# Neurons --- **Neurons** are cells found in [[neural tissue]] that communicates with other cells over special connections called **[[synapse]]**. Neurons have different jobs, depending on what types of signals they carry. [[peripheral nervous system|Sensory neurons]] carry sensory information for the PNS Neurons activate when a certain threshold of stimuli is met, triggering an [[action potential]] that sends a chain reaction all the way to the other end of the system. ![[neuron.png]] ## Neuron Structure The neuron has a distinctive structure of three parts, the main cell body, the axon and the dendrites. Different neurons have different features, but there are some similarities across all neurons The **cell body** or **soma** is the central portion of the cell where most of the inner workings live—such as the cytoplasm, nucleus, and organelles. The **dendrites** are the branch-like arms that reach out from the main body. The **axon** is the long, slender tail-like piece of the neuron. The nerve impulses move along the axon, away from the body of the cell. Axons are often bundled units that function together. In the [[central nervous system|CNS]] they are called "tracts" and in the [[peripheral nervous system|PNS]] they are called "[[nerves]]". At the end of the axon there are processes called **axonal terminals** which have sac-like endings called **synaptic bulb**—which is what releases neurotransmitters into the [[synapse]]. Between the soma and the axon is the **axon hillock**. Some tracts or nerves are protected and electrically insulated by a [[myelination]]. But even those that are are not myelinated down the whole length. There are gaps where the myelination is missing, which is actually imporant for allowing the charge to propigate ## Types Neurons come in one of four types of basic configurations: multipolar, bipolar, pseudo-bipolar and anaxonic. - **Multipolar** neurons have many dendrites that extend the [[cell|soma]] off of the soma, and a single prominent axon. - **Bipolar** neurons have a single dendrite and a single axon. - **Pseudo-bipolar** neurons may look like bipolar neurons, but it's actually comprised of a single process that sort of splits off in two directions. - **Anoxonic** neurons lack an axon and just have a bunch of dendrites all over. ![[basic neurons.png]] ## Axonal Transport Neurons can transport materials along its axion in either direction, but the means are quite different. - **Anterograde transport** is transport along the axon starting in the soma and moving towards the axonal terminals. This type of transport is very important as the soma is where all the [[ribosome|ribosomes]] are that can make the substances that are released in the synaptic bulbs. - **Retrograde transport** is transport along the axon starting at the axonal terminals and moving towards the terminals. For example, broken down neurotransmitters that have been [[neurotransmitter|reabsorbed]] are sent back to the soma by retrograde transport. There is also an ability of neurons to send retrograde messages this way. - **Fast axonal transport** can go in either direction, and it is facilitated by [[microtubules]] >[!health] Rabies! >The [[rabies]] virus reaches your central nervous system via a cut anywhere on your body by hitching a ride on the axonal transport of your neurons. ## Neural Action Potentials The neuron's main job is to send [[action potential|action potentials]]. Like action potentials vary from type-of-cell to type-of-cell, so to do they vary from neuron to neuron. However, there are some overall similarities we can describe. The graph of a neural action potential follows the standard graph pretty closely. The [[resting membrane potential|RMP]] is generally around -70mv and the [[action potential|threshold potential]] is around -55mv. At the peak of depolarization the [[membrane potential]] reaches about +30mv before repolarization begins. The action potential is triggered when the neuron receives enough [[graded potential#EPSPs|EPSPs]] to a) counter any [[graded potential#IPSPs|IPSPs]] and b) reach the threshold potential. ![[action potential - neuron.png]] ### Conduction of Action Potentials The neuron is divided into four main sections with regards to action potentials: the **receptive segment**, the **initial segment**, the **conductive segment** and the **transmissive segment** The **receptive segment** is made up of the soma and the dendrites. This segment forms many [[synapse|synapses]] from one or more other neurons. In the plasma membrane there are specially [[channel proteins|chemically-gated channels]] that bind to different [[neurotransmitter|neurotransmitters]], that allow in or out [[sodium|Na+]], [[potassium|K+]], or [[chloride|Cl-]] ions in order to generate [[graded potential#IPSPs|IPSPs]] or [[graded potential#EPSPs|EPSPs]], depending. Action potentials are not generated here because there are no *voltage*-gated sodium channels. The **initial segment** is the axon hillock, and this is where IPSPs and EPSPs total together and if the EPSPs are strong enough, trigger the [[action potential]]. The **conductive segment** is the axon, where the action potentials [[action potential#Conduction|conduct]] all the way down to the axonal terminals and finish at the synaptic bulbs. Unmyelinated axions perform the slow and steady [[action potential#Conduction|continuous conduction]], but [[myelination|myelinated]] axons can perform the faster, jumpier saltatory conduction (see below). The **transmissive segment** is the synaptic bulbs. ![[neuron pumps and channels.png]] ## Saltatory Conduction **Saltatory conduction** is a way of making the internal charge "jump" from one axonal hillock to the next. Essentially the myelanation makes it so the membrane can't depolarize, so somehow the charge zipps really fast between them. If they didn't have the gaps, the ion gradient would just end up dispersing and fizzling out. ___