THE NERVOUS SYSTEM

THE NERVOUS SYSTEM

• Overall Function• COMMUNICATION• Works with the

endocrine system in regulating body functioning, but the nervous system is specialized for SPEED

NEURONS

• A neuron is the functional unit of the nervous system

• Neurons are specialized for transmitting signals from one location in the body to another

• Neurons consist of a large cell body (contain a nucleus and other organelles), and neuronal processes• Axons

• Conduct messages AWAY from cell body• Dendrites

• Conducts messages TOWARD cell body

NEURON STRUCTURE

PARTS OF THE NEURON

• Cell body: this is where most of the neuron’s organelles (including the nucleus) are located

• Dendrites: highly branched extensions from the cell body that RECEIVE signals from other neurons

• Axon: a large extension from the cell body that TRANSMITS signals to other neurons or “effector” cells

• Axon hillock: where the axon joins the cell body• Myelin sheath: a fatty layer of cells that “insulates”

the axon (not present in most invertebrates)• Synaptic terminal: the branching ends of the axon

that release a “neurotransmitter” to send a message• Synapse: the space between the synaptic terminal

and the effector cell

SUPPORTING CELLS OF THE NERVOUS SYSTEM

• Glia is the term given to the many cells that support the neurons in the nervous system• Astrocytes: provide structural support for

neurons in the CNS. They also regulate extracellular ion concentrations (important when we talk about membrane potentials)

• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS): responsible for creating the myelin sheath on the axon

ORGANIZATION OF THE NERVOUS SYSTEM

• Organisms have different types of nervous systems based on their complexities• The simplest organisms will have a web-like

arrangement of nerves throughout the body the act as a nerve net• These organisms are able to react to stimuli, but do

not show any higher activity• Example: Hydra

• A little more complicated organism also have bundled fiber-like extensions of neurons called nerves, along with nerve nets• This allows nerve nets to control more complex

movements• Example: Sea star

ORGANIZATION OF THE NERVOUS SYSTEM: MORE COMPLICATED ORGANISMS

• Central Nervous System (CNS)• Consists of brain

and spinal chord• In more primitive

organisms, this could include a cluster of neurons (called ganglia) along a ventral nerve and a brain

• Peripheral Nervous System (PNS)• Consists of all of the

peripheral nerves that connect with the CNS

CENTRAL AND PERIPHERALNERVOUS SYSTEMS

• The central nervous system consists of the brain and spinal cord• This is where integration

occurs• Made of interneurons

• The peripheral nervous system consists of the nerve cells that communicate signals between the CNS and the rest of the body• Sensory neurons

• Carry info from the sensory receptors to the brain

• Motor neurons• Carry info from the brain to

effector cells (to do whatever the brain said!)

OTHER DIVISIONS OF THE NERVOUS SYSTEM

• Autonomic Nervous System• Regulates internal

environment (digestion, cardiovascular, excretion and hormone release

• Called the “involuntary” nervous system

• Three parts:• Sympathetic• Parasympathetic• Enteric

• Somatic Nervous System• Carries signals to and

from the skeletal muscles

• Responds to external stimuli

• Called the “voluntary” nervous system

AUTONOMIC NERVOUS SYSTEM

• Sympathetic: corresponds to increased arousal or energy output (fight or flight response)• Increased heart rate• Dilate blood vessels and respiratory passages• Convert glycogen to glucose• Release epinephrine (adrenaline)• Inhibits digestion

• Parasympathetic: corresponds to self-maintenance and relaxation (“rest” and “digestion”)• Opposite of sympathetic nervous system

• Enteric: network of neurons responsible for digestion (digestive tract, pancreas, and gallbladder)

INFORMATION PROCESSING

• Regardless of the complexity of the nervous system, there are 3 general stages to information processing:• Sensory input• Integration• Motor output/effect

COMMUNICATION LINES

Stimulus(input)

Receptors(sensory neurons)

Integrators(interneurons)

motor neurons

Effectors(muscles, glands)

Response(output)

MAJOR NERVOUS SYSTEM PROCESSES

• Input• The conduction of signals from sensory neurons

to integration centers in the nervous system• Detect external stimuli (light, sound, heat, smell,

touch, taste)• Detect internal conditions (blood pressure, blood

CO2 levels, muscle tension)• Integration

• The process by which the information from the environmental stimulation of the sensory receptors is sent and interpreted by interneurons in the CNS• The complexity of the CNS has to do with the

amount of connections between interneurons

MAJOR NERVOUS SYSTEM PROCESSES

• Motor Output• The conduction of signals from the processing

center of the CNS to the motor neurons which communicate with muscle cells or gland cells (effector cells) that actually carry out the body’s responses to stimuli

ACTION POTENTIALS: HOW THE NERVES CONDUCT SIGNALS

• In order to actually TRANSMIT a signal, the voltage (charge) across the membrane (membrane potential) has to change• A signal will cause the ion channels to open, letting some

of the ions (Na+, K+) through, trying to achieve EQUILIBRIUM

• This depolarizes the membrane• This causes the signal to be passed along the neuron,

which is known as an ACTION POTENTIAL (like a wave of electricity)

RESTING POTENTIAL: NOT TRANSMITTING A SIGNAL

• Resting Potential: charge difference across the plasma membrane of a neuron when not transmitting signals

• Fluid just outside cell is more positively charged than fluid inside because of large negatively charged proteins in the cytoplasm

• Potassium (K+): Higher inside than outside

• Sodium (Na+): Higher outside than inside

• Potential is measured in millivolts

• Resting potential is usually about -60mV to -80mV (inside of the membrane is “-” and outside is “+”)

RESTING POTENTIAL

• The resting potential of a neuron creates an ionic gradient• Remember the concentration gradient in the H+ pump to

make ATP• There are many open potassium ion channels in the

plasma membrane and few sodium ion channels (ungated)• This causes a net flow of Na+ and K+ across the

membrane• This is what creates the voltage (flow of ions)

• To maintain the levels of Na+ and K+, the cells utilize the sodium-potassium pump (remember active transport)

GATED ION CHANNELS

• Neurons also have 3 gated ion channels (controls the flow of ions)• Stretch-gated ion channels: sense

stretching of the cell and cause the gates to open

• Ligand-gated ion channels: open or close when a specific chemical binds to the channel

• Voltage-gated ion channels: open or close when the membrane potential changes

ACTION POTENTIALS: TRANSMITTING A SIGNAL

• Depending on external stimuli, gated ion channels can open or close• Some stimuli can cause a hyperpolarization which makes the

membrane potential of the cell greater than resting potential• Example: opening K+ gated channels allows the movement of K+

out of the cell (remember: at rest K+ is more concentrated inside the cell)

• Increases membrane potential to -92 mV (losing “+” out of cell)

• Some stimuli can cause a depolarization which makes the membrane potential of the cell less than resting potential• Example: opening Na+ gated channels allows the movement of

Na+ into the cell (remember: at rest Na+ is more concentrated outside the cell

• Decreases membrane potential to +62 mV (gaining “+” in cell)

ACTION POTENTIALS: TRANSMITTING A SIGNAL

• A change in membrane potential is called a graded potential

• Action potentials are either ALL or NOTHING• Either there is enough change in the voltage to pass

the message along, or there isn’t• The neuron either “fires” or it doesn’t fire• In order to “fire”, the membrane potential must hit a

threshold (the membrane voltage that sets the reaction)

• If the threshold is reached, then the neuron undergoes an action potential (these are what carries a signal along the axon)

ALL OR NOTHING

• All action potentials are the same size

• If stimulation is below threshold level, no action potential

occurs

• If it is above threshold level, cell is always depolarized to

the same level

• Action potential is initiated at the axon hillock and travels

down the axon to the axon terminal

STRUCTURE OF A NEURON

dendrites

cell body

TRIGGER ZONE

INPUT ZONE

CONDUCTING ZONE

OUPUT ZONEaxon

axon endings

ACTION POTENTIAL

• Step 1: Neuron is in the resting potential, the gated-ion channels are closed

• Step 2: A stimulus causes some Na+ ion channels to open allowing Na+ to diffuse through the membrane. This causes the membrane to be depolarized. The depolarization causes even more Na+ ion channels to open (positive feedback) until a threshold is reached in the membrane potential

• Step 3: Once the threshold is reached, positive feedback progresses at a rapid rate to create an action potential (the voltage that allows the membrane to conduct the signal)

ACTION POTENTIAL

• Step 4: After the action potential is reached, the Na+ gates close, preventing the influx of any more Na+ ion. At the same time, the K+ ion channels open. This allows the K+ ions to diffuse out of the membrane (high concentration of K+ inside the membrane compared to outside). This release of K+ ions rapidly lowers the membrane potential.

• Step 5: As the membrane potential lowers, it falls a little below the resting potential, undershoot The K+ ion channels close and the membrane eventually returns to its resting potential

STEPS IN THE ACTION POTENTIAL

• An action potential is very quick (each one only takes 1-2 milliseconds

• After an action potential, it takes a little bit of time to return all of the Na+ and K+ concentrations to their original levels

• Na+ / K+ pumps the Na+ and K+ back to original positions

• During this time, a second action potential cannot by initiated (refractory period)

RECORDING OF ACTION POTENTIALaction potential

threshold

resting membrane potential

Time (milliseconds)

Mem

bra

ne

po

ten

tial

(m

illi

volt

s)

-40

-70

-20

0

+20

0 1 2 3 4 5Figure 34.6b

Page 583

TRANSMITTING SIGNAL ALONG AXON

• Transmitting the signal• In order to propagate the signal, the membrane potential

must be depolarized along the length of the axon• To make this occur, when the Na+ is being let into the

cell (depolarization) in one part of the axon, it creates an electric current that causes depolarization in an adjacent area

• Behind the zone of depolarization is where the membrane is returning to resting potential (repolarization)

• The refractory period prevent the action potential from being sent “backwards” along the neuron

ACTION POTENTIAL

Na+

Na+Na+

Na+

Na+

Na+

K+

K+

K+

Na+Na+

K+ K+K+

K+

Na+

1 2

3 4Figure 34.5dPage 583

SPEED OF CONDUCTION

• In general, the speed of a signal along an axon is dependent on a few things• The smaller the axon diameter, the slower the speed of signal

conduction• Simple invertebrates (worms) may have conduction speeds of

centimeters/second

• Larger axon diameters allow increased speed of signal conduction• Complex invertebrates (squid or octopi) have conduction speeds of

about 100 meters/second

• In the vertebrate axon, there is a myelin sheath which increases speed due to insulation

• There are gaps in the myelin sheath (Nodes of Ranvier), where the depolarization can “jump” to. This greatly increases conduction rate (about 120 meters/second)

COMMUNICATION BETWEEN NEURONS

NEURON TO NEURON COMMUNICATION

• As the action potential travels along the axon it stops at the axon terminal (synaptic terminal)• Action potentials do not travel between

different neurons• Yet, it is still necessary to send the “signal”

from one neuron to the next• To do this, there has to be a way to send a

signal across the space that exists between one neuron and another (synaptic cleft or gap junction)

CHEMICAL SYNAPSE

• Gap between axon

terminal of one neuron and

dendrite of adjacent

neuron

• Action potential in axon

ending of presynaptic cell

causes voltage-gated

calcium channels to open

• Flow of calcium into

presynaptic cell causes

release of

neurotransmitter into

synaptic cleft

synaptic vesicle

plasma membrane of axon ending of presynapic cell

plasma membrane of postsynapic cell

synaptic cleft

membrane receptor

Figure 34.7aPage 584

NEUROTRANSMITTERS

• Neurotransmitters are substances that carry the “message” across the synapse

• Important neurotransmitters:• Acetylcholine

(bridges gaps between motor neurons & muscle cells),

• norepinephrine, dopamine, serotonin work in CNS

SYNAPTIC TRANSMISSION

• Neurotransmitter

diffuses across cleft and

binds to receptors on

membrane of

postsynaptic cell

• Binding of

neurotransmitter to

receptors opens ion

channels in the

membrane of

postsynaptic cell

ION GATES OPEN

ions

neurotransmitter

receptor for neurotransmitter

gated channel protein

SYNAPTIC TRANSMISSION

• Enzymes in synaptic cleft will degrade neurotransmitters after action potential is initiated on the post-synaptic cell. The neurotransmitters are recycled after they are broken down.

• Example: Acetylcholine is broken down by the enzyme acetylcholine esterase

INDIRECT SYNAPTIC TRANSMISSION

• The neurotransmitter does not bind directly to an ion channel gate.

• Instead, it activates a signal transduction pathway (Remember cell signaling . . . again)• Utilizes a second messenger (AMP to

cAMP . . . again)• These signals take longer to activate, but

last for a longer period of time

NERVE

• A bundle of axons enclosed within a connective tissue sheath

axon

myelin sheath

nerve fascicle

REFLEXES

• Automatic movements made in response

to stimuli

• In the simplest reflex arcs, sensory

neurons synapse directly on motor

neurons; interneurons in CNS aren’t

involved.

• Most reflexes involve an interneuron

STRETCH REFLEX

STIMULUSBiceps

stretches.

ResponseBiceps

contracts.

motor neuron

sensory neuron

STRUCTURE OF THE SPINAL CORD

spinal cord

ganglion

nerve

vertebra

meninges(protectivecoverings)

Figure 34.19aPage 593

DIVISIONS OF BRAIN

Division Main Parts

Forebrain

Midbrain

Hindbrain

CerebrumOlfactory lobesThalamusHypothalamusLimbic systemPituitary glandPineal gland

Tectum

PonsCerebellumMedulla oblongata

anterior end of thespiral cord

Figure 34.20Page 594

CEREBROSPINAL FLUID

• Surrounds the spinal cord

• Fills ventricles within the brain

• Blood-brain barrier controls which solutes enter the cerebrospinal fluid

ANATOMY OF THE CEREBRUM

• Largest and most complex part of human

brain (Responsible for thinking & higher level

functions)

• Outer layer (cerebral cortex) is highly folded

• A longitudinal fissure divides cerebrum into

left and right hemispheres

• Corpus collosum connects the two

hemispheres

LOBES OF THE CEREBRUM

Temporal

FrontalParietal

Occipital

Primary motor cortex Primary somatosensory cortex

LIMBIC SYSTEM

• Controls emotions and has role in memory

(olfactory tract) cingulate gyrus thalamus

amygdala

hippocampus

hypothalamus

OTHER PARTS OF THE BRAIN

• Cerebellum - Controls muscle coordination and posture

• Medulla oblongata- Controls heart rate & breathing rate

VARIATIONS IN NERVOUS SYSTEMS AMONG ANIMALS

EXAMPLE: PROBLEM WITH NERVOUS SYSTEM

• Multiple Sclerosis:

• A condition in which nerve fibers lose their myelin

• Slows conduction

• Symptoms include visual problems, numbness, muscle weakness, and fatigue