3.1) Communication in the Nervous System
Key Learning Goals
Identify the various parts of the neuron and the main functions of glial cells.
Describe the neural impulse, and explain how neurons communicate at chemical synapses.
Discuss some of the functions of acetylcholine and the monoamine neurotransmitters.
Discuss how gamma-aminobutyric acid (GABA), glutamate and endorphins are related to behaviour.
Nervous Tissue: The Basic Hardware
• Nervous system is living tissue composed of cells that fall into two major categories: neurons and glia.
• Behaviour depends on rapid information processing.
• The nervous system is a complex communication network in which signals are constantly being transmitted, received and integrated.
Neurons
• Neurons are individual cells in the nervous system that receive, integrate and transmit information.
▪ They're the basic chains of communication within the nervous system.
▪ Most neurons communicate only with other neurons.
- Some neurons also receive signals from outside the nervous system (from sensory organs) or carry messages from the nervous system to the
muscles that move the body.
• Soma (cell body) contains the nucleus and most of the other structures common to most cells (soma is Greek for ‘body’).
▪ Rest of the neuron is devoted exclusively to handling information.
▪ Neurons have a number of branched structures called dendritic trees (dendrite is Greek for ‘tree’).
▪ Each individual branch is a dendrite.
- Dendrites are the parts of a neuron that are specialised to receive information.
- Most neurons receive information from many other cells – sometimes thousands of others – and so have extensive dendritic trees.
Structure of the Neuron
▪ Neurons are the communication links of the nervous system.
▪ These are the key parts of a neuron
- specialised receptor areas (dendrites),
- cell body (soma),
- axon fibre along - which impulses are transmitted,
- terminal buttons - which release chemical messengers that carry signals to other neurons.
▪ Neurons vary considerably in size and shape and are usually densely interconnected.
• From the many dendrites, information flows into the cell body and then travels away from the soma along the axon (from the Greek for ‘axle’).
▪ Axon is a long, thin fibre that transmits signals away from the soma to other neurons, or muscles or glands.
▪ Axons vary in length, and may communicate with a number of other cells.
• Axons are generally wrapped in cells with a high concentration of a white, fatty substance called myelin.
▪ The myelin sheath acts as insulating material and aids in accelerating the transmission of signals that move along axons.
▪ If an axon’s myelin sheath deteriorates, its ability to conduct signals is less effective.
- Loss of muscle control that is seen in the disease multiple sclerosis is due to a degeneration of myelin sheaths, leading to an interruption of the
signal, which affects movement.
• The axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals called neurotransmitters.
▪ These chemicals serve as messengers that can activate nearby neurons. The points at which neurons interconnect are called syn apses.
▪ A synapse is a junction where information is transmitted from one neuron to another (synapse is from the Greek for ‘junction’).
Information is received at the dendrites, passed through the soma and along the axon, and transmitted to the dendrites of other cells at meeting points called
Chapter 3 - Biology of Behaviour Page 1
, Information is received at the dendrites, passed through the soma and along the axon, and transmitted to the dendrites of other cells at meeting points called
synapses.
Misconception
Neurons are responsible for all the information processing in the nervous system.
Reality
Until recently, it was thought that the transmission and integration of informational signals was the exclusive role of the neurons. However, newer research has
demonstrated that glial cells also play an important role in information processing.
Glia
• Glia are cells found throughout the nervous system that provide various types of support for neurons.
▪ Glia (which literally means ‘glue’) tend to be much smaller than neurons but are much more abundant within the human brain.
- Glial cells appear to account for over 50% of the brain’s volume.
- Glial cells serve many supportive functions, such as the provision of certain nutrients for neurons, insulation and the removal of waste products.
▪ Myelin sheaths that encase some axons are derived from special types of glial cells.
- Glial cells also play a complicated role in the development of the nervous system in the human embryo.
• Recent research indicates that glial cells may also play a role in sending and receiving chemical signals.
▪ Some types of glia can detect neural impulses and send signals to other glial cells.
▪ Neuroscientists are now trying to figure out how this signalling system works with the neural communication system.
- Halassa and Haydon argue that glia cells modulate the signalling of neurons, decreasing or increasing synaptic activity.
- Nedergaard and Verkhratsky point out that glial cells shield synapses from the ‘chatter’ of neighbouring neurons, thereby enhancing the signal-
to-noise ratio in the nervous system.
• Some of the early findings and theorising have proved very interesting.
Recent research suggests that glial cells may play an important role in memory formation and that the gradual deterioration of glial tissue might contribute to
Alzheimer’s disease.
▪ Other research suggests that glial cells play a crucial role in the experience of chronic pain.
- Impaired neural glial communication may also contribute to psychological disorders such as schizophrenia and mood disorders.
• Although glial cells may have a contributing role with regard to information processing in the nervous system, the bulk of this work is handled by the neurons.
The Neuron and Neural Impulse
• Branched structure is called a dendritic tree, and each individual branch is a dendrite.
▪ Dendrites are the parts of a neuron that are specialized to receive information.
• Long fiber is the axon.
▪ Axons are specialized structures that transmit information to other neurons or to muscles or glands.
• Soma (cell body) of the neuron contains the cell's nucleus.
• Information flows into the neuron through the dendrites, and away from the neuron along the axon.
• Most human axons are wrapped in a myelin sheath.
▪ Myelin is a white, fatty substance that serves as an insulator around the axon and speeds the transmission of signals.
- In people suffering from multiple sclerosis, some myelin sheaths degenerate, slowing or preventing nerve transmission to certain muscles.
• The axon ends in a cluster of terminal buttons.
• Inside each terminal button are sacs (or vesicles) containing neurotransmitters.
▪ These chemicals serve as messengers that may activate neighboring cells.
Chapter 3 - Biology of Behaviour Page 2
,• Inside each terminal button are sacs (or vesicles) containing neurotransmitters.
▪ These chemicals serve as messengers that may activate neighboring cells.
- The junction where information is passed from a neuron to another cell is called a synapse.
• The neuron at rest is a tiny battery, a store of potential energy.
▪ Inside and outside the axon are fluids containing electrically charged atoms and molecules called ions.
- Positively charged sodium (Na+) and potassium (+K) ions and negatively charged chloride ions are the principal molecules involved in the nerve
impulse.
• When the neuron is not conducting an impulse, it's in a resting state.
▪ The cell membrane is polarized–negatively charged on the inside and positively charged on the outside.
▪ The charge difference across the membrane can be measured with a pair of microelectrodes connected to an oscilloscope.
- In a resting neuron, this difference, called the resting potential, is about –70 millivolts
• When the neuron is stimulated, sodium gates in its cell membrane open briefly, allowing positively charged sodium ions to run into the axon and causing an
upward spike on the oscilloscope screen.
• The change in charge causes potassium (+K) gates to open, and positively charged potassium (+K) ions flow out of the neuron.
▪ This outflow of potassium reestablishes a negative charge inside the axon and causes a drop on the oscilloscope screen.
• The brief reversal in the charge difference across the neuron membrane, which appears as a spike on the oscilloscope screen, is called an action potential or
neural impulse.
How does an action potential travel along an axon?
• As sodium gates close and potassium gates open at one point on the membrane, sodium gates are triggered to open at the next point along the axon membrane.
Chapter 3 - Biology of Behaviour Page 3
, How does an action potential travel along an axon?
• As sodium gates close and potassium gates open at one point on the membrane, sodium gates are triggered to open at the next point along the axon membrane.
▪ It's this sequential opening of sodium gates at adjacent points that moves the action potential along the axon.
• After the firing of an action potential, some time is needed before the sodium gates are ready to open again.
▪ Until they're ready to reopen, the neuron cannot fire.
▪ The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin.
- This "down time" isn't very long, only 1 or 2 milliseconds.
• The size of an action potential is not affected by the strength of the stimulus—a weaker stimulus does not produce a weaker action potential.
▪ If the neuron receives a stimulus of sufficient strength, it fires, but if it receives a weaker stimulus, it doesn't.
- This is referred to as the "all-or-none law."
• Even though the action potential is an all-or-nothing event, neurons can convey information about the strength of a stimulus.
▪ They do so by varying the rate at which they fire action potentials.
- Generally, a weaker stimulus, such as light pressure on the skin, will produce a slower volley of neural impulses than would a stronger stimulus,
such as greater pressure.
The Neural Impulse: Using Energy to Send Information
The Neuron at Rest: A Tiny Battery
• Hodgkin and Huxley learned that the neural impulse is a complex electrochemical reaction.
• Both inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions.
• Positively charged sodium (Na+) and potassium ions and negatively charged chloride (+K) ions flow back and forth across the cell membrane.
• Negatively charged chloride (Cl-) ions flow back and forth across cell membrane.
• BUT ions DO NOT cross at the same RATE (Semi-permeable Membrane).
• Difference in flow rates leads to a slightly higher concentration of ions inside the cell.
• = Resting potential of a neuron is its stable, negative charge when the cell is inactive (–70 millivolts) – not stimulated.
• Sodium channels are closed.
The electrochemical properties of the neuron allow it to transmit signals. The electric charge of a neuron can be measured with a pair of electrodes connected to
an oscilloscope, as Hodgkin and Huxley showed with a squid axon. Because of its exceptionally thick axons, the squid has frequently been used by scientists
studying the neural impulse. (a) At rest, the neuron’s voltage hovers around –70 millivolts. (b) When a neuron is stimulated, a brief jump occurs in its voltage,
resulting in a spike on the oscilloscope recording of the neuron’s electrical activity. This change in voltage, called an action potential, travels along the axon like
a spark travelling along a trail of gunpowder.
Chapter 3 - Biology of Behaviour Page 4
Key Learning Goals
Identify the various parts of the neuron and the main functions of glial cells.
Describe the neural impulse, and explain how neurons communicate at chemical synapses.
Discuss some of the functions of acetylcholine and the monoamine neurotransmitters.
Discuss how gamma-aminobutyric acid (GABA), glutamate and endorphins are related to behaviour.
Nervous Tissue: The Basic Hardware
• Nervous system is living tissue composed of cells that fall into two major categories: neurons and glia.
• Behaviour depends on rapid information processing.
• The nervous system is a complex communication network in which signals are constantly being transmitted, received and integrated.
Neurons
• Neurons are individual cells in the nervous system that receive, integrate and transmit information.
▪ They're the basic chains of communication within the nervous system.
▪ Most neurons communicate only with other neurons.
- Some neurons also receive signals from outside the nervous system (from sensory organs) or carry messages from the nervous system to the
muscles that move the body.
• Soma (cell body) contains the nucleus and most of the other structures common to most cells (soma is Greek for ‘body’).
▪ Rest of the neuron is devoted exclusively to handling information.
▪ Neurons have a number of branched structures called dendritic trees (dendrite is Greek for ‘tree’).
▪ Each individual branch is a dendrite.
- Dendrites are the parts of a neuron that are specialised to receive information.
- Most neurons receive information from many other cells – sometimes thousands of others – and so have extensive dendritic trees.
Structure of the Neuron
▪ Neurons are the communication links of the nervous system.
▪ These are the key parts of a neuron
- specialised receptor areas (dendrites),
- cell body (soma),
- axon fibre along - which impulses are transmitted,
- terminal buttons - which release chemical messengers that carry signals to other neurons.
▪ Neurons vary considerably in size and shape and are usually densely interconnected.
• From the many dendrites, information flows into the cell body and then travels away from the soma along the axon (from the Greek for ‘axle’).
▪ Axon is a long, thin fibre that transmits signals away from the soma to other neurons, or muscles or glands.
▪ Axons vary in length, and may communicate with a number of other cells.
• Axons are generally wrapped in cells with a high concentration of a white, fatty substance called myelin.
▪ The myelin sheath acts as insulating material and aids in accelerating the transmission of signals that move along axons.
▪ If an axon’s myelin sheath deteriorates, its ability to conduct signals is less effective.
- Loss of muscle control that is seen in the disease multiple sclerosis is due to a degeneration of myelin sheaths, leading to an interruption of the
signal, which affects movement.
• The axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals called neurotransmitters.
▪ These chemicals serve as messengers that can activate nearby neurons. The points at which neurons interconnect are called syn apses.
▪ A synapse is a junction where information is transmitted from one neuron to another (synapse is from the Greek for ‘junction’).
Information is received at the dendrites, passed through the soma and along the axon, and transmitted to the dendrites of other cells at meeting points called
Chapter 3 - Biology of Behaviour Page 1
, Information is received at the dendrites, passed through the soma and along the axon, and transmitted to the dendrites of other cells at meeting points called
synapses.
Misconception
Neurons are responsible for all the information processing in the nervous system.
Reality
Until recently, it was thought that the transmission and integration of informational signals was the exclusive role of the neurons. However, newer research has
demonstrated that glial cells also play an important role in information processing.
Glia
• Glia are cells found throughout the nervous system that provide various types of support for neurons.
▪ Glia (which literally means ‘glue’) tend to be much smaller than neurons but are much more abundant within the human brain.
- Glial cells appear to account for over 50% of the brain’s volume.
- Glial cells serve many supportive functions, such as the provision of certain nutrients for neurons, insulation and the removal of waste products.
▪ Myelin sheaths that encase some axons are derived from special types of glial cells.
- Glial cells also play a complicated role in the development of the nervous system in the human embryo.
• Recent research indicates that glial cells may also play a role in sending and receiving chemical signals.
▪ Some types of glia can detect neural impulses and send signals to other glial cells.
▪ Neuroscientists are now trying to figure out how this signalling system works with the neural communication system.
- Halassa and Haydon argue that glia cells modulate the signalling of neurons, decreasing or increasing synaptic activity.
- Nedergaard and Verkhratsky point out that glial cells shield synapses from the ‘chatter’ of neighbouring neurons, thereby enhancing the signal-
to-noise ratio in the nervous system.
• Some of the early findings and theorising have proved very interesting.
Recent research suggests that glial cells may play an important role in memory formation and that the gradual deterioration of glial tissue might contribute to
Alzheimer’s disease.
▪ Other research suggests that glial cells play a crucial role in the experience of chronic pain.
- Impaired neural glial communication may also contribute to psychological disorders such as schizophrenia and mood disorders.
• Although glial cells may have a contributing role with regard to information processing in the nervous system, the bulk of this work is handled by the neurons.
The Neuron and Neural Impulse
• Branched structure is called a dendritic tree, and each individual branch is a dendrite.
▪ Dendrites are the parts of a neuron that are specialized to receive information.
• Long fiber is the axon.
▪ Axons are specialized structures that transmit information to other neurons or to muscles or glands.
• Soma (cell body) of the neuron contains the cell's nucleus.
• Information flows into the neuron through the dendrites, and away from the neuron along the axon.
• Most human axons are wrapped in a myelin sheath.
▪ Myelin is a white, fatty substance that serves as an insulator around the axon and speeds the transmission of signals.
- In people suffering from multiple sclerosis, some myelin sheaths degenerate, slowing or preventing nerve transmission to certain muscles.
• The axon ends in a cluster of terminal buttons.
• Inside each terminal button are sacs (or vesicles) containing neurotransmitters.
▪ These chemicals serve as messengers that may activate neighboring cells.
Chapter 3 - Biology of Behaviour Page 2
,• Inside each terminal button are sacs (or vesicles) containing neurotransmitters.
▪ These chemicals serve as messengers that may activate neighboring cells.
- The junction where information is passed from a neuron to another cell is called a synapse.
• The neuron at rest is a tiny battery, a store of potential energy.
▪ Inside and outside the axon are fluids containing electrically charged atoms and molecules called ions.
- Positively charged sodium (Na+) and potassium (+K) ions and negatively charged chloride ions are the principal molecules involved in the nerve
impulse.
• When the neuron is not conducting an impulse, it's in a resting state.
▪ The cell membrane is polarized–negatively charged on the inside and positively charged on the outside.
▪ The charge difference across the membrane can be measured with a pair of microelectrodes connected to an oscilloscope.
- In a resting neuron, this difference, called the resting potential, is about –70 millivolts
• When the neuron is stimulated, sodium gates in its cell membrane open briefly, allowing positively charged sodium ions to run into the axon and causing an
upward spike on the oscilloscope screen.
• The change in charge causes potassium (+K) gates to open, and positively charged potassium (+K) ions flow out of the neuron.
▪ This outflow of potassium reestablishes a negative charge inside the axon and causes a drop on the oscilloscope screen.
• The brief reversal in the charge difference across the neuron membrane, which appears as a spike on the oscilloscope screen, is called an action potential or
neural impulse.
How does an action potential travel along an axon?
• As sodium gates close and potassium gates open at one point on the membrane, sodium gates are triggered to open at the next point along the axon membrane.
Chapter 3 - Biology of Behaviour Page 3
, How does an action potential travel along an axon?
• As sodium gates close and potassium gates open at one point on the membrane, sodium gates are triggered to open at the next point along the axon membrane.
▪ It's this sequential opening of sodium gates at adjacent points that moves the action potential along the axon.
• After the firing of an action potential, some time is needed before the sodium gates are ready to open again.
▪ Until they're ready to reopen, the neuron cannot fire.
▪ The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin.
- This "down time" isn't very long, only 1 or 2 milliseconds.
• The size of an action potential is not affected by the strength of the stimulus—a weaker stimulus does not produce a weaker action potential.
▪ If the neuron receives a stimulus of sufficient strength, it fires, but if it receives a weaker stimulus, it doesn't.
- This is referred to as the "all-or-none law."
• Even though the action potential is an all-or-nothing event, neurons can convey information about the strength of a stimulus.
▪ They do so by varying the rate at which they fire action potentials.
- Generally, a weaker stimulus, such as light pressure on the skin, will produce a slower volley of neural impulses than would a stronger stimulus,
such as greater pressure.
The Neural Impulse: Using Energy to Send Information
The Neuron at Rest: A Tiny Battery
• Hodgkin and Huxley learned that the neural impulse is a complex electrochemical reaction.
• Both inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions.
• Positively charged sodium (Na+) and potassium ions and negatively charged chloride (+K) ions flow back and forth across the cell membrane.
• Negatively charged chloride (Cl-) ions flow back and forth across cell membrane.
• BUT ions DO NOT cross at the same RATE (Semi-permeable Membrane).
• Difference in flow rates leads to a slightly higher concentration of ions inside the cell.
• = Resting potential of a neuron is its stable, negative charge when the cell is inactive (–70 millivolts) – not stimulated.
• Sodium channels are closed.
The electrochemical properties of the neuron allow it to transmit signals. The electric charge of a neuron can be measured with a pair of electrodes connected to
an oscilloscope, as Hodgkin and Huxley showed with a squid axon. Because of its exceptionally thick axons, the squid has frequently been used by scientists
studying the neural impulse. (a) At rest, the neuron’s voltage hovers around –70 millivolts. (b) When a neuron is stimulated, a brief jump occurs in its voltage,
resulting in a spike on the oscilloscope recording of the neuron’s electrical activity. This change in voltage, called an action potential, travels along the axon like
a spark travelling along a trail of gunpowder.
Chapter 3 - Biology of Behaviour Page 4
