The Nervous System

General Functions:

1) Input
2) Processing
3) Output

General Macroanatomy:

Central Nervous System (CNS): Consists of the Brain and Spinal cord.

Peripheral Nervous System (PNS): All nervous system tissue outside the CNS.

General Microanatomy:

Nervous tissue is made up of 2 main groups:

Neurons: Deal with moving and processing information. They fall into 3 main groups:
  • Sensory Neurons: Bring information from the body and environment towards the CNS.
  • Motor Neurons: Usually send motor information from the CNS out into the body.
  • Interneurons: Are found in the CNS and deal with processing information from sensory neurons and other interneurons.

The 3 main parts of a neuron are shown below:

  • Dendrites: Move messages towards the cell body (input)
  • Soma: contains the cell's organelles and helps process excitatory and inhibitory messages (processing)
  • Axon: Moves messages away from the cell body (output)

Neurolgia: These are support cells that help neurons. We are only concerned with:
  • Oligodendrocytes: These cells wrap the axons of neurons in a fatty layer known as a myelin sheath. This sheath helps speed up the signals that move through neurons

Resting and Action Potentials

Resting Potential occurs in cells when there are more positive ions outside of the cell membrane than inside. Due to this imbalance there is a measurable voltage (-70mV). This imbalance is maintained by sodium
potassium pumps that continuously move sodium out (3 ions at a time) and potassium in (2 ions at a time) to the cell. The first minute of this clip is decent.

A neuron can be excited to threshold potential. This occurs by changes in the cell allowing positively charged ions into the cell. This moves the voltage more towards zero (called depolarization). If the voltage hits threshold potential an action potential is generated.

Once threshold is reached, (1) sodium channels in the cell membrane open up. This causes sodium to move into the cell (remember sodium ions are at a higher concentration outside the cell as compared to inside the cell. Diffusion occurs meaning that substances move down their concentration gradient (from high concentration to low concentration).). This influx of sodium causes the voltage to move towards (and past) zero voltage (known as depolarization).
(2) Sodium channels shut and potassium channels open. This causes potassium to move out of the cell (again diffusion. Remember that potassium is at a high concentration inside the cell and a low concentration outside the cell). This is referred to as repolarization.
(3) The potassium channels shut and the sodium potassium pump restores the cell membrane to resting potential.
Heres a good clip of an action potential.

The graph shows a hyperpolarization. This prevents the particular area of the cell membrane from generating another immediate action potential. That allows action potentials to travel in one direction.

All action potentials look the same (same amplitude and time). This is referred to as the All-or-none princple. A neuron can adjust its response to stimuli by changing the frequency (how many times per second) that an action potential is sent.

An action potential on one part the cell membrane causes the nearby parts of the cell membrane to get closer (and maybe reach) to threshold. This allows the action potential 'signal' to spread or travel throughout the entire cell. This is how an action potential moves down an axon... but how does it move from one neuron to the other?

Synaptic Transmission

This process allows the axon of one neuron (known as the presynaptic neu
ron) to communicate with the dendrites of another neuron (the postsynaptic neuron). The signal can either be excitatory (this opens sodium channels and helps move the voltage more positive... towards threshold) or inhibitory (this opens potassium channels and helps move the voltage more negative... away from threshold).

Step 1: Action potential reaches the synaptic knob. This releases neurotransmitter into the synaptic cleft (or synapse).

Step 2: The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic dendrite. Depending on which receptors are bound (this depends on which type of neurotransmitter is released), this depolarizes or hyperpolarizes the dendrite of the postsynaptic neuron.

Step 3: Enzymes (like acetylcholinesterase) will remove the neurotransmitter in the synaptic cleft, ending the signal.

Neuronal Pools

A neuronal pool is a group of neurons working together towards a specific function.

(b) Divergence: Occurs when one neuron sends information to many. external image image024.gifExample: A stubbed toe sensory signal can cause a sensation of pain, the desire to run around, and a verbal 'curse word'.

(a) Convergence: Occurs when many neurons can influence one. Example: Seeing a flat round object, smelling delicious cheese, bread and tomato sauce, and hearing the word 'Pizza' can cause feelings of hunger.

(d) Parallel Processing: Occurs when several neuronal pools receive the same input, but these neuronal pools do different processes with it at the same time. Example: Running the pacer can cause increases in breathing rate, decreases in blood flow to the digestive system, increases in heart rate, constant readjustment of your eyes, and rapid cycling of your leg muscles in a specific motor pattern (run).

Reflex Arcs

A reflex is an involuntary process that occurs to help maintain homeostasis.
It consists of a sensory neuron detecting a stimulus and then activating a motor neuron which causes a response. For example, the eye detects motion coming rapidly towards it, the sensory neurons involved activate muscle neurons which close the eye lids and jerk the head backwards (a 'flinch').

Spinal Reflexes are processed in the spinal cord and do not involve the brain. Most of our reflexes tested in the lab are spinal reflexes.