8.2%20Structures%20and%20Processes%20of%20the%20%20%20%20%20%20Nervous%20System

1
8.2 Structures and Processes of the
Nervous System
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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)

2
An Overview of the Nervous System
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

3
Cells of the Nervous System
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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).
4
The Structure of a Neuron
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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 cells 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  
5
The Structure of a Neuron
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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)

6
Classifying Neurons
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• Structurally, neurons are classified based on
  the number of processes that extend from the cell
  body.

Continued  
7
Classifying Neurons
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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  
8
Classifying Neurons
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2
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  
9
The Reflex Arc
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• -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.

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The Electrical Nature of Nerves
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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
  cells plasma membrane.
•  

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Resting Membrane Potential
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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  
12
Resting Membrane Potential contd
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

•  
• The process of generating a resting membrane
  potential of -70 mV is called polarization.

Continued  
13
Resting Membrane Potential contd
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2
14
Sodium-Potassium Pump
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

15
Action Potential
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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  
16
Action Potential
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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  
17
Action Potential
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2
The graph shows the changes that occur to
membrane potential as an action potential travels
down an axon.
18
Action Potential Step 1
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• An action potential is triggered when the
  threshold potential is reached.

19
Action Potential Step 2
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Action Potential Step 3
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Action Potential Step 4
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Action Potential
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Signal Transmission Across a Synapse
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Signal Transmission Across a Synapse Step 1
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• The nerve impulse travels to the synaptic
  terminal.

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Signal Transmission Across a Synapse Step 2
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• Synaptic vesicles containing neurotransmitters
  move toward and fuse with the presynaptic
  membrane.

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Signal Transmission Across a Synapse Step 3
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Signal Transmission Across a Synapse Step 4
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.

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Neurotransmitters
UNIT 4
Chapter 8 The Nervous System and Homeostasis
Section 8.2

• 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.