Neurons are specialized cells responsible for transmitting information throughout the human body via electrical and chemical signals. They consist of three main parts: soma, dendrites, and axon, and can be classified into efferent, afferent, and interneurons based on their function. The communication between neurons occurs at synapses, which can be electrical or chemical, each serving different roles in transmitting signals.

1. Neuron

2. • Neurons are responsible for carrying information throughout the human body.
Using electrical and chemical signals, they help coordinate all of the necessary
functions of life.

4. • Neurons can only be seen using a microscope and can be split into three parts:
• Soma (cell body) — this portion of the neuron receives information. It contains the
cell’s nucleus.
• Dendrites — these thin filaments carry information from other neurons to the soma.
They are the “input” part of the cell.
• Axon — this long projection carries information from the soma and sends it off to
other cells. This is the “output” part of the cell. It normally ends with a number of
synapses connecting to the dendrites of other neurons.

5. Types of neurons
• Connection
• Efferent neurons — these take messages from the central nervous system (brain and
spinal cord) and deliver them to cells in other parts of the body.
• Afferent neurons — take messages from the rest of the body and deliver them to the
central nervous system (CNS).
• Interneurons — these relay messages between neurons in the CNS.

6. • Function
• Sensory — carry signals from the senses to the CNS.
• Relay — carry signals from one place to another within the CNS.
• Motor — carry signals from the CNS to muscles.

7. How do neurons carry a message?
• If a neuron receives a large number of inputs from other neurons, these signals add up until they exceed
a particular threshold.
• Once this threshold is exceeded, the neuron is triggered to send an impulse along its axon — this is
called an action potential.
• An action potential is created by the movement of electrically charged atoms (ions) across the axon’s
membrane.
• Neurons at rest are more negatively charged than the fluid that surrounds them; this is referred to as the
membrane potential. It is usually -70 millivolts (mV).
• When the cell body of a nerve receives enough signals to trigger it to fire, a portion of the axon nearest
the cell body depolarizes — the membrane potential quickly rises and then falls (in about 1,000th of a
second). This change triggers depolarization in the section of the axon next to it, and so on, until the rise
and fall in charge has passed along the entire length of the axon.
• After each section has fired, it enters a brief state of hyperpolarization, where its threshold is lowered,
meaning it is less likely to be triggered again immediately.
• Most often, it is potassium (K+) and sodium (Na+) ions that generate the action potential. Ions move in
and out of the axons through voltage-gated ion channels and pumps.

8. Myelin
• Most axons are covered by a white, waxy substance called myelin.
• This coating insulates nerves and increases the speed at which impulses travel.
• Myelin is created by Schwann cells in the peripheral nervous system and
oligodendrocytes in the CNS.
• There are small gaps in the myelin coating, called nodes of Ranvier. The action
potential jumps from gap to gap, allowing the signal to move much quicker.
• Multiple sclerosis is caused by the slow breakdown of myelin.

10. How synapses work
• Neurons are connected to each other and tissues so that they can communicate
messages; however, they do not physically touch — there is always a gap
between cells, called a synapse.
• Synapses can be electrical or chemical. In other words, the signal that is carried
from the first nerve fiber (presynaptic neuron) to the next (postsynaptic neuron) is
transmitted by an electrical signal or a chemical one.

11. Chemical synapses
• Once a signal reaches a synapse, it triggers the release of chemicals
(neurotransmitters) into the gap between the two neurons; this gap is called the synaptic
cleft.
• The neurotransmitter diffuses across the synaptic cleft and interacts with receptors on
the membrane of the postsynaptic neuron, triggering a response.
• Chemical synapses are classified depending on the neurotransmitters they release:
• Glutamergic — releases glutamine. They are often excitatory, meaning that they are
more likely to trigger an action potential.
• GABAergic — release GABA (gamma-Aminobutyric acid). They are often inhibitory,
meaning that they reduce the chance that the postsynaptic neuron will fire.
• Cholinergic — release acetylcholine. These are found between motor neurons and
muscle fibers (the neuromuscular junction).
• Adrenergic — release norepinephrine (adrenaline).

12. Electrical synapse
• Electrical synapses are less common but are found throughout the CNS. Channels called
gap junctions attach the presynaptic and postsynaptic membranes. In gap junctions, the
post- and presynaptic membranes are brought much closer together than in chemical
synapses, meaning that they can pass electric current directly.
• Electrical synapses work much faster than chemical synapses, so they are found in
places where quick actions are necessary, for instance in defensive reflexes.
• Chemical synapses can trigger complex reactions, but electrical synapses can only
produce simple responses. However, unlike chemical synapses, they are bidirectional —
information can flow in either direction.