Neurons

Neural signals can be transmitted much more quickly, in the range of milliseconds,  than can hormonal signals, in the range of seconds, minutes, or hours.

 

Neural signals can be sent at speeds up to 100 meters per second.

 

 

Neural signalling is an all-or-nothing  action, whereas hormonal signalling is an action that can be continuously variable as dependent upon hormone concentration.

Interneurons connect neurons to other neurons within the same region of the brain, or spinal cord in networks.

A typical neuron consists of a cell body, dendrites, and an axon.

Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometers and branching multiple times, giving rise to a complex dendritic tree.

An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 meter in humans or even more in other species.

Nerve fibers are often bundled into fascicles.

In the peripheral nervous system, bundles of fascicles make up nerves.

The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon.

Axon may branch hundreds of times before it terminates.

At most synapses, signals are sent from the axon of one neuron to a dendrite of another.

There are neurons, however that lack dendrites, or have no axon.

Synapses can occur axon to axon or dendrite to dendrite.

All neurons are electrically excitable.

Each neuron maintains a voltage gradient across its membrane by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane.

Each neuron generates intracellular/extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium.

Cross-membrane voltage changes can alter the function of voltage-dependent ion channels.

If the voltage changes by a large amount, an electrochemical pulse, an action potential is generated,

Action potentials travel rapidly along the cell’s axon, and activates synaptic connections with other cells when it arrives.

In most cases, neurons are generated by special types of stem cells.

Adult brain neurons generally do not undergo cell division.

Neurogenesis largely ceases during adulthood in most areas of the brain.

Astrocytes are star-shaped glial cells that can turn into neurons by virtue of the stem cell characteristic pluripotency.

Evidence for generation of substantial numbers of new neurons has been demonstrated in two brain areas, the hippocampus and olfactory bulb.

Along the axon are a number of protuberances labeled as myelin sheaths.

The nervous system is constituted by neurons and glial cells,

Glial cells give neurons structural and metabolic support.

Majority of neurons belong to the central nervous system, but some reside in peripheral ganglia, and many sensory neurons are situated in sensory organs such as the retina and cochlea.

The neuron cell body is usually compact, while the axon and dendrites are filaments that extrude from it.

Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometers from the soma.

The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances, giving rise to hundreds of branches.

An axon usually maintains the same diameter as it extends.

The soma may give rise to numerous dendrites, but never to more than one axon.

Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon.

With a typical synapse, contact occurs between the axon of one neuron and a dendrite or soma of another.

Synaptic signals may be excitatory or inhibitory.

The net excitation received by a neuron generates a brief pulse called an action potential.

The action potential, originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes.

Neurons do not lack a soma, but there are neurons that lack dendrites, and others that lack an axon.

In addition to the typical axodendritic and axosomatic synapses, there are axoaxonic and dendrodendritic synapses.

The key to neural function is the synaptic signaling process

Synaptic signaling is partly electrical and partly chemical.

The cell body of the neuron is enclosed by a plasma membrane.

The plasma membrane has a bilayer of lipid molecules with many types of protein embedded in it.

A lipid bilayer includes ion channels that permit electrically charged ions to flow across the membrane, and ion pumps that actively transport ions from one side of the membrane to the other.

The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline.

Voltage differential, functions as a source of energy for an assortment of voltage-dependent protein processes that is embedded in the membrane, and provides a basis for electrical signal transmission between different parts of the membrane.

Neurons communicate by chemical and electrical synapses, as neurotransmission, also called synaptic transmission.

Electrical synapses are direct, cytoplasmic connections between neurons.

Neurotransmitter release triggered by the action potential of the excitable membrane of the neuron, known as a wave of depolarization.

Neurons are highly specialized for the processing and transmission of cellular signals, that are performed in different parts of the nervous system.

Neurons exist in a wide variety of shapes, sizes, and electrochemical properties.

The soma of a neuron can vary from 4 to 100 micrometers in diameter.

The soma is the body of the neuron.

The soma contains the nucleus, and most protein synthesis occurs here.

The nucleus can ranges in size from 3 to 18 micrometers in diameter.

The dendrites of a neuron are cellular extensions with many branches.

This overall shape and structure is referred to as a dendritic tree.

The majority of input to the neuron occurs via the dendritic spine.

The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length.

The axon carries nerve signals away from the body of the neuron.

The axon can also carry some types of information back to it.

Many neurons have only one axon.

But an axon may undergo extensive, enabling communication with many target cells.

The axon hillock is where the axon emerges from the soma.

The axon hillock is part of the neuron that has the greatest density of voltage-dependent sodium channels.

The axon hillock is the most easily excited part of the neuron, and is the spike initiation zone for the axon.

The axon hillock has the most negative action potential threshold.

The axon and axon hillock are generally involved in information outflow

The axon and axon hillock can also receive input from other neurons.

The axon terminal contains synapses.

Synapses are specialized structures where neurotransmitter chemicals are released to communicate with target neurons.

Axons and dendrites in the central nervous system are typically only about one micrometer thick.

Axons and dendrites in the peripheral nervous system may be much thicker.

The body or soma is usually about 10–25 micrometers in diameter.

The body or soma is often is not much larger than its cell nucleus.

The longest axon reaches from the base of the spine to the toes. and can be over a meter long.

Sensory neurons can have axons that run from the toes to the posterior column of the spinal cord, over 1.5 meters in adults.

Fully differentiated neurons do not undergo mitosis.

Neurons throughout the brain can originate from neural stem cells through the process of neurogenesis, and are particularly concentrated in the subventricular zone and subgranular zone.

Nissl bodies are microscopic clumps of endoplasmic reticulum and associated ribosomal RNA seen when nerve cell bodies are stained with a basophilic dye.

Nissl bodies are involved in protein synthesis, indicating that nerve cells are very metabolically active.

The cell body of a neuron is supported by structural proteins called neurofilaments, which are assembled into larger neurofibrils.

Some neurons also contain pigment granules, such as neuromelanin and lipofuscin, both of which accumulate with age.

Other structural proteins that are important for neuronal function are actin and the tubulin of microtubules.

Actin is predominately found at the tips of axons and dendrites during neuronal development.

Axons almost never contain ribosomes.

Dendrites contain granular ribosomes, in diminishing amounts as the distance from the cell body increases.

Neurons exist in a number of different shapes and sizes.

Neurons can be classified by their morphology and function.

Neurons are grouped into two types; type I with long axons used to move signals over long distances and type II with short axons.

Type I neurons, represented by spinal motor neurons, consists of a cell body called the soma and a long thin axon covered by the myelin sheath.

Around the cell body is a branching dendritic tree that receives signals from other neurons.

The end of the axon has branching terminals, the axon terminal, that release neurotransmitters into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron.

Types of neurons:

1 Unipolar neuron

2 Bipolar neuron

3 Multipolar neuron

4 Pseudounipolar neuron

Most neurons characterized as:

Unipolar or pseudounipolar: dendrite and axon emerging from same process.

Bipolar: axon and single dendrite on opposite ends of the soma.

Multipolar: two or more dendrites, separate from the axon:

Golgi I: neurons have long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.

Golgi II: neurons whose axonal process projects locally.

Anaxonic refers to neurons whose axon cannot be distinguished from dendrites.

Unique cells include:

Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and cerebellum.

Betz cells, large motor neurons.

Lugaro cells, interneurons of the cerebellum.

Medium spiny neurons, most neurons in the corpus striatum.

Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron.

Pyramidal cells, neurons with triangular soma, a type of Golgi I.

Renshaw cells, neurons with both ends linked to alpha motor neurons.

Unipolar brush cells, interneurons with unique dendrite ending in a brush-like tuft.

Granule cells, a type of Golgi II neuron.

Anterior horn cells, motoneurons located in the spinal cord.

Spindle cells, interneurons that connect widely separated areas of the brain.

Afferent neurons convey information into the central nervous system and are also called sensory neurons.

Efferent neurons transmit signals from the central nervous system to the effector cells and are also called motor neurons.

Interneurons connect neurons within the central nervous system.

Neurons affect other neurons by neurotransmitters that are released that bind to chemical receptors.

The effect upon the postsynaptic neuron is determined by the type of receptor that is activated.

Receptors can be classified broadly as excitatory, inhibitory, or modulatory.

The two most common neurotransmitters in the brain are glutamate and GABA.

Glutamate acts have effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors.

GABA acts on several different types of receptors, but all have effects that are inhibitory.

Cells that release glutamate referred to excitatory neurons.

Cells that release GABA referred to as inhibitory neurons.

Over 90% of the neurons in the brain release either glutamate or GABA.

A single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others.

Parvalbumin-expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex.

Somatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.

Neurons can be classified according to their electrophysiological characteristics:

Some neurons are typically constantly active.

Neurons that fire in bursts are called phasic.

Some neurons have. high firing rates- some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells.

Acetylcholine is released from presynaptic neurons into the synaptic cleft.

Acetylcholine acts as a ligand for both ligand-gated ion channels and metabotropic muscarinic receptors.

Acetylcholine is synthesized from choline and acetyl coenzyme A.

GABA is one of two neuroinhibitors in the CNS, the other being Glycine.

GABA has a homologous function to ACh, gating anion channels that allow Cl− ions to enter the post synaptic neuron.

Cl−ions causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative.

For an action potential to fire, a positive voltage threshold must be reached.

GABA is synthesized from glutamate neurotransmitters by the enzyme glutamate decarboxylase.

Glutamate is one of two primary excitatory amino acid neurotransmitter, the other being aspartate.

Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA.

Dopamine is connected to mood and behavior, and modulates both pre and post synaptic neurotransmission.

Loss of dopamine neurons in the substantia nigra has been linked to Parkinson’s disease.

Dopamine is synthesized from the amino acid tyrosine.

Tyrosine is catalyzed into levadopa (or L-DOPA) by tyrosine hydroxlase, and levadopa is then converted into dopamine by amino acid decarboxylase.

Serotonin (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory.

Of the four 5-HT receptor classes, 3 are GPCR and 1 is ligand gated cation channel.

Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase.

A lack of 5-HT at postsynaptic neurons has been linked to depression.

Drugs that block the presynaptic serotonin transporter are used for treatment, such as fluoxetine and sertraline.

Neurons communicate with one another via synapses, where the axon terminal of one cell contacts another neuron’s dendrite, soma or, less commonly, axon.

Neurons such as Purkinje cells in the cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells.

Other neurons, such as the magnocellular neurons of the supraoptic nucleus, have only one or two dendrites, each of which receives thousands of synapses.

Synapses can be excitatory or inhibitory and either increase or decrease activity in the target neuron, respectively.

The process of synaptic chemical transmission occurs when an action potential reaches the axon terminal, it opens voltage-gated calcium channels, allowing calcium ions to enter the terminal.

Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft.

The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron.

Cytosolic calcium in the axon terminal triggers mitochondrial calcium uptake, which activates mitochondria to produce ATP to support continuous neurotransmission.

Each of the one hundred billion neurons of the brain has on average 7,000 synaptic connections to other neurons.

Estimated that the brain of a three-year-old child has about 1 quadrillion synapses.

This number declines with age, stabilizing by adulthood.

Estimates vary for an adult, ranging from 100 to 500 trillion synapses.

A signal propagating down an axon to the cell body and dendrites of the next cell.

The cell membrane of the axon and soma contain voltage-gated ion channels that allow the neuron to generate and propagate an electrical signal.

Signals are generated and propagated by charged-carrying ions including sodium (Na+), potassium (K+), chloride (Cl−), and calcium (Ca2+).

Stimuli that can activate a neuron leading to electrical activity include: pressure, stretch, chemical transmitters, and changes of the electric potential across the cell membrane.

Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential.

To minimize metabolic expense while maintaining rapid conduction, many neurons have insulated sheaths of myelin around their axons.

Myelin sheaths are formed by glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

The myelin sheath enables action potentials to travel faster than in unmyelinated axons while using less energy.

The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier, which contain a high density of voltage-gated ion channels.

Demyelination of axons in the central nervous system occurs in multiple sclerosis.

Some neurons do not generate action potentials, but instead generate a graded electrical signal,.

Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances.

The conduction of nerve impulses is an example of an all-or-none response.

Greater intensity of stimulation does not produce a stronger signal but can produce a higher frequency of firing.

There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing.

Tonic receptors respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second.

Other receptor types are quickly adapting or phasic receptors, where firing decreases or stops with steady stimulus.

The pacinian corpuscle has concentric layers around the axon terminal, and when pressure is applied so that the corpuscle is deformed, a mechanical stimulus is transferred to the axon, which fires.

If the pressure is steady at the pacinian corpuscle there is no more stimulus.

Such neurons respond with a transient depolarization during the initial deformation and again when the pressure is removed, changing the corpuscle’s shape again.

The adult human brain contains about 85-86 billion neurons, of which 16.3 billion are in the cerebral cortex and 69 billion in the cerebellum.

Neuron disorders:

Charcot–Marie–Tooth disease (CMT) is an inherited disorder of neurons characterized by loss of muscle tissue and touch sensation.

Alzheimer’s disease (AD), a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes.

Parkinson’s disease (PD), is a degenerative disorder of the central nervous system that often impairs motor skills and speech.

Myasthenia gravis is a neuromuscular disease manifested by fluctuating muscle weakness and fatigability during simple activities.

Weakness in myasthenia graviis typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine.

Guillain–Barré syndrome – demyelination, or the loss of the myelin sheath insulating the nerves, impairing conduction of signals along the nerve can be impaired, leading to certain neurodegenerative disorders like multiple sclerosis and chronic inflammatory demyelinating polyneuropathy.

Axonal injuries lead to acute axonal degeneration.

Neurogenesis occurs only for a minority of cells, and a vast majority of neurons comprising the neocortex were formed before birth and persist without replacement.