NOTES: CH 48Neurons, 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 fragments2) plasmodesmata3) ???????4) ???????5) ???????6) rubisco7) oxaloacetate8) islets of Langerhans9) Batesian mimicry10) 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+) channelActivation

gateInact. 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) Undersho

ot

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…

http://www.blackwellpublishing.com/matthews/actionp.html

“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: **acetylcholine

 Neuromuscular junction; can be inhibitory or excitatory

 epinephrine

 

 norepinephrine

 

 dopamine

 

 serotonin

 

 endorphins

 Decrease 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 ashormones; “fight or flightresponse”

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 fragments2) plasmodesmata3) ????????4) acetylcholinesterase5) ????????6) rubisco7) oxaloacetate8) islets of Langerhans9) Batesian mimicry10) nodes of Ranvier