1. Nervous System: Central Nervous System:
(The center of integration and control)
1. The brain
2. The spinal cord
Peripheral Nervous System:
The nervous system outside of the brain and spinal cord

2. Basic Nerve cell Structure: Neurons
Dendrites
Cell body
Axon
Axon terminal
The functional and structural unit of the nervous system is the neuron.
It is specialized to conduct information from one part of the body to another.
There are different types of neurons but most have certain structural and functional characteristics in common:
Dendrites: Provide a large surface area for connecting with other neurons, and carry nerve impulses towards the cell body.
Cell Body
Axon: A single long axon carries the nerve impulse away from the cell body.
Axon Terminal
Most neurons have many companion cells called Schwann cells, which wrap their cell membrane around the axon many times in a spiral to form a thick insulating lipid layer called the myelin sheath.
Nodes of Ranvier: space between successive Schwann cells along axon... the inter-node area is a non-myelinated area
Nerve impulse can be passed from the axon of one neuron to the dendron of another at a synapse. A nerve is a discrete bundle of several thousand neuron axons.

3. 3 main types of Neurons: sensory neuron motor neuron relay neuron
Humans have three types of neuron:
Sensory neurons have long axons and transmit nerve impulses from sensory receptors all over the body to the central nervous system (Brain and Spinal Cord).
Motor neurons also have long axons and transmit nerve impulses from the central nervous system (Brain and Spinal Cord) to effectors (muscles and glands) all over the body.
Interneurones (also called connector neurons or relay neurons) are usually much smaller cells, with many interconnections. Carry information between other neurons only found in the brain and spinal cord.
sensory neuron
relay neuron
motor neuron

4. Conduction of Nerve Impulses:
Online animation
Stimuli (think of them as energy forms) are detected by the receptors and turned into an nerve impulse (chemical energy).
Nerve impulses from sensory nerves are conducted to the central nervous system along sensory neurons.
The impulse is sent to the relay neurons that move it around inside the central nervous system (brain and spine).
Motor neurons take the relayed nerve impulse to the effectors (often muscles) which then produce the response.

5. Nerve impulses are conducted along the neuron

6. Online video
Online animation

7. Resting Potential: Na/K Pump+
The Na/K pump actively transport Na ions out of the cell and K ions in. For every 3 Na ions pumped out, only 2 K ions are pumped in. The sodium-potassium pump creates a concentration and electrical gradient for Na+ and K+, which means that K+ tends to diffuse (‘leak’) out of the cell and Na+ tends to diffuse in. BUT, the membrane is much more permeable to K+, so K+ diffuses out along its concentration gradient much more faster than Na diffuses in, thus inside become more negative. A higher concentration of organic anions is found on the inside of the membrane than on outside. Normal diffusion of Cl into the cell. -

8. Na+ and Cl- are more concentrated outside the cell
K+ and organic anions (organic acids and proteins) are more concentrated inside.

9. The Action Potential Activation gates
of the Na+ channels are open, but the K+ channels
remain closed. Na+ ions rush into the cell, and
the interior of the cell becomes more positive.
Na+ close and potassium channels
open. K+ ions leave the cell and
the loss of positive charge causes
the inside of the cell to become
more negative than the outside.
A stimulus opens some Na+ channels.
If the Na+ influx achieves threshold potential,
then additional Na+ gates open, triggering an action potential.
When the cell membranes are stimulated, there is a change in the permeability of the membrane to sodium ions (Na+).
The membrane becomes more permeable to Na+ and K+, therefore sodium ions diffuse into the cell down a concentration gradient. The entry of Na+ disturbs the resting potential and causes the inside of the cell to become more positive relative to the outside.
As the outside of the cell has become more positive than the inside of the cell, the membrane is now DEPOLARISED.
When enough sodium ions enter the cell to depolarise the membrane to a critical level (threshold level) an action potential arises which generates an impulse.
Throughout depolarisation, the Na+ continues to rush inside until the action potential reaches its peak and the sodium gates close.
Both Na+ & K+ channels are closed, and the
membrane’s resting potential is maintained.
Na+ channels are closed,
but the slower K+ remain open. Within a
millisecond, the resting state is restored.

10. Nerve impulse along a non-myelinated neuron
animation

11. Synaptic Transmission
animation
The junction between two neurons is called a synapse.
An action potential cannot cross the synaptic cleft between neurons, and instead the nerve impulse is carried by chemicals called neurotransmitters.
These chemicals are made by the cell that is sending the impulse (the pre-synaptic neuron) and stored in synaptic vesicles at the end of the axon.
The cell that is receiving the nerve impulse (the post-synaptic neuron) has chemical-gated ion channels in its membrane, called neuroreceptors.
These have specific binding sites for the neurotransmitters

12. Synaptic Transmission
animation
At the end of the pre-synaptic neuron there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell.
2. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis.
3. The neurotransmitters diffuse across the synaptic cleft.
4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open. In the example shown these are sodium channels, so sodium ions flow in.
5. This causes a depolarisation of the post-synaptic cell membrane, which may initiate an action potential.
6. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neuron by endocytosis and used to re-synthesis more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on.