Transcript File - Ms. Ferrari`s Classroom

UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
8.2 Structures and Processes of the
Nervous System
The nervous system performs the vital role of regulating
body processes and structures to maintain homeostasis.
The nervous system has two major divisions:
• the central nervous system (CNS)
• the peripheral nervous system (PNS)
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
An Overview of the Nervous System
The central nervous system (CNS) consists of the brain and
spinal cord. The CNS integrates and processes information
sent by nerves.
The peripheral nervous system
includes nerves that carry
sensory messages to the CNS
and nerves that send messages
from the CNS to the muscles
and glands (effectors). The
peripheral nervous system consists
of the autonomic and somatic systems.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Cells of the Nervous System
The nervous system is composed of two main types of cells:
• neurons: basic structural and functional units of the
nervous system. They respond to stimuli, conduct
electrochemical signals, and release regulating chemicals.
Neurons are organized into tissues called nerves.
• glial cells: support neurons by nourishing them, removing
wastes, and defending against infection. They also
function as structural support cells.
Glial cells, shown in green in
this micrograph, support
neurons (shown in orange).
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
The Structure of a Neuron
In general, neurons share four common features:
• dendrites: short, branching terminals that receive impulses
and relay the impulses to the cell body
• a cell body: contains the nucleus and is the site of the
cell’s metabolic reactions
• an axon: conducts impulses away from the cell body and
varies in length from 1 mm to over 1 m
• branching ends: found on dendrites and axons, they
increase the surface area available for receiving and
sending information
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
The Structure of a Neuron
Axons are enclosed in a
fatty, insulating layer
called the myelin sheath
(protects neurons and
increases the rate of nerve
impulse transmission).
They are composed of
Schwann cells (a type of
glial cell)
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Classifying Neurons
Structurally, neurons are classified based on the number of
processes that extend from the cell body.
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Classifying Neurons
Functionally, neurons are classified as one of three types:
•
•
•
sensory neurons: receive input and transmit impulses
from sensory receptors to the CNS
interneurons: found in the CNS; link btw. sensory and
motor neurons; they process incoming sensory input
and relay outgoing motor information
motor neurons: transmit info. from the CNS to
effectors (muscles, glands, organs)
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Classifying Neurons
This diagram shows how a sensory neuron, an interneuron, and a motor neuron are arranged
in the nervous system. (The breaks indicate that the axons are longer than shown.)
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
The Reflex Arc
-An involuntary reflex action in response to a stimulus.
-Allows a rapid response, occurring in about 50 ms.
-Brain centres are not activated until after the response.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
The Electrical Nature of Nerves
Neurons use electrical signals to communicate with other
neurons, muscles, and glands.
The signals, called nerve impulses, involve changes in the
amount of electric charge across a cell’s plasma membrane.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Resting Membrane Potential
In a resting neuron, the cytoplasmic side of the
membrane (inside the cell) is negative, relative to the
extracellular side (outside the cell). This charge
separation across the membrane is a form of potential
energy called membrane potential. (-70mV)
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Resting Membrane Potential cont’d
The process of generating a resting membrane potential of -70
mV is called polarization.
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Resting Membrane Potential cont’d…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Sodium-Potassium Pump
The sodium-potassium pump is the most important factor
that contributes to the resting membrane potential. This system
uses ATP to transport 3 Na ions (Na+) out and 2 potassium
ions (K+) into the cell. The overall result of this process is a
constant membrane potential of -70 mV.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
Recall that a neuron is polarized due to the charge
difference across the membrane. Depolarization occurs
when the cell becomes less polarized (the membrane
potential is reduced to less than the resting membrane
potential of -70 mV).
During depolarization, the inside of the cell becomes less
negative relative to the outside of the cells. An action
potential causes depolarization to occur.
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
An action potential is the movement of an electrical
impulse along the plasma membrane of an axon.
It is an “all-or-none” response.
If a stimulus causes the axon to depolarize to a certain
level (the threshold potential), an action potential occurs.
Threshold potentials are usually close to -50 mV.
Note: The strength of an action potential does not change
based on the strength of the stimulus.
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
The graph shows the changes that occur to membrane potential as an action potential travels down
an axon.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 1
An action potential is
triggered when the
threshold potential is
reached.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 2
Voltage-gated sodium
(Na+) channels open
when the threshold
potential is reached.
Sodium ions move down
their concentration
gradient and rush into
the axon, causing
depolarization of the
membrane. The
membrane potential
difference is now
+40 mV.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 3
Voltage-gated sodium
channels close due to
change in membrane
potential. Voltage-gated
potassium (K+) channels
open. Potassium ions
move down their
concentration gradient
and exit the axon,
causing the membrane to
be hyperpolarized to
-90 mV.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 4
Voltage-gated potassium
channels close. The
sodium-potassium pump
and naturally occurring
diffusion restore the
resting membrane
potential of -70 mV. The
membrane is now
repolarized.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
After an action potential occurs, the membrane cannot be
stimulated to undergo another action potential. This brief
period of time (usually a few milliseconds) is called the
refractory period of the membrane.
The events that occur in an action potential continue down
the length of the axon until it reaches the end, where it
initiates a response at the next cell.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a Synapse
The junction between two neurons, or between a neuron and
an effector, is called a synapse.
Neurons are not directly connected. They have a small gap
between them called the synaptic cleft. A nerve impulse,
however, cannot jump from one neuron to another across the
cleft.
How does the nerve impulse proceed from the presynaptic
neuron (sends out the info.) to the postsynaptic neuron (
receives the info)?
Chemical messengers, neurotransmitters carry the nerve
impulse across the synapse from one neuron to another.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a Synapse: Step 1
The nerve impulse travels to the synaptic terminal.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a Synapse: Step 2
Synaptic vesicles containing neurotransmitters move toward
and fuse with the presynaptic membrane.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a Synapse: Step 3
Synaptic vesicles release neurotransmitters into the synaptic
cleft by exocytosis. Neurotransmitters diffuse across the
synapse to reach the postsynaptic neuron or the cell membrane
of an effector.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a Synapse: Step 4
Neurotransmitters bind to specific receptor proteins on the
postsynaptic membrane. The receptor proteins trigger ion
channels to open. Depolarization of the postsynaptic membrane
occurs, and an action potential is initiated if the threshold
potential is reached.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Neurotransmitters
Neurotransmitters have either excitatory or inhibitory effects
on the postsynaptic membrane. Excitatory molecules, like
acetylcholine, cause action potentials by opening sodium
channels. Inhibitory molecules cause potassium channels to
open, causing hyperpolarization.