Biology 30
Unit 1:
The nervous system plays a key role in homeostasis.
It acts as a relay center between the external environment and the internal environment.
The nervous system is a complex communication system which sends messages from one area of the body to another. The messengers are electrochemical messengers: hormones and nerve impulses. Nerve impulses are much faster than hormones.
There are more than 100 billion nerves cells in the brain and each nerve cell can have up to 100,000 connections that means 10’15 possible routes.
Nervous system is made up of 2 types of cells:
1. Neurons: functional units of the nervous system
2. Glial cells (glue): support for neurons, also nourishes, removes wastes and defends against infection
Most nerves lie deep in the body to be protected from injury except the ulnar nerve (elbow) which causes the tingling sensation when triggered.
Three types or neurons:
1. Sensory neuron
2. Interneuron
3. Motor neuron
Sensory-interneurons-CNS-motor neurons
Reflex arc- uses fewer neurons to transmit a message therefore it is much faster.
Types of reflexes
1. Knee jerk reflex (patellar)
2. Flexor reflex
3. Achilles reflex (walking)
4. Babinski under 11/12
5. Abdominal reflex
Order of events?
1. Touch receptor is stimulated
2. Sensory information is relayed from the sensory neuron to spinal cord
3. Sensory information is received by interneurons in the spinal cord sent to brain
4. Sensory information is relayed to the motor neuron
5. Motor neuron activates muscle cell to contact

The structure of a neuron:
Neurons exist in many different shapes and sizes however all neurons are similar in their basic shape and components
All neurons have:
Dendrites- short branching ends that receive a nerve impulse
Cell body- part of the neuron that contains the nucleus
Axon- extension of the cell body that carries impulses away from the cell body
Can be enclosed by a fatty insulation(myelin sheath)
(myelinated or unmyelinated) that protects the neuron and speeds the rate of the nerve impulse transmission.
-Myelin sheath- made of Schwann cells (glial cells)
-Nodes of Ranvier- gap between Schwann cells
Grey matter: unmyelinated. Nerve cells that don’t have a myelin sheath
White matter: myelinated. Nerve cells that have a myelin sheath
Neurilemma- Membrane that surrounds the axon and the Schwann cells which allows neurons to regenerate
An electrochemical impulse occurs when neurons establish a voltage difference between the inside and the outside of the cell membrane

Resting membrane
-When the neuron is not being stimulated
-The neuron is said to be POLARIZED
-Potassium (K) ions are inside the cell membrane and sodium (NA) outside
-Usual electrical potential is -70mV

Acton potential
-The neuron is being stimulated
-The cell membrane is more permeable to sodium therefore sodium (Na) ions rush inside the cell
-Cell membrane is DEPOLARIZED
-The usual action potential is -55mV
Action potential is -55mV once it begins sodium rushes in until we get a +35mV charge

Steps to an action potential:
1. Neuron receives a stimulus
2. The cell membrane becomes more permeable to sodium (Na)
3. Sodium gates open and sodium rushes in by diffusion, requires lots of ATP
4. Inside is now +35mV, an action potential has been reached and the sodium gates close (now the cell is depolarized)
The cell now needs to re-polarize:
-A sodium-potassium pump moves potassium ions at the cell and sodium ions in using ATP (active transport)
-Only takes about 1 to 10 ms and this is called the Refractory period
*Nerves cannot be activated until the cell has repolarized*
Sometimes potassium moves inside the cell slower than sodium moves out. This causes even more negatively charged cell membrane called Hyperpolarization

**In myelinated neurons nerve impulses travel by SALTATORY CONDUCTION. Jumping from one node of Ranvier to the next. This allows for faster transition**