Biological bases of behaviours

The Neuron

What are neurons?

The basic units of the nervous system that coordinate thinking, feeling, & acting

What is the function of neurons

Neurons carry info between cells, muscles, & organs to enable body functions

How many neurons are in the human body?

The human body contains 10-100 billion neurons

What are neurons

Excitable cells that create & propagate neural impulses to transmit information

What is the function of dendrites?

Receive info from other neurons & send it to the cell body

What is the function of the SOMA

The soma (cell body) contains nucleus & processes incoming info

What does the axon do?

Transmits signals away from the soma to other neurons

What is the myelin sheath?

A fatty substance that encases axons, speeding up signal transmissions

What is the function of terminal buttons

Secrete neurotransmitters, which help transmit signals to other neurons

What is a synapse

The gap where info is transmitted between neurons

Sensory (Afferent) Neurons

• Carry info from sensory receptors to the brain

• Responsible for sensations

Interneurons

• Connect neurons together

• Found in the brain & spinal cord

Motor (Efferent) Neurons

• Send commands from interneurons to muscles

• Control voluntary actions (e.g. dancing) & bodily functions (e.g. heartbeat)

Glial Cells (Support Cells)

• Make up >50% of brain volume

• Support neurons; non-excitable cells

Function of Glial Cells

• Supply nourishment to neurons

• Remove waste

• Form myelin sheaths for insulation

Additional Roles of Glial Cells

• Detect neural impulses & send signals to other glial cells

• Play a role in memory formation

Glial Cells & Disease

• Deterioration may contribute to Alzheimer’s disease

What is a Neural Impulse

• An electrochemical reaction

• How neurons communicate with one another

Ions involved in Neural Impulse

• Positive ions: Sodium (Na⁺) & Potassium (K⁺)

• Negative ion: Chloride (Cl⁻)

Ion Movement in Neurons

• Ions flow across the membrane at different rates

• The inside of the neuron is more negative than the outside

Resting Potential

A stable negative charge inside the neuron when at rest

How does a neural impulse move through a neuron

• When activated, positive ions (e.g. sodium) rush into the neuron

• The neuron becomes less negative or positive

Direction of Action Potential

• Travels along the axon in ONE direction only

• Channels then close, & resting potential is restored

Absolute Refractory Period

• Minimum time before another action potential can begin

• Lasts 1-2 milliseconds

Relative Refractory Period

• Higher threshold needed for a new action potential

“All-or-None” Principle

• A neural impulse either fires or it doesn’t

• No partial firing

Action Potential Size

Always the same size, regardless of stimulus strength

How is Stimulus Strength Conveyed

• Stronger stimulus = faster firing rate of action potentials

• Example: Pin prick vs. car running over your toe

What is the Synapse

The point where a neural impulse is transmitted from one neuron to another

Synaptic Cleft

• A gap that separates 2 neurons

• Neurotransmitters jump the gap to send signals

What does the Presynaptic Neuron do

Sends signals

What does the postsynaptic neuron do

Receives signals

Neurotransmitter Release

Action potential triggers the release of neurotransmitters

Where are Neurotransmitters Stored

Stored in the synaptic vesicles inside the presynaptic neuron

How are neurotransmitters released

Vesicles fuse with the membrane & release neurotransmitters into the synaptic cleft

Neurotransmitter Binding

Neurotransmitters bind to specific receptor sites on the postsynaptic neuron

What is Postsynaptic Potential (PSP)

• A voltage change at the receptor site

• Caused when a neurotransmitter binds to a receptor

Effect of PSP on Neural Impulse

• PSPs vary in size

• Can increase or decrease the chance of a neural impulse in the receiving cell

What is Excitatory PSP

Positive voltage shift (more likely to fire)

What is inhibitory PSP

Negative voltage shift (less likely to fire)

Duration of PSP

Lasts only milliseconds

Neurotransmitters Fate after PSP

Neurotransmitters drift away, are broken down by enzyms, or undergo reuptake

Neural Connections

• Each neuron is connected to thousands of other neurons

• Neurons integrate signals from multiple sources

Excitatory PSP in Neural Networks

Enough excitatory PSPs → Action potential fires

Inhibitory PSP in Neural Networks

Many inhibitory PSPs → Cancel excitatory effects

Neural Networks & thought

• Thoughts depend on interconnected neurons

• Neurons are linked by synapses

Synaptic Pruning

• Less active synapses are removed over time

What does Synaptic Pruning do

Help improve neural efficiency