03. Brain Communication

Hyperpolarisation

Further polarise the cell, make it more negative. Inhibits the likelihood of action potential occurring

Depolarisation

Make it more positive, towards neutral. Excitatory and increases the chance of the cell generating an action potential

Post synaptic potentials

Resting membrane potential is influenced by incoming signals from other cells - hyperpolarisation or depolarisation.

Travel across the neuron almost instantaneously, as they travel they decrease in size

Integration of Signals

The balance between excitatory and inhibitory PSP input determines whether an action potential fires

Action Potentials

The firing of a neuron, transmitting a signal

Action Potential Generation

When the integration of input achieves the threshold of excitation at the axon hillock

Action Potential Phases

1. Depolarisation

2. Peak

3. Repolarisation

4. Hyperpolarisation

Function of Action Potentials

Large swings in opposite polarity, non-decremental so are able to carry the signal for long distances

Propagation

An electrical signal or nerve impulse travels along a neuron’s axon. One-way movement of the action potential along the cell membrane

Synaptic Terminal

• The junction between a terminal button and another neuron

• The sender is the presynaptic neuron

• The receiver is the postsynaptic neuron

Electrical Synapses

• The result of a narrow gap between the pre- and postsynaptic neurons, known as a gap junction

• This small gap permits electrical signals to pass directly from one cell to the next, making the system faster and bidirectional

Chemical Synapses

• Depends on the release of chemicals from presynaptic cell, which are received and have an effect on post synaptic cell

• The gap between synapses is large, and the post synaptic membrane contains receptors that receive the chemical transmitters

Transmitter release

• Neurones contain bubble like structures that are filled with chemicals called vesicles

• These chemicals act as a form of transmission or communication in the post-synaptic neurone

NT Receptors

• Membrane spanning proteins

• The part exposed to the extracellular space recognises and binds the transmitter to bring about a function that has an effect on the target cell

Neurotransmitters

A chemical released by the presynaptic neurone to bring about an effect in the post-synaptic neurone

Amino Acid Neurotransmitters

• The molecule building blocks of proteins

• Obtained from proteins we eat or synthesise

• Found at fast-acting direct synapses

• Glutamate - most prevalent excitatory NT

• GABA - most prevalent inhibitory NT

Neuropeptides

Large molecules

Over 100 identified, loosely grouped

Desensitisation of Neurotransmitters

Receptors cannot withstand constant exposure to NTs, if they are overexposed their ability to respond is impaired

Mechanisms for clearing unused NTs

• Enzymatic Degradation

• Reuptake

• Diffusion

• Glia

Enzymatic Degradation

• Enzymes break down the neurotransmitter into its constituent parts

• These parts can no longer activate a receptor, but can be re-absorbed into the cells to be reused

Reuptake

• The presynaptic cell reabsorbs the neurotransmitter after it contacts a receptor

• Repackaged to be used again

Agonist Chemicals

Increase or promote activity

Antagonist Chemicals

Decrease or inhibit activity

Cell development

• Zygote, begins process of cell proliferation

• 2 blastomeres

• 4 blastomeres

• Morula, ball of cells

• Blastulae, hollow ball of cells

Neural Plate

• First observable development of the nervous system

• 3 weeks after conception, patch of the ectoderm becomes distinguishable as the neural plate

• Develops to form neural groove and neural tube

Neural Tube

• The anterior end develops 3 swellings that become the forebrain, midbrain and hindbrain

• These then develop into distinct brain structures and systems

Cell Migration

• Once cells have been created, they migrate to an appropriate location

• They are still immature neurones at this stage, no axons or dendrites

• Glia make scaffolding for migration

• After migration cells aggregate to form various neural structures

Cell Differentiation

• Once neurones aggregate, differentiation occurs

• Axons and dendrites begin to grow as the cells develop depending on their purpose and location

Neuronal Death

• More neurones are produced during gestation than required

• Superfluous cells die

• Either pre-programmed (aptosis) or synaptic rearrangement (unnecessary connections die, necrosis)

Synaptogenesis

• Formation of synaptic connections

• Integral to brain activity and communication

• Many synapses formed are eventually lost, overproduction allows for plasticity and learning

Myelination

Increases the speed of axonal conduction and parallels functional development

Dendritic Branching

• Rapid process

• Generation of new dendrites which leads to creation of new synapses

Synaptic Pruning

• ‘Use it or lose it’ principle

• Synapses that are frequently used have strong connections while the rarely used synapses are eliminated

• Carried out by microglia

Sensory Deprivation

Animals reared in the dark have fewer synapses in visual areas as adults, and have problems perceiving depth

Sensory Enrichment

Thicker cortices with more dendrites and more synapses per neurone