Nervous System

Electrical Conduction & Impulse to Control Body Function

Brain

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

• These neurons each have connections to thousands of neighboring neurons

• Average adult brain has between 100 – 500 trillion synapses

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

Neurons

• Brain Processes Electrical Impulses

• Neurons Transfer Electrical Impulses from Brain to All Portions of the Body

• Brain is the Control Panel – Neurons are the Wires to External Devices (although both portions composed of neurons)

Components of Neurons

• 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

Neurons

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

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

Neuron Communication

• Nerve cells convert electrical signal (action potential) to a chemical signal (neurotransmitter (NT) molecules).

• Dozens of neurotransmitters have been identified, with different functions.

Neuron Communication

• NTs are released by one nerve cell and bind to receptors on the post-synaptic membrane of the next nerve cell.

• The chemical signal is then converted back into an electrical signal.

Nerve Activation

Electrical Conductivity

• Myelin – insulation

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

• Difference in charge accumulation between inside and outside of cell

• The action potential is a self-propagating event that begins at a dendrite and travels down the axon to the end of the neuron.

Ion Separation in Cells

• Ion pumps in cell membrane

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

• Action potential of 70 mV across cell membrane (positive outside and negative inside)

Neuron Signals1. Neuron receives impulse2. Sodium gates open on cell3. Positively charged Na+ ions

flood into cell4. Negative charge inside cell

and active cell pumping pull in the positive charge

5. As action potential decreases, more sodium channels open up allowing in more positive charge – positive feedback.

6. Action potential changes from -70 mV to +40 mV.

7. At +40 mV sodium channels close – negative feedback

Neuron Signals

• Na channels in the cell membrane are voltage dependent. As Na ions flow through one channel, the altered voltage causes the adjacent Na channel to open. This series of events continues, thus propagating an action potential along the cell membrane.

• This propagation can occur at 100 m/s (over 200 miles/hour)!

Spinal Cord

• A bundle of nerves which connect the brain to the body

• Each portion of the spinal cord innervates a different part of the body

Spinal Cord Function

• Cervical – diaphragm & arms/hands

• Thoracic – chest muscles & abdominal muscles

• Lumbar – Legs• Sacral – Bowel &

Bladder

Nerve Injury & SCI

• Myelin damage – nerves intact but signals impaired. Multiple sclerosis, Guillan-Barre, leukodystrophy.

• Physical break or compression in spinal cord. Level of injury indicates level of function.

• Nerve cells in the central nervous system (brain and spinal cord) do not grow back if damaged – unlike some other types of cells (e.g. skin).

Myelin Disorder