The Peripheral nervous system Anatomy & Physiology/Nervous System

Neurons Nerve cells are called neurons These cells are specialized to transmit electrical impulses from one part of the body to another. Although they can vary in structure and appearance, all neurons have the same basic features:

Diagram of a Neuron

Nerve impulses Electrical signals move through the body along pathways of neurons that branch out from the brain and spinal cord. These pathways form complex networks that allow the brain to communicate almost instantly with any part of the body.

Nerve Fibers Bundles of nerve fibers in the body are called nerves, while in the brain they are called “tracts” The corpus callosum is an example of a tract. In the brain, the white matter is made of myelinated tracts, while the grey matter is made of un-myelinated tracts

Nerve Classification There are many types of nerves in the body and two main ways to group or classify them: 1. Functional Classification = grouping neurons based on the direction the impulse travels 2. Structural Classification = grouping neurons based on the number of parts connected to the cell body

Functional Groups There are three main functional groups of neurons: Sensory (Afferent) Neurons = neurons that carry information from receptors in the skin or organs towards the brain. Motor (Efferent) = neurons that carry information away from the brain and to the muscles or glands. Association (Inter) Neurons = neurons that connect motor and sensory neurons on long neural pathways.

Structural Groups There are three main structural groups of neurons: Multipolar neurons = neurons with an axon and several dendrites attached to the cell body. These are the most common neurons Bipolar neurons = neurons with one axon and one dendrite attached to the cell body. Unipolar neurons = neurons with a short “stem” attached to the cell body. Generally an axon and several dendrites will extend out from that stem.

Generating Impulses Many different types of stimuli excite neurons to become active and generate an impulse or electrical signal: Light excites neurons in the eyes Sound excites neurons in the ears Pressure excites neurons in parts of the skin Most neurons in the body are excited by chemicals called neurotransmitters.

neurotransmitters Neurotransmitters = chemicals that are stored in the axon terminals of association neurons and used to transmit nerve impulses from one neuron to another. Some examples of common neurotransmitters include: Dopamine Seratonin GABA Adrenaline

polarization Regardless of how the neuron is stimulated, the result in always the same. A resting (non-stimulated) neuron is polarized, which means there are more positively charged ions outside the neuron membrane than there are inside.

depolarization Normally, sodium (Na+) ions cannot move back and forth freely across the neuron membrane. When the neuron becomes stimulated (by a neurotransmitter or any other stimulus), the sodium channels in the membrane open. As the channels open, sodium rushes in (following the rules of diffusion) and the charges even out, the neuron is said to be depolarized.

Action potential Depolarization of the neuron generates a long distance electrical impulse called an action potential. This impulse is an all or nothing response, meaning that it either happens or it doesn’t. An action potential never goes part way along a neuron or weakens with distance.

Repolarization Almost immediately after the rush of sodium ions into the cell, the membrane again becomes impermeable to sodium ions. Sodium-Potassium pumps in the neuron membrane are activated and the original concentrations of sodium ions are restored. Until the neuron becomes repolarized it cannot conduct another impulse.

Crossing the synapse When an action potential reaches the end of a neuron, it does not continue across the synapse, or space between neurons. Instead, the action potential triggers vesicles filled with neurotransmitters to bind to the neuron membrane and release the neurotransmitter into the synapse. The neurotransmitters cross the synapse and bind to specific receptors on the receiving neuron. This stimulates a new action potential.

Graphing an action potential