NOTES: CH 48

Neurons, Synapses, and

Signaling

A nervous system has three overlapping

functions:

1) SENSORY INPUT: signals from sensory receptors to integration centers

2) INTEGRATION: information from sensory receptors is interpreted and associated with appropriate responses

3) MOTOR OUTPUT: conduction of signals from the integration center to effector cells (muscle cells or gland cells)

*CENTRAL NERVOUS SYSTEM (CNS)

integration center

brain and spinal cord

*PERIPHERAL NERVOUS

SYSTEM (PNS)

made up of nerves

(ropelike bundles

of neurons)

nerves communicate

motor and sensory

signals to and from CNS

and rest of body

Two Main Classes of Cells:

1) NEURONS:

functional unit of the nervous system

transmits signals from one location to another

made up of: cell body, dendrites, axon

many axons are enclosed by an insulating

layer called the MYELIN SHEATH

include: sensory neurons,

interneurons,

motor neurons

2) GLIAL CELLS (“GLIA”) -

SUPPORTING CELLS

10 to 50 times more numerous than

neurons

provide structure; protect, insulate,

assist neurons

example: Schwann cells and

oligodendrocytes form myelin sheaths

in the PNS and CNS, respectively

MYELIN SHEATH:

produced by Schwann cells in the

peripheral nervous system;

gaps between successive Schwann

cells are called NODES OF

RANVIER….

***the #10 term!!!

NODES OF RANVIER!***word #10 on my list!!!

1) Okazaki fragments

2) plasmodesmata

3) ???????

4) ???????

5) ???????

6) rubisco

7) oxaloacetate

8) islets of Langerhans

9) Batesian mimicry

10) nodes of Ranvier

2) GLIA (SUPPORTING CELLS)

example: astrocytes: responsible for

blood-brain barrier

Nerve

cells

Astrocyte

ACTION POTENTIALS & NERVE

IMPULSES

all cells have an electrical charge difference

across their plasma membranes; that is,

they are POLARIZED.

this voltage is called the MEMBRANE

POTENTIAL (usually –50 to –100 mV)

inside of cell is negative relative to outside

arises from differences in ionic

concentrations inside and outside cell

**A- = group of anions

including proteins,

amino acids, sulfate,

phosphate, etc.; large

molecules that cannot

cross the membrane

and therefore provide

a pool of neg. charge

that remains in the

cell

the sodium-

potassium pump

uses ATP to

maintain the ionic

gradients across the

membrane

(3 Na+ out; 2 K+ in)

How is this potential maintained?

the “resting potential”

of a nerve cell is approx.

–70 mV

neurons have special

ion channels (GATED

ION CHANNELS) that allow the cell

to change its membrane potential

(a.k.a. “excitable” cells)

when a stimulus reaches a neuron, it

causes the opening of gated ion

channels

(e.g.: light photoreceptors in the eye;

sound waves/vibrations hair cells in

inner ear)

HYPERPOLARIZATION: membrane

potential becomes more negative (K+

channel opens; increased outflow of K+)

DEPOLARIZATION: membrane potential

becomes less negative

(Na+ channel opens; increased inflow

of Na+)

**If the level of depolarization reaches the

THRESHOLD POTENTIAL, an ACTION

POTENTIAL is triggered.

ACTION

POTENTIALS

(APs):

the nerve impulse

all-or-none event;

magnitude is

independent of the

strength of the

stimulus

5 Phases of an A.P.:

1) Resting state

2) Depolarizing phase

3) Rising phase of A.P.

4) Falling phase of AP (repolarizing phase)

5) Undershoot

Phase of A.P.

State of Voltage-Gated Sodium (Na+) Channel State of Voltage-Gated Potassium (K+) channel

Activation gate Inact. Gate Entire channel

1) Resting closed open closed closed

2 & 3) Depolari-zation

open open open closed

4) Repolar-ization

open closed closed open

5) Undershoot

closed closed closed open

**during the undershoot, both Na+ channel

gates are closed; if a second depolarizing

stimulus arrives during this time, the

neuron will NOT respond

(REFRACTORY PERIOD)

strong stimuli result in greater frequency

of action potentials than weaker stimuli

How do action potentials “travel” along an

axon?

the strong depolarization of one action

potential assures that the neighboring region

of the neuron will be depolarized above

threshold, triggering a new action potential,

and so on…

“Saltatory Conduction”

SYNAPSE: junction

between a neuron and

another cell; found

between:

-2 neurons

-sensory receptor

& sensory neuron

-motor neuron & muscle

cell

-neuron & gland cell

Motor Neuron and Muscle Cell

Presynaptic cell = transmitting cell

Postsynaptic cell = receiving cell

Electrical Synapses: allow action potentials to

spread directly from pre- to postsynaptic cell

*connected by gap junctions (intercellular

channels that allow local ion currents)

**Most synapses are…

Chemical Synapses: cells are separated by a

synaptic cleft, so cells are not electrically

coupled; a series of events converts:

elec. signal chem.signal elec.signal

HOW???...

NEUROTRANSMITTERS: intercellular

messengers; released into synaptic cleft when

synaptic vesicles fuse with presynaptic

membrane

specific receptors for neurotransmitters project

from postsynaptic membrane; most receptors

are coupled with ion channels

neurotransmitters are quickly broken down by

enzymes so that the stimulus ends

the electrical charge caused by the binding of

neurotransmitter to the receptor can be:

EPSP (Excitatory Postsynaptic Potential):

membrane potential is moved closer to

threshold (depolarization)

IPSP (Inhibitory Postsynaptic Potential):

membrane potential is hyperpolarized (more

negative)

most single EPSPs are not strong

enough to generate an action potential

when several EPSPs occur close

together or simultaneously, they have

an additive effect on the postsynaptic

potential: SUMMATION

-temporal vs. spatial

Examples of neurotransmitters:

**acetylcholineNeuromuscular junction; can

be inhibitory or excitatory

epinephrine

norepinephrine

dopamine

serotonin

endorphinsDecrease perception of pain by

CNS; (heroin & morphine

mimic endorphins)

dop. & ser. both affect sleep, mood,

attention, learning; LSD &

mescaline bind to ser. & dop.

receptors

epin. & norep. also function as

hormones; “fight or flight

response”

Neurotransmitters: Ach

ACETYLCHOLINE: triggers

skeletal muscle fibers to contract…

so, how does a muscle contraction

stop???

Neurotransmitters: Ach

a muscle contraction ceases when

the acetylcholine in the synapse of

the neuromuscular junction is

broken down by the enzyme…..

wait for it………………….

ACETYLCHOLINESTERASE!!

It’s term #4!!!!!

ACETYLCHOLINESTERASE = the

enzyme the breaks down the

neurotransmitter acetylcholine.

ACETYLCHOLINESTERASE!***word #4 on my list!!!

1) Okazaki fragments

2) plasmodesmata

3) ????????

4) acetylcholinesterase

5) ????????

6) rubisco

7) oxaloacetate

8) islets of Langerhans

9) Batesian mimicry

10) nodes of Ranvier