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NERVOUS SYSTEM
NERVOUS SYSTEM
NERVOUS SYSTEM: TRUE OR FALSE?
HUMANS ONLY USE 10% OF THEIR BRAIN OR LESS False!
THE BRAIN USES 20% OF THE ENERGY WE CONSUME EACH DAY True!
MENTAL ABILITIES ARE SEPARATED INTO DISCRETE LEFT AND RIGHT “HALVES”
False!
Broad generalizations are often made in popular psychology about one side or the other having characteristic labels such as "logical" or "creative". These labels need to be treated carefully; although a lateral dominance is measurable, these characteristics are in fact existent in both sides,[1]
and experimental evidence provides little support for correlating the structural differences between the sides with functional differences.[2]
NEW NEURONS CANNOT BE CREATED False!
THE BRAIN MAKES UP ONLY 2% OF OUR BODY WEIGHT True!
THE BRAIN DOES NOT CHANGE AFTER GROWTH THAT OCCURS DURING ADOLESCENCE
False!
A NERVE IS A TYPE OF CELL False!
THE BRAIN USES THE MAJORITY OF THE OXYGEN WE BREATHE
False!
THE LONGEST CELLS IN THE HUMAN BODY ARE NEURONS
True! The sciatic nerve can be over a meter long!
THE HUMAN BRAIN IS FIRM AND GREY False! (well, sort
of… it’s about as firm as tofu!)
TYPES OF NERVOUS SYSTEM CELLS
Functions of the Nervous System:
WHAT IS THE NERVOUS SYSTEM? WHAT ARE THE FUNCTIONS OF THE NERVOUS SYSTEM? A system of cells, tissue, and organs that
regulate the body’s responses to external and internal stimuli.Communication between organ systems.Provides info about environmental conditions
to all internal organs.Translates environmental stimuli to
messages understood by the cells External stimuli: environmental factors that
influence metabolic changes in a cell or physiological changes in tissues and organs.
Internal stimuli: cell secretions used to communicate info about a cell’s jobs and needs.
NEURONS & NEUROGLIA Neural tube – developmental
struct Stem cells
Neurons & neurogliaNeurons:
excitable cells receive, interpret, and transmit
external and internal stimuli.Neuroglia:
maintain the excitability & health of neurons.
Don’t take part in communication.Supportive.
TYPES OF NERVOUS SYSTEM CELLS: NEURAL CREST CELLS
• Derived from the neural tube
• Bidirectional communication with neuroglia and neurons.
• Play a role in the development of the nervous system.
ANATOMY OF A NEURON Categorized by their cell anatomy and
mode of communication
ANATOMY OF A NEURON Common features:
Axon: Long process Extends from cell body from the axon hillock Transfers impulses to the terminus. Usually one per neuron. Some have branches (collaterals) that reach out to
other neurons. Job: initiate the electrical signal that will be
transmitted from the axon to glands, muscles, other neurons.
ANATOMY OF A NEURON CONT’D…Terminus: Releases neurotransmitters—transmit
info from one neuron to another. Cells must possess neurotransmitter
receptors if they are to respond to the stimulus
ANATOMY OF A NEURON
Cell body (soma): contains nucleus & organelles, ER and Golgi bodies that produce specialized enzymes and secretions needed for nerve cell communication.
Dendrite: antennae. Receive stimuli from several sources
Terminus: Releases
neurotransmitters
Axon: long process that comes off the body; transfers impulses to the terminus. Job: transmit electrical signal to glands, muscles, other neurons.
Axon Hillock: where the axon originates
SYNAPSE
Neurons don’t directly touch the cells with which they communicate
Form a synapse: the junction where an impulse is transmitted from one neuron to another.
ANATOMY OF A SYNAPSE
1. Synaptic Cleft: the space between the terminus of one neuron and the dendrites of another.
2. Pre-synaptic neuron: produces the neurotransmitter
3. Post-synaptic neuron: receives the neurotransmitter
4. Neurotransmitter: most pre-synaptic neurons produce 1 kind.
5. Receptor: Post-synaptic neurons an have a variety of neurotransmitter receptors.
SHAPES OF NEURONS TELL US ABOUT THEIR FUNCTIONS
NEUROGLIA AND STEM CELLS Make up bulk of cells in the nervous system Closely associated with neurons High lipid content
White in appearanceVulnerable to improper diet
Many types…
TYPES OF NEUROGLIA Astrocytes Ependymal cells Microglia OligodendrocytesNot pictured: Radial glia Satellite cells Schwann cells
ASTROCYTES A.k.a. macroglia Largest class Star-shaped, w/ many branches, or feet Often associate w/ blood vessels
Control types of materials that pass from blood to neurons
Protects neurons from harmful agentsCreates blood-brain barrierMostly found in brain, spinal cord
EPENDYMAL CELLS Primary secretory cells Line cavities of brain, spinal column Produce cerebrospinal fluid (CSF)
Bathes, nourishes, protects brain, spinal cord
Cilia help circulate CSF
MICROGLIA Highly variable Found throughout nervous system Many carry out phagocytosis,
removing infectious agents, repair damage
Others produce secretions that maintain neuron health, assist in healing
Malfunctions often produce disorders
OLIGODENDROCYTES Large, w/ numerous branching
processes Wrap around axons of neurons
Form an insulating cover (myelin sheath) Found only in brain, spinal cord Speeds up nerve transmission
RADIAL GLIA Found in developing nervous system Provide framework for growing
interconnections In adults, assist maintenance of brain
and eyesCommunicate “needs” of these cells
SATELLITE CELLS Small, numerous Cover surface of neurons outside brain,
spinal cord Help maintain chemical environment May help w/ nerve cell repair
SCHWANN CELLS Form myelin sheath around axons of
neurons outside of brain, spinal cord (in PNS)
Gaps between cells called nodes of RanvierHelp speed transmission
NEURON PHYSIOLOGY
Functions of the Nervous System:
REVIEW OF DIFFUSION Materials diffuse from high low concentration
Membranes act as a barrier to diffusion… they can be “selective” about what can pass
In general, things that are large/charged need special “permission” to pass through the membraneThey need a channel/gate that gives them a
pathway Ions, like Sodium (Na+), Potassium (K+), and
Chloride (Cl-) are normally not allowed through
OPPOSITES ATTRACT! In general, the following are true
about ions:They will repel each other (likes repel)They will be attracted to an opposite
charge
HOW DO NEURONS COMMUNICATE WITH ONE ANOTHER? Neurons are excitable! They transmit a signal that was received
by the dendrites/cell body down through the axon
Cytoplasm must be ready! Neurons transmit information to other
cells via an action potential
MEET THE PROTEINS!
Na+ gatedchannel
K+ gatedchannel
Na+/K+ Pump
K+ pore(leaks)
WHAT HAS TO HAPPEN TO A NEURON BEFORE IT GENERATES OR PROPAGATES AN ACTION POTENTIAL? Must maintain an excitable condition
called resting potential.Chemically unstable conditionSodium ion concentration higher outside cell
than inside Creates a diffusion potential; sodium
“wants” to enterPotassium ions higher inside cell than outsideA.k.a. a “salty banana”Sodium/potassium pump maintains this potential
Na+
RESTING PHASE = POLARIZED!
Na+Na
+Na+
Na+
Na+
Na+Na
+
Na+
Na+Na
+Na+
Na+
Na+
Na+Na
+
Na+ Na
+
Na+
Na+
Na+
K+
K+ K+K+K+ K+
K+K+
K+K+
K+
K+
K+
- PROTEIN -
- PROTEIN -
- PROTEIN -
- PROTEIN -
- PROTEIN -
- PROTEIN -
FIRST LOOK: ACTION POTENTIAL ANIMATION Animation
WHAT ARE THE FOUR STAGES OF AN ACTION POTENTIAL? Debatable… some have 6 phases
DepolarizationRepolarizationHyperpolarizationRecovery phase
PHASE 1: DEPOLARIZATION Cytoplasm’s charge starts at ~ -70 mV Dendrites receive stimulus from a.
another cell or b. the environment Sodium channels open, allowing rapid
influx If enough channels open, cytoplasm’s
charge reaches -55 mV = thresholdRequired for an action potential to
propagate, or travel, across the cell membrane
ALL-OR-NOTHING! At threshold, more Na+ channels open Charge of cytoplasm increases to +30
mV
Each depolarized segment of axon depolarizes the adjacent segment… like falling dominoes
SIDE NOTE… Potassium gated ion channels are
also stimulated to open during a depolarization!They are slower to respondThey don’t fully open enough to allow K+
ions to flow out until the sodium gates have both opened AND closed!
PHASE 2: REPOLARIZATION Sodium channels closed, and potassium
channels finally open K+ ions diffuse outward, causing the cell’s
interior to become more negative (lost + ions)
Neuron is becoming repolarized.
HYPERPOLARIZATION Repolarization is rapid! Cell moves past resting potential (-70
mv) and overshoots, reaching -90 mV.K+ gated ion channels are slow to close as
well…
This is hyperpolarization…
HYPERPOLARIZATION K+ gates on K+ channel proteins are
slow to close, allowing this hyperpolarization
Why does this occur?1. Prevents neuron from becoming
stimulated during repolarization period2. Prevents action potential from
travelling both forward AND backward… becomes a unidirectional signal.
= REFRACTORY PERIOD
RECOVERY PHASE Sodium/Potassium pumps return cell to
resting potential (Na+ outside, K+ inside)
Some cells send a second impulse before recovery is complete = tetany
SALTATORY CONDUCTION Action potentials relatively slow (5 25
m/second) To increase velocity, neurons’ axons are
myelinated. Reduces amount of membrane that must be
depolarizedStimulus “jumps” from node to node10 120 meters/second!- - - -
WORD CHALLENGE! Axon Cytoplasm Dendrite Depolarization Diffusion Potential Hyperpolarization Influx K+ gated ion
channels K+ ion Na+ gated ion
channels Na+ ion Na+/K+ pump Outflux Refractory period Repolarization
Working with your partner, write a “story” that describes an action potential.
NEURON-TO-NEURON COMMUNICATION When the terminus depolarizes, calcium
ions diffuse into terminusStimulates movement of vesicles towards
terminal knobs Vesicles fuse w/ cell membrane, releasing
contentsThese vesicles contain neurotransmitters
Neurotransmitters diffuse across synaptic cleft, binding to matching receptors on post-synaptic neuron
4 STAGES OF NEUROTRANSMITTER COMMUNICATION 1. Synthesis and storage of
neurotransmitters 2. Neurotransmitter release 3. Neurotransmitter binding to post-synaptic
receptors 4. Inactivation of neurotransmitters
Synthesis occurs in nerve cell body, transferred to terminus
Inactivation occurs by degradation or reuptake (for recycling)… many drugs affect these processes
NEUROTRANSMITTER SUMMARY
TWO TYPES OF NEUROTRANSMITTERS Chemical signals that transfer action
potential from affector (sensory neuron receptor) to an effector (motor neuron, muscle, gland)
Can be excitatory or inhibitory Excitatory: helps depolarize post-
synaptic neuron (move interior closer to threshold)
Inhibitory: hyperpolarize post-synaptic neuron (move interior farther from threshold)
MAJOR CLASSES OF NEUROTRANSMITTERS Amino acids:
Usually in brain, spinal columnAspartate, gamma-aminobutyric acid
(GABA) (inhibitory), glutamate (excitatory), glycine (inhibitory)
Catecholamines: Excitatory; made from tyrosineEx. Epinephrine, norepinephrine, dopamine
(both excitatory/inhibitory)Associated w/ stress
MAJOR CLASSES OF NEUROTRANSMITTERS Cholinergics: Excites muscle cells;
made from dietary fats, other metabolic compoundsAcetylcholine most common
Monoamines: Related to catecholaminesSerotonin (made from tryptophan)
Inhibits catecholamine NT’sHistamine: associated w/ pain sensations,
stress
TYPES OF NEURON COMMUNICATION
Function of the Nervous System
TYPES OF SYNAPSE ARRANGEMENTS Key term: Innervate = supply a body part w/
nervous stimulation Ex: Gland, muscle, neuron
Types of neural pathways (focus on the term!) Axo-dendritic synapse: terminus dendrite
connection Axo-somatic synapse: terminus nerve cell
body connection Axo-axonic synapse: terminus axon
connection Reverberating pathway (brain)
REVERBERATING PATHWAYS Neurons can stimulate themselves
repeatedly until another stimulus stops it
Linked to important pathways in brainEmotions, learning, memory
Breakdown in these pathways leads to disordersEx. Epilepsy (uncontrolled excitatory
activity)
EPSP VS. IPSP Type of communication that takes place
between two neurons also significant:Excitatory postsynaptic potential
(EPSP) = action potential generated In some pathways, may require multiple,
simultaneous EPSP’s to create an action potential
Inhibitory postsynaptic potential (IPSP) = action potential prevented Hyperpolarizes the membrane
Many neurons have both EPSP and IPSP connections – allows decision-making in brain!
REFLEXESFunctions of the Nervous System:
INNERVATION SEQUENCE Sensory neuron (w/ receptor)
interneuron (in spinal cord) motor neuron
OR… Afferent neuron interneuron Efferent
neuron
WHAT IS A REFLEX? Instantaneous, involuntary response to
a stimulus No intervention/conscious control
required Neurons arranged in a reflex arc
HOW DO REFLEXES BEGIN? Stimulus excites an affector
Carry out physiological job = transductionConvert a stimulus (touch/pain) into a
message that can be relayed to cellsPart of sensory nerve’s dendritesTransfers response to interneuron, which
relays information to motor neuron Motor neuron stimulates effector, which carries
out task of the reflex Interneurons communicate w/ brain
= certain reflexes can be “trained”, like urination and bowel movements
PATHOLOGY OF THE NERVOUS SYSTEM
Function of the Nervous System:
TYPES OF NERVE CELL DISEASES Infectious: causes by microorganisms Degenerative: progressive
deterioration of a cell/tissue Congenital: embryological/maturation
errors Toxicological: poisons that affect cell
metabolism/communication Traumatic: injuries resulting
INFECTIOUSMost common: bacterial Release toxins into blood
Can inflame, kill neurons, neuroglia Affect neuron communication
Ex. Botulism – toxin blocks action of acetylcholine Produces flaccid paralysis (no muscle contraction)
Ex. Tetanus – toxin enhances acetylcholine Prevents muscle relaxation
Endotoxins: produced as bacteria replicate, dieCause immediate death to neuroglia and
neurons
ENDOTOXINS Commonly cause diseases Examples
Encephalitis – inflammation of brainMeningitis – inflammation of membranes
surrounding brain, spinal cord Fungal toxins similar to those from
bacteria
NEUROTROPHIC MICROBES Enter and infect nervous system cells Varied:
ProtistaViruses: herpes, rabiesViroidsPrions: Mad cow/BSE/Creutzfeldt-Jakob
Kill cells outright/produce inflammation Carried by mosquitoes, biting insects
DEGENERATIVE DISORDERSAmylotrophic lateral sclerosis (ALS) a.k.a. Lou Gehrig’s disease
Faulty mitochondria Gradual loss of muscle function
DemyelinationLoss of neuroglia around axons, bodies of
neuronsCauses: metabolic, loss of blood flow
Results in slower neural impulses, eventual degeneration
Ex: Multiple sclerosis
CONGENITAL Krabbe’s disease
Lack enzyme (galactosylceramide beta-galactosidase) that prevents accumulation of toxic wastes in nerve cells
Buildup of harmful fats Abnormal neuron functioning, diminished
neuroglia maturation Hirschsprung’s disease
Affects large intestine neuronsNerve cells stop growing during
development, causing loss of function of LI
TOXINS Variety of sources:
LeadArsenic, cyanide (pesticides) – block cellular
respiration, disabling neuronsTetrodotoxins: inhibits flow of sodium into
nerve cells
TRAUMATIC INJURY Neurons cannot be replaced once they
die* Injured neurons can be repaired
Intact neuroglia must be nearbyCan replicate if only a small number are
killedRebuild damaged components of neuronsRedirect axons to original positions
Encouraged by growth factors Stem cells show promise
AGING OF THE NERVOUS
SYSTEM
WHY NO REPLACEMENT OF DAMAGED NEURONS? Mitosis is rare!
Cells are so specialized, to divide would mean de-differentiating!
Remember neurons originate from stem cells, not other neurons
= Neurons and neuroglia stay with you throughout life
= They accumulate damage over your lifespan
METABOLISM-DAMAGE CONNECTION The higher the cell’s metabolism, the
greater the buildup of metabolic “oxidizing” byproductsThese come from mitochondriaCan alter DNA metabolic errors that can be fatal
Alcohol, drug abuse, smoking, air pollution accelerate cell aging
GO WITH THE FLOW… As one ages, consistent blood flow to
tissues is lost Neurons are highly susceptible to this
High metabolic needsObtain nutrients, ions for action potentialsMaterials needed for NT’s
Become less responsive to stimuliGlands, muscles, neurons
TONIC FOR THE SOUL? Loss of tonic control
= regular nerve communication with glands, muscles
Without tonic control…Lose mobilityLoss of balance, postureLose muscle mass
INCREASE IN LATENCY Due to increased age Refractory period longer = fewer action potentials
Slows down impulses to muscles, delays sensory communication to brain, body
Wastes collect: plaques, tanglesAmyloid proteins = plaqueTangles = changes in cell’s cytoplasm,
changing shape Lipofuscin = fatty, brown pigment that
builds up; indicator of nerve cell pathology
