Biological Bases of Behaviour' Lecture 3: Brain Cells and Neural Communication

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Biological Bases of Behaviour. Lecture 3
Brain Cells and Neural Communication
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Learning Outcomes.

• At the end of this lecture you should be able to
  
• 1. Describe the key elements of a nerve cell
  (neuron).
• 2. Describe the main support cells of the CNS.
• 3. Explain what is meant by the term 'membrane
  potential'.
• 4. Explain how an action potential is initiated
  and conducted down the axon

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1. Neurons.

• According to Williams Herrup (1988) the adult
  human brain contains around 100 billion neurons
  (nerve cells).
• These are specialised cells which receive and
  transmit information.
• They vary in size and shape but all consist of
  the same basic structures

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Structure of a Neuron.

• a) Cell body (soma).
• Contains the nucleus, which houses the
  chromosomes.
• The bulk of the cell consists of cytoplasm - a
  jelly like substance containing structures which
  carry out certain functions

soma
nucleus
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Structures in the Cytoplasm.

• Mitochondria Extract energy from the breakdown
  of nutrients and provide energy in the form of
  adenosine triphosphate (ATP).
• Endoplasmic reticulum Store and transport
  chemicals through the cytoplasm. 2 forms
• i) Rough endoplasmic reticulum contain ribosomes
  which are involved in protein synthesis.
• ii) Smooth endoplasmic reticulum transport
  substances around the cytoplasm and produce lipid
  (fat).
• Golgi apparatus A special type of endoplasmic
  reticulum breaks down substances no longer
  required by the cell.
• The plasma membrane separates the inside of the
  cell from the outside, it is selectively
  permeable with charged ions only able to pass
  through protein channels.

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b) Dendrites.

• These are the information-receiving parts of a
  neuron.
• Dendrites receive chemical information across a
  tiny gap called a synapse.
• The surface of a dendrite is lined with synaptic
  receptors.
• Outgrowths called dendritic spines increase the
  surface area available for synaptic
  communication.

dendrites
Dendritic spines
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c) Axon.

• This is the information-sending part of the
  neuron
• A neural impulse (action potential) flows along
  the axon.
• Many vertebrate axons are covered with an
  insulating substance called a myelin sheath.
• This consists of segments separated by
  unmyelinated regions called nodes of Ranvier.

myelin
Axon
Node of Ranvier
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The Information Flow.

• Action potentials flow along the axon to the
  presynaptic terminals.
• Axons that send information to the periphery are
  called efferent axons (e.g. motor neurons).

Presynaptic terminals
Flow of information
A motor neuron
Muscle fibre
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The Information Flow, continued.

• Axons that receive information from the periphery
  are called afferent axons (e.g. sensory endings
  in the skin).
• Thus, motor neurons act as efferents from the
  nervous system, sensory neurons act as afferents
  into the nervous system.
• So, efferent out, afferent in.

Sensory endings
Information flow
Cross section of skin
A sensory neuron
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d) Presynaptic Terminals.

• At the end of an axon are the presynaptic
  terminals (or terminal buttons).
• When an action potential reaches the terminal
  buttons they secrete a transmitter substance
  which travels across the synapse to the next
  neuron in the chain.
• The neurotransmitter either excites or inhibits
  the postsynaptic receptors (dendrites) of another
  neuron.
• Thus an individual neuron receives information
  via its dendrites from the terminal buttons of
  axons from other neurons, the terminal buttons of
  its axon send information to other neurons.

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Presynaptic Terminals.
Axons from other nuerons influence neuron A
Neuron B
Neuron A
Message flows down axon of neuron A to influence
neuron B
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2. Support Cells.

• Neurons have a high metabolic rate and must be
  constantly supplied with oxygen and glucose or
  they will die.
• The various support cells are thus very
  important.
• Glial cells hold neurons in place, control their
  supply of chemicals, insulate them, and remove
  neurons that have died. There are several forms
• i) Astrocytes (astroglia) Provide physical
  support to neurons and clear up debris (called
  phagocytosis).
• ii) Oligodendrocytes Produce myelin in the CNS.
  In the PNS the same function is provided by
  Schwann Cells. These digest dying cells and then
  guide the axons to re-grow to a limited extent.
• This does not happen in the CNS so that nerve
  damage (e.g. in spinal neurons) is to be
  permanent.

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Electrical Activity Within a Neuron.

• A microelectrode is placed in the axon of a giant
  squid.
• An electrode is placed in the surrounding medium.
• Both are connected to a voltmeter.
• The inside of an inactive axon is negatively
  charged with respect to the outside.
• This resting potential is
• -70mV.

voltmeter
microelectrode
electrode
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The Action Potential.

• A positives charge applied to the inside of the
  axon makes it more positive (depolarisation).
• If a sufficiently strong charge is applied then
  the threshold of excitation is reached, and the
  neuron produces an action potential.
• Here the membrane potential is rapidly reversed
  and becomes strongly positive (up to 40mV) with
  respect to the exterior.
• The membrane potential quickly returns to normal,
  but first it briefly overshoots its resting
  potential and drops to around -75mV
  (hyperpolarisation).
• This entire process takes about 2msec.

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The Action Potential.
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The Membrane Potential.

• The electrical charge within the axon results
  from the balance between two opposing forces
• i) Diffusion Molecules distribute themselves
  evenly throughout the medium in which they
  reside.
• ii). Electrostatic pressure Particles with the
  same electrical charge repel one another while
  particles with the opposite charge attract one
  another.
• The environment inside the axon and in the fluid
  surrounding it contain different ions.
• Organic ions (A-) only found inside the axon.
• Potassium (K) found predominantly inside the
  axon.
• Chloride ions (Cl-) found predominantly outside
  the axon.
• Sodium ions (Na) found predominantly outside the
  axon.

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Resting Potential.

• The axonal membrane is selectively permeable.
• At rest, ion channels permit potassium and sodium
  to pass through slowly.
• Most of the sodium channels remain closed.
• The sodium-potassium pump expels sodium ions and
  draws in potassium ions in the ratio of 3 sodium
  out to 2 potassium in.
• During an action potential the sodium channels
  open and allow sodium ions to flood into the
  axon.

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Events During the The Action Potential.
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After the Action Potential.

• Neurons may have different thresholds of
  excitation but all obey the rule that once the
  threshold is reached, an action potential is
  triggered this is called the all-or-none
  rule.
• Following the action potential, the sodium gates
  remain closed for around 1ms and so further
  action potentials cannot be triggered regardless
  of the stimulation.
• This is called the absolute refractory period.
• The sodium gates then open but the potassium
  gates remain open for a further 2-4ms ensuring
  the no action potentials can be generated.
• This is called the relative refractory period.
• The axon cannot cope with repeated excitation as
  the sodium-potassium pump cannot keep up, as a
  result sodium accumulates within the axon and no
  more action potentials are possible. Scorpion
  venom keeps open the sodium channels and causes
  paralysis.

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Conduction of the Action Potential in
Unmyelinated Axons.

• Each point along the axon membrane generates the
  action potential. The next area of membrane is
  depolarised, reaches its threshold and generates
  another action potential. In this manner the
  action potential passes down the axon like a
  wave.

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Conduction of the Action Potential in
Myelinated Axons.

• These axons are covered with an insulating layer
  of myelin, separated by small unmyelinated gaps
  (nodes of Ranvier).
• Action potentials travel down the axon reducing
  in strength until they reach the next node where
  another action potential is triggered.

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Saltatory Conduction.

• The jumping of action potentials from one node to
  another in myelinated axons is referred to as
  saltatory conduction.
• There are two advantages to this
• 1. Energy is saved as sodium-potassium pumps are
  only required at specific points along the axon.
• 2. Conduction of an action potential is much
  faster within a myelinated axon (around 120 m/sec
  as opposed to around 35 m/sec) in unmyelinated
  ones.