The Nervous System

Neurons

• Neurons are main cells in nervous system, as well as some of the largest cells in your body (one extends from base of your spine to tip of your big toe!)

• Function: to carry electrical signals from brain to all parts of your body

• Brain is the “control panel” – Neurons are the “wires” to external devices

Types of Neurons

• A neuron that carries signals from tissue to the brain is a sensory neuron or afferent neuron.

• A neuron that carries signals from the brain to tissue is a motor neuron or efferent neuron.

Neuron Geometry• The dendrite receives signals from other cells• The axon relays signals from the neuron to the next

cell• The spaces between neurons where signals are

transferred from cell to cell are called synapses

The Brain

• Contains about 100 billion neurons – these are your brain cells

• These neurons each have connections to thousands of neighboring neurons (through synapses)

• Average adult brain has between 100 – 500 trillion synapses

• A child’s brain has about 1 quadrillion synapses!

Why Does the Signal Travel?

• Motion of electrical impulse along a neuron is called an action potential

• For this to happen, there must be a difference in charge accumulation between inside and outside of cell

(a.k.a. a voltage difference)

Ion Separation in Cells

• In most cells, ion pumps in the cell membrane distribute ions differentially between the inside and outside of the cell.

• Ion pumps: For every two positively charged potassium ions pumped into cell, three positively charged sodium ions are pumped out

• This creates an voltage difference of 70 mV across cell membrane (more positive charges are outside than inside)

So How Do Neurons Signal?1. Neuron receives impulse

2. Sodium gates open on cell

3. Positively charged sodium ions flood into cell (since the inside is already less positively charged and since cell pumps them in)

4. As the charge evens out, voltage difference decreases, and more sodium channels open up allowing in even more positive charge – positive feedback loop.

5. Voltage changes from -70 mV to +40 mV.

6. At +40 mV sodium channels close – negative feedback loop

All the sodium ions flooding into the cell creates a force that pushes the sodium ions away to adjacent parts of the cell (like charges repel)

wave of positivity flies down neuron cell

this then signals the next cell

Unfortunately, nerve cells are not very resilient…

Nerve cells in the central nervous system (brain and spinal cord) do not grow back if damaged (at least not currently–we’ll talk about the potential for stem cells to help with this next time) – unlike most other types of cells

Spinal Cord FunctionEach portion of the spinal cord innervates a different part of the body:

• Cervical – diaphragm & arms/hands• Thoracic – chest muscles &

abdominal muscles• Lumbar – Legs• Sacral – Bowel & Bladder

If the spinal cord is damaged, some signals will not get sent and received

Nerve InjuryMyelin (insulator around nerve cell) damage – nerves intact but signals impaired.

Examples: Multiple sclerosis, Guillan-Barre, leukodystrophy.

Spinal Cord InjuryPhysical break or compression in spinal cord. Level of injury indicates level of function.

And again, nerve cells in the central nervous system do not grow back if damaged (at least not currently) – unlike most other types of cells

Next time…

• We’ll talk about the basics of stem cells

• Debate and human cloning papers due next Wednesday