UNIT II: THE HUMAN BRAIN

How many genes are in the human bod?

• 20,000-25,000!

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How many cells are in the human brain?

• 1 trillion!• Two categories:– Glial Cells– Neurons

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Glial Cells• Three Functions: – Provide scaffolding to support

both developing and mature neurons.

– Wrap around neurons to form insulation from electrical signals

– Releasing chemicals that influence neuron growth

• Make up 90% of cells in brain!

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Neurons• 100 billion cells - both sends and receives messages and

signals (up to 268 mph!)

• Allows us to:– Receive sensory information– Control muscle movement– Regulate digestion– Release hormones– Complete mental processes (thinking, etc.)

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Neurons (cont)

• Each neuron has hundreds of connections– Approximately 100 trillion neural connections in

the brain• Which is more: the world’s communications,

or the brain’s?• 0.00005%

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Neuron Growth

• Humans born with all our neurons that slowly die over our lifetime.

• What two areas of the brain does new research suggest can regrow?– Hippocampus and olfactory bulb

• Humans do have the ability to rewire our brains -> recover from traumatic event

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Alzheimer’s Disease• Excessive buildup of protein amyloid destroys

neurons faster than body can repair them.• All people produce amyloid, Alzheimer’s patients

cannot get rid of it– Teeth plaque

• Only “cure” right now is to slow or stop amyloid buildup

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Neurons – Structure and Function

1. Cell body – egg-shaped structure that provides fuel, manufactures chemicals, maintains working order in the neuron, contains neurons DNA

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Neurons – Structure and Function

2. Dendrites – Branchlike extensions; receive signals from cells or other neurons and pass them to the cell body.

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Neurons – Structure and Function3. Axon – single threadlike structure, carries

messages to other neurons, organs, or muscles. Can be the length of the width of a human hair, or over 3 feet!

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Neurons – Structure and Function

4. Myelin Sheath – looks like separate tube-like segments composed of fatty tissue. Prevents interference from other electrical signals and helps signals travel faster.

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Neurons – Structure and Function

5. Terminal bulbs – look like tiny bubbles at the end of axon’s branches. Acts like a tiny container that stores neurotransmitters. Do not actually touch any neighboring organs, muscles, or cells.

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Neurons – Structure and Function

6. Synapse – Infinitely small gap between terminal bulb and its neighboring organ, muscle, or other neural cells. Terminal bulbs eject neurotransmitters into the synapse to send messages.

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What’s Wrong Here?

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Reattaching Limbs

• Limbs can be reattached because of something we call nerves.

• In whole body except brain/spinal cord• String-like bundles of axons and dendrites• Carry messages from senses, skin, muscles,

and organs

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Reattaching Limbs

• Nerves (but not neurons) have the ability to regrow • Gives us the ability to reattach limbs• Difference between nerve and neuron:– Neuron is individual cell– Nerve is a group of neurons

• Think of nerves as the superfast lane of highway neurons use to get signals to your brain

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Neurotransmission

• Action Potential (tiny electrical current that propels the message through the neuron) travels down the pre-synaptic neuron to terminal bulbs.

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Neurotransmission (cont.)

• Action Potential cannot cross synapse.• To send message, AP causes vesicles to move to

membrane of terminal.

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Neurotransmission (cont.)

• Vesicles attach to membrane and release its contents (neurotransmitters, or NT) into synapse.

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Neurotransmission (cont.)

• Binding of NT to post-synaptic neuron causes AP to activate in next cell.

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Neurotransmission (cont.)• Once message is received, NT release from the receptors. • Some are broken down by enzymes in synapse, others are

sucked back into pre-synaptic neuron by transporter proteins. This is called “reuptake”.

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Neurotransmission (cont.)• This process repeats itself every time a signal is sent, at

speeds well over 200 mph! No wonder you are tired at the end of the day!

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Neurotransmitters• Definition: any of about a dozen chemicals that are made by

neurons and used to communicate to other neurons during physical or mental activities.

• 12 kinds? Don’t they get confused where to go, like a traffic jam?

• Each neurotransmitter has a specific chemical key which allows it open or close a receptor like a lock in a door

• NT can be either excitatory (open lock and turn on neuron) or inhibitory (close lock and turn off neuron)

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