perception of color or brightness
photoreceptors are found in retina
structure of eye focuses completely on the task of focusing light onto retina

light-sensitive inner layer of eye

light passes through clear protective layer called cornea
light then passes through the pupil, and into the interior of the eye

pupil is opening to the iris

then travels to lens

transparent, biconvex structure that helps to focus light on retina

muscles attached to lens change the shape of the lens to bend light rays so that they focus on retina
light hitting retina causes chemical changes in photosensitive cells of retina
retina has two forms of photosensitive cells: rods and cones
rod cells

highly sensitive to light, letting them respond in dim light and dark conditions
cannot detect color
the darker conditions become, the less color things seem to have

cone cells

responds to different wavelengths of bright light to initiate nerve impulse
responsible for sharpness of images
do not respond well in poor light conditions
three different kinds of cone cells
contains pigment that absorbs energy from different wavelengths of light to initiate nerve impulse
activation of visual pigments opens ion channels on membrane of cone or rod cell
leads to action potential, carried by millions of neuron axons, that make up optic nerve to visual centers of the brain
brain integrates nerve impulses from cone cells and perceives world in all the colors of the visual spectrum

Hearing

sense of sound perception that results from movement of tiny hair fibers in inner ear
hairs detect motion of membrane which vibrates in response to changes in air pressure
can also be detected as vibrations that are conducted through body
sound wave frequencies that are too low or too high are detected as vibrations
pinna

folds of cartilage surrounding outer ear canal

sound waves gathered by pinna and channeled down auditory canal

tube-shaped opening of ear that ends at tympanic membrane, or eardrum

sound waves traveling through ear canal hits eardrum, causing it to vibrate
wave information travels across middle air cavity to the three, tiny delicate bones: hammer, anvil and stirrup
transfers eardrum vibrations to another membrane called oval window

separates middle ear from inner ear
inner ear contains cochlea

cochlea

coiled tube that is filled with watery liquid
moves in response to vibrations coming from middle ear through oval window
hair cells (mechanoreceptors) bend, releasing neurotransmitter
causes and action potential in neurons of auditory nerve
travels along auditory nerve to structures in brainstem
loud noise can kill hair cells
common cause of partial hearing loss
hairs never grow back once they are destroyed

Balance and the ears

ears are also in charge of sense of balance
semicircular canals are three fluid-filled, interconnected tubes found inside each ear
canal filled with fluid called endolymph

cilia lines each canal

movement of head and body cause endolymph in canals to move about
hair cells sense strength and direction of fluid's movement, and sends electrical signals to cerebellum

interprets information and responds to help keep body's sense of balance

when sense of balance is interrupted, it causes dizziness and nausea
balance can be upset by inner ear infection, or a number of other medical conditions
can be temporarily disturbed by rapid and repetitive movement

Taste

four types of taste receptors on tongue
taste stimuli sends information to different region of the brain
detects sweet, salt, sour, and bitter
umami, a fifth receptor, was confirmed in 2000

detects amino acid glutamate, which causes a savory, "meaty" flavor in foods

chemoreceptors of mouth are taste cells found in bundles called taste buds
most are embedded within tiny papillae, or bumps, that cover the tongue
each receptor has a different way of detecting certain compounds and starting action potential, alerting the brain
compounds bind to receptors in taste cells and stimulate neurons in taste buds
action potential moves along facial nerves to thalamus, then to taste center of cerebral cortex for interpretation by the brain
tongue can feel temperature, coolness (minty), spiciness, and fattiness (greasy)

Smell

other "chemical" sense
chemoreceptors of smell are called olfactory receptors
40 million olfactory receptor neurons line the nasal passages
different molecules bind to and excite specific olfactory receptors
combination of excitatory signals makes up what we identify as "smell"
signals from receptors travel along nerves to olfactory bulb in the brain

moves to smell center in frontal lobe of cerebral cortex
olfactory neurons in nose differ fro most other neurons
die and regenerate on regular basis

sense of taste and smell are closely linked
nasal cavity connects to mouth at back of throat
olfactory receptors and taste receptors both contribute to the flavor of food

Touch, pressure, and pain

sense of pressure perception, generally felt in skin
variety of pressure receptors that respond to variations in pressure and tension
mechanoreceptors most numerous on tongue, lips, face, palms, and soles of feet
nociceptors

respond to potentially damaging stimuli
mostly found in external parts of body: skin, cornea, and mucous membranes
also found in muscles, joints, and some internal organs
classified according to stimuli to which they respond

thermal, mechanical, or chemical

thermal receptors: activated by potentially harmful heat or cold
mechanical receptors: respond to excess pressure, squeezing, or bending

Drugs and the nervous system

drugs

andy chemical or biological substance that affects body's structure of functions
can be used to treat many illnesses or disorders

medicine

drug that is taken to cure or reduce symptoms of an illness

drugs can be abused for the effects they have on the CNS
psychoactive drug

substance that affects CNS by altering cognitive function
results in change of how a person feels, thinks, perceives, and acts
coffee or tea contains psychoactive drug caffeine
psychoactive drugs affect how neurons communicate with each other
can alter neurotransmission by blocking receptor protein
some effects are beneficial, like taking a prescribed painkiller to ease pain of broken bone
some are harmful, like if a person takes a strong painkiller long after their broken bone

Drug abuse

repeated use of drug without advice or guidance of medical professional

used for reasons other than what drug was originally intended for
if a person uses the drug continuously, they may find they cannot function normally without the drug
called physical dependence
psychological dependence: emotionally or mentally needing a drug to be able to function normally
eventually need to take larger doses of drug to get desired effect, known as building a tolerance to the drug

person who is abusing a drug may eventually lose control of drug-taking behavior, and resort to stealing and lying to get money or drugs

addiction
drug addict's life revolves around getting more drugs to feed their habit

some people can lead to drug overdose if they keep on increasing the dose

generally considered harmful and may lead to death

drug dependence and addiction are caused by changes in the way neurons in the CNS send and receive neurotransmitters
dependency and addiction are treated as brain disorders by medical professionals
stimulants

cocaine, nicotine, amphetamine
increases activity of sympathetic nervous system, central nervous system, or both
generally increase heart rate, blood pressure, and increase sense of alertness
caffeine are used medicinally to increase or maintain alertness, and to counteract fatigue
high doses can be fatal

hypnotics

depressants
alcohol, codeine, barbiturates, and benzodiazepines
decrease activity of central nervous system
slows down brain function and gives drowsy or calm feeling
taking too much can lead to slow breathing and slow heart rates, and can lead to death
depressants increases activity of inhibitory neurotransmitters GABA
promotes sleep
slows down brain function and causes drowsy and calm feeling
generally prescribed to relieve symptoms of anxiety or insomnia

hallucinogens

also known as psychedelic drugs
do not increase or decrease a certain feeling or emotion
induce experiences, such as sensory distortions and "out of body experiences"
different from those of ordinary consciousness
often called trance-like states
has been linked to potential for brain damage

drugs that increase activity on particular neurotransmitters are called agonists
drugs that reduce neurotransmitter activity are called antagonists

work by interfering withs synthesis
also blocks postsynaptic receptors so neurotransmitters cannot bind to them

different drugs affect different parts of the brain

How addiction happens

neurobiological theory of addiction proposes that certain chemical pathways are changed in the brain of an addicted person
most abused drugs affect brain structures in the limbic system

called the brain rewards system
provides feeling of pleasure that motivates person to perform certain activities over and over again
dopamine released at synapses by neurons when person has a pleasurable experience
this mechanisms has evolved to ensure survival of organisms

some drugs (cocaine, nicotine, amphetamines, and alcohol) increase the amount of dopamine in the limbic structure

tricks body into thinking that drug is good and important for survival

drugs that affect brain reward system are highly addictive

nicotine is highly addictive

many psychoactive substances are abused for their mood and perception altering effects
drugs that have no medical uses and high potential for abuse are usually illegal
not all drugs are physically addictive

Jasmine's Biology Blog

Wednesday, April 9, 2014

Nervous System

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