neurons Flashcards
(18 cards)
neurons
Neurons are cells that send nerve impulses (electrical messages) through the body.
They may look different depending on their type or function, but they all share basic parts:
A cell body (with a nucleus)
One or more processes (branches that carry signals)
cell bodies
The cell body is the metabolic center of the neuron. Its transparent nucleus contains a large nucleolus. The cytoplasm surrounding the nucleus contains the usual organelles, except that it lacks centrioles (which confirms the amitotic nature of most neurons). The rough ER, called Nissl (nis′l) bodies, and neurofibrils (intermediate filaments that are important in maintaining cell shape) are particularly abundant in the cell body.
The nucleloulus inside the nucleus is the ribosome factory and the ribosomes stay in the nissl factory to make proteins
processes
Length:
Some neuron processes (especially axons) can be very long—up to 3 feet (about 1 meter) or more, not dendrites.
Two types of processes:
Dendrites
Many branching fibers extending toward the cell body.
They receive incoming signals from other neurons.
Usually, there are multiple dendrites per neuron.
Axon
Only one axon per neuron.
It carries nerve impulses away from the cell body to:
Muscles,
Glands, or
Other neurons (at their dendrites or cell bodies).
axon
An axon occasionally gives off collateral branches along its length, but it always branches at the end into many smaller extensions. These endings are called axon terminals.
Inside the axon terminals are tiny sacs called vesicles, which hold neurotransmitters—the chemicals that help send signals.
The axon carries nerve impulses away from the neuron’s cell body. When the impulse reaches the axon terminals, it triggers the release of neurotransmitters into the space between neurons (called the synaptic cleft) or between a neuron and another type of cell, like a muscle cell.
-the impulses have to be converted into the neurotransmiters to go into the synaptic cleft.
synapse: synaptic cleft
Each axon terminal is separated from the next neuron or its target by a tiny gap called the synaptic (sĭ-nap′tik) cleft. Such a functional junction, where an impulse is transmitted from one neuron to another, is called a synapse (syn = to clasp or join). Although they are close, neurons never actually touch other neurons or target cells. You will learn more about synapses and the events that occur there a bit later.
myelin sheats
Myelin
A whitish, fatty material that covers most long nerve fibers.
Acts like insulation on electrical wires.
Functions:
Protects and insulates the axon.
Increases the speed of nerve impulse transmission.
🔹 Schwann Cells (only in the Peripheral Nervous System)
Make the myelin sheath by wrapping themselves around the axon.
Think of it like wrapping a jelly roll: each Schwann cell wraps around the axon many times.
As it wraps, the inner parts (cytoplasm) get squeezed out, leaving layers of membrane.
The final product is a tight myelin sheath.
🔹 Neurilemma
The outermost part of the Schwann cell, outside the myelin sheath.
Contains most of the Schwann cell’s remaining cytoplasm.
Important for nerve repair in the PNS.
🔹 Nodes of Ranvier
Gaps between Schwann cells in the myelin sheath.
These gaps help speed up the impulse even more by allowing the impulse to “jump” from node to node (this is called saltatory conduction).
⚡ Analogy:
Think of a nerve fiber like an electrical wire:
Myelin = insulation on the wire.
Schwann cells = the tape or rubber that wraps the insulation around the wire.
Neurilemma = the final outer layer of that tape.
Nodes of Ranvier = little gaps in the insulation that actually help electricity travel faster by allowing jumps.
oligondrocytes
As mentioned previously, myelinated fibers are also found in the central nervous system. Oligodendrocytes form CNS myelin sheaths (see Figure 7.3d). In the PNS, it takes many Schwann cells to make a single myelin sheath; but in the CNS, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. Thus, in the CNS, one oligodendrocyte can form many myelin sheaths. Although the myelin sheaths formed by oligodendrocytes and those formed by Schwann cells are similar, the CNS sheaths lack a neurilemma. Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the central nervous system.
multiple sclerosis (MS)
The importance of myelin insulation is best illustrated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, the electrical current is short-circuited and may “jump” to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The affected person may have visual and speech disturbances, lose the ability to control his or her muscles, and become increasingly disabled. Multiple sclerosis is an autoimmune disease in which the person’s own immune system attacks a protein component of the sheath. As yet there is no cure, but injections of beta interferon (a hormonelike substance released by some immune cells) appear to hold the symptoms at bay and provide some relief. Other drugs aimed at slowing the autoimmune response are also being used, though further research is needed to determine their long-term effects.
neuron bodies
Nuclei are found in the brain and spinal cord, and they are involved in processing information.
Each nucleus usually has a specific job, like:
Controlling movement (e.g., basal nuclei in the brain)
Regulating emotions
Processing sensory information
Coordinating reflexes
✅ Think of CNS nuclei as control centers for different functions — like stations inside a central processing hub (your brain/spinal cord).
🟠 What Ganglia Do (PNS)
Ganglia are found outside the CNS, often near the spinal cord or along nerves.
They mainly act as relay points or processing centers for information coming into or going out of the CNS.
Examples:
Sensory ganglia (like the dorsal root ganglion) hold the cell bodies of sensory neurons. These neurons carry signals from the body to the CNS.
Autonomic ganglia help control automatic functions like heart rate, digestion, and breathing.
✅ Think of ganglia as relay stations on the edge of the network — collecting info from the body or sending out commands.
Nuclei (CNS) and ganglia (PNS) are made up of neuron cell bodies.
Neuron cell bodies contain rough ER (called Nissl bodies in neurons).
This is where the proteins are made, which are essential for:
Neurotransmitter production
Cell maintenance
Growth and repair
These clusters (nuclei and ganglia) act as processing or control centers:
They analyze, sort, or relay incoming information.
Then they send signals to the right place — other neurons, muscles, or glands.
tracts vs nerves
Tracts (CNS):
Bundles of nerve fibers (axons) inside the Central Nervous System.
Can carry sensory or motor signals.
Example: the corticospinal tract carries motor commands from the brain to the spinal cord.
🔸 Nerves (PNS):
Bundles of nerve fibers (axons) in the Peripheral Nervous System.
These are what you usually think of as “nerves” — like the sciatic nerve or optic nerve.
gray vs white matter
As a general rule, the white matter consists of dense collections of myelinated fibers (tracts), and gray matter contains mostly unmyelinated fibers and cell bodies.
when they say mylanaited they mean axons.
dendrites ends
Sensory Neurons:
Their dendrite endings (the receiving ends) are connected to sensory receptors.
These receptors are specialized to detect changes in your environment (like heat, pressure, or touch).
🧠 Types of Sensory Receptors Mentioned:
1. Cutaneous Sense Organs (in the skin):
Detect touch, pressure, temperature, and pain.
Example: Receptors in your skin that let you feel a hot surface or a soft fabric.
- Proprioceptors (in muscles and tendons):
Detect body position and movement — like knowing where your arm is even with your eyes closed.
Help with balance and coordination.
- Pain Receptors:
Are actually bare nerve endings (not complex structures).
They are the most numerous receptors.
Important for protection, because pain alerts your brain to potential danger or injury.
📝 Important Note:
Even non-painful receptors (like touch or temperature) can cause pain if the stimulus is too strong — like extreme heat or pressure.
🔁 Summary:
propia
The proprioceptors detect the amount of stretch, or tension, in skeletal muscles, their tendons, and joints. They send this information to the brain so that the proper adjustments can be made to maintain balance and normal posture. Propria comes from the Latin word meaning “one’s own,” and the proprioceptors constantly advise our brain of our own movements.
effrent pathways
They carry impulses away from the CNS to:
Muscles (to make them move)
Glands (to make them secrete something)
Viscera (internal organs, like the stomach or heart)
These are called efferent neurons:
Think: “E” for Exit — signals exit the CNS.
Their cell bodies are usually located inside the CNS (in the brain or spinal cord), even though their axons can reach far into the body.
interneurons
These are the “middlemen” in the nervous system.
They connect sensory neurons (which bring information into the CNS) to motor neurons (which send commands out from the CNS).
They are only found inside the CNS (brain and spinal cord).
Their cell bodies are located in the CNS — just like motor neurons.
multi neurons
Have many processes — 1 axon and 2 or more dendrites.
Most common type of neuron in the body.
Found in:
Motor neurons
Interneurons
Location: Mainly in the CNS.
✅ Function: Carry out motor commands and processing.
bipolar neurons
Have two processes — 1 axon and 1 dendrite.
Rare in adults.
Found only in some special sense organs:
Retina of the eye
Inner ear
Olfactory area of the nose
✅ Function: Specialized for sensory processing in the senses.
Unipolar Neurons
One single process leaves the cell body but quickly splits into two branches:
One goes to the body (peripheral process)
One goes to the CNS (central process)
Found in:
Sensory neurons in the PNS (especially in ganglia)
✅ Weird fact: Although it looks like one long axon, one part receives info (acts like a dendrite), and the other sends it (acts like an axon). So the signal can move both toward and away from the cell body.
