Transcript File - Biology with Radjewski

Chapter 49
Table of Contents
Section 1 Neurons and Nerve Impulses
Chapter 49
Section 1 Neurons and Nerve
Impulses
What is a neuron?
• The scientific name for a nerve cell
• is made up of a cell body, dendrites, and axons.
• A cell body is the nucleus of a neuron and most of its
organelles.
• Dendrites extend from the cell body and are covered by a
membrane.
• The axon is a long membrane-bound projection that
transmits information away from the cell body in the form of
electrical signals.
• These electrical signals are called action potentials
More on the axon
• The end of an axon is called the axon terminal – It may contact
and communicate with:
– Muscle cells
– A gland cell
– Another neuron
• Axons are covered by a lipid membrane called the myelin
sheath (ring a bell?)
– It insulates the axon
– It speeds up the transmission of action potentials
– Produced by Schwann cells that surround the axon
– Gaps in the myelin sheath are called the Nodes of Ranvier
Let’s label a neuron
Chapter 49
Section 1 Neurons and Nerve
Impulses
Neuron Communication
• Neurons do not touch each other or other cells
• There is a small gap between neurons called a synaptic
cleft. That region or junction is called synapses.
– This is where neurons communicate
– The signaling activity of the nervous system is made up
of electrical activity within neurons and chemical flow
between neurons.
• These synapses do not communicate by touch, but by
releasing chemicals, or neurotransmitters, into the synaptic
cleft.
• At a synapse, the transmitting neuron is called the
presynaptic neuron and the receiving cell is called a
postsynaptic cell
Chapter 49
Section 1 Neurons and Nerve
Impulses
Nerve Impulses
• All cells have an electrical charge inside them that is different
from outside the cell
– A membrane potential is a difference in the electrical
charge across a cell membrane.
• A membrane potential can change with an addition or
removal of ions within the cell.
• Ions move in and out of the cell by passing through proteins
that act as ion channels. Whether the ion channels are open
or closed affects the membrane potential.
• Just like with batteries, membrane potentials are expressed
as voltage
Chapter 49
Section 1 Neurons and Nerve
Impulses
Resting Potential
• A neuron is at rest when it is not sending or
receiving a signal.
– When at rest, the neuron is more negatively
charged on the inside than on the outside of
the cell body.
• Inside the cell has negatively charged
proteins and K+ ions
• Outside the cell, there is an accumulation of
Na+ ions (due to the sodium potassium
pump)
• The electric potential across the cell membrane
when the neuron is at rest is called the resting
potential. It is about -70 millivolts.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Action Potential
• When a dendrite or cell body is stimulated, the
permeability of the neuron’s membrane changes
suddenly.
– A stimulus triggers the cell membrane to
suddenly become permeable to Na+ ions and
they rush into the cell
– The membrane potential reverses and the
neuron’s cell body becomes more positively
charged than the exterior of a cell.
• This reverse in polarity begins an action
potential.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Action Potentials continued
• After the first segment of the neuron is stimulated the
segment next to the first will become stimulated.
• The action potential will continue away from the cell body.
• Soon the voltage gated channels for Na+ close and the
voltage gated channels for K+ open.
• Soon after being stimulated, the interior of the neuron
begins to become more and more negative.
• This signals the end of the action potential.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Refractory Period
• A neuron cannot generate another action
potential until it has returned to its resting
potential.
• The period in which a neuron cannot send a
signal is called the refractory period.
– Returning the neuron to its resting potential
requires energy.
– Neurons need a continuous supply of ATP to
keep the sodium potassium pump operating.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Conduction of a Nerve Impulse
Chapter 49
Section 1 Neurons and Nerve
Impulses
Action Potential
Click below to watch the Visual Concept.
Visual Concept
Chapter 49
Section 1 Neurons and Nerve
Impulses
Communication Between Neurons
• Once an action potential reaches the axon terminal, it
releases neurotransmitters into the synaptic cleft. These
neurotransmitters bind to receptors proteins and open the
ion channels of the new neuron cell.
• If enough ion channels are opened, the action potential will
continue through the new neuron. If not, the nervous signal
will be terminated.
• After the neurotransmitters have opened the ion channels,
they will be cleared out of the synaptic cleft by being
reabsorbed by the neuron that released them or broken
down by enzymes.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Release of Neurotransmitter
Click below to watch the Visual Concept.
Visual Concept
Chapter 49
Section 1 Neurons and Nerve
Impulses
Synaptic Transmission