Neurone

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Vidya Acharya, Kim Jackson, Stacy Schiurring, Rucha Gadgil, Manisha Shrestha and Khloud Shreif  

Introduction

The central nervous system is comprised of two main types of cells: glia, which support, protect, and regulate neurons, and neurons, which are the primary computing elements. Neurons transmit information as electrical signals along their axons, connecting with other neurons at synapses.

Three main morphological patterns of neurones are described, depending on the relationship of the cell body to the axon and dendrites

• Multipolar neurons: most common type of neuron. They are located in the central nervous system (brain and spinal cord) and in autonomic ganglia. Multipolar neurons have a single axon and many dendrites extending from the cell body. A typical multipolar neuron consists of soma or cell body, an axon, and dendrites. The axon while the dendrites are receiving afferent signals from their surroundings.
• Bipolar: relatively rare. They are sensory neurons found in olfactory epithelium, the retina of the eye, and ganglia of the vestibulocochlear nerve. Shape consists of one axon and one dendrite that extend from the cell body in opposite directions.
• Unipolar/pseudounipolar: its shape comprises of a single short process that extends from the cell body and then splits into two branches in opposite directions; one branch travels to the peripheral nervous system (PNS) for the sensory reception, and the other to the CNS (central process). These neurons have no dendrites as the branched axon serving both functions. Localization: dorsal root ganglion and sensory ganglia of cranes nerves, and some mesencephalic nucleus

Structure

Neurones vary in morphology and size substantially, but all share a number of features .

The neuron’s shape has a unique feature. Besides the cell body or soma, neurons have specialized thin branches know as dendrites and axons. Neurons receive input from other neurons (afferent signals from their surroundings) through dendrites and communicate information to other cells through axons (transmitting efferent signals).

Cell body

• Contains the nucleus and is the site of synthesis of virtually all neuronal proteins and membranes. Some proteins are synthesized in dendrites, but no proteins are made in axons and axon terminals, which do not contain ribosomes. Proteins and membranes that are required for renewal of the axon and nerve termini are synthesized in the cell body and assembled there into membranous vesicles or multiprotein particles.
• Perikaryon: cytoplasm surrounding the nucleus

Axon

• Axons carry efferent signals
• They terminate in an axon terminal wherein the neurotransmitters, neuromodulators, or neurohormones are released in the conversion of the electrical signal to a chemical signal which can cross the synapse or neuromuscular junction.
• The axon is specialized for conducting a specific type of electric impulse known as an action potential, which travels outward from the cell body toward the axon terminus. An action potential consists of a series of rapid changes in voltage, or electric potential, across the plasma membrane. In a resting (nonstimulated) state, the electric potential across the axonal membrane is approximately −60 mV, meaning the inside is negative relative to the outside. This resting potential is similar to the membrane potential found in most non-neuronal cells. At the peak of an action potential, the membrane potential can rise to as much as +50 mV (making the inside positive), resulting in a net change of about 110 mV. This depolarization of the membrane is quickly followed by a rapid repolarization, which returns the membrane potential to its resting value. These characteristics set action potentials apart from other types of changes in electric potential across the plasma membrane, enabling them to propagate along an axon without diminishing in strength.
• Single process of variable length (even over a metre)
• Axonal transport:  the transportation of materials along the axon of a neuron via the flow of the jellylike fluid (axoplasm) it contains. Transport may be directed away from the cell body (anterograde) or back toward the cell body (retrograde). Also called axoplasmic flow.  Carried out by proteins such as kinesin and dynein..
• Terminates in terminal buttons which release neurotransmitters
• Most of the volume of our brain is occupied by axons, which form a complex network called white matter. Like a maze of airways linking cities around the world, white matter manages communication and co-ordination between the various areas where populations of neurons process information. These areas are located in different parts of the brain, sometimes close to each other, sometimes far away: this is the principle of distributed computing.

Dendrite

• Most neurons have multiple dendrites, which extend out-ward from the cell body and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them inward, in the direction of the cell body. Neuronal cell bodies can also form synapses and thus receive signals.
• Dendrites play a role in protein synthesis and independent signaling functions with other neurons.
• Particularly in the central nervous system, neurons have extremely long dendrites with complex branches. This allows them to form synapses with and receive signals from a large number of other neurons, perhaps up to a thousand.
• Electric disturbances generated in the dendrites or cell body spread to the axon hillock. If the electric disturbance there is great enough, an action potential will originate and will be actively conducted down the axon.

Synapses

Chemical synapse

Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. The synapse consists of:

1. A presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles. At the presynaptic ending an electrical impulse will trigger the migration of vesicles containing neurotransmitters toward the presynaptic membrane. The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft.
2. A synaptic cleft or space between the presynaptic and postsynaptic ending.
3. A postsynaptic ending that contains receptor sites for neurotransmitters. The neurotransmitter molecules diffuse across the synaptic cleft where they can bind with receptor sites on the postsynaptic ending. When a neurotransmitter binds to a receptor on the postsynaptic side of the synapse, it changes the postsynaptic cell's excitability: it makes the postsynaptic cell either more or less likely to fire an action potential. If the number of excitatory postsynaptic events is large enough, they will add to cause an action potential in the postsynaptic cell and a continuation of the "message."

Many psychoactive drugs and neurotoxins can change the properties of neurotransmitter release, neurotransmitter reuptake and the availability of receptor binding sites.

Types of Neurons

3 types of Neurones exist and are programmed to do different things.

1. Motor neurones carry signals from the CNS to the outside parts of the body.
2. Sensory Neurons: The sensory neurones are the portion of the nervous system responsible for processing input from the environment. Beginning with detection through the transfer of stimuli to the central nervous system, the peripheral nerves and their associated receptors rapidly relay information. The peripheral nervous system consists of the somatosensory nervous system and autonomic nervous system: The sensory pathway of the somatosensory system involves spinal nerves which transmit information about the external environment to the spinal cord; The autonomic nervous system has visceral sensory neurons which are responsible for monitoring the internal environment and eliciting appropriate changes in effector organs to maintain homeostasis (visceral sensory nerves transmit pain, stretch, temperature, and chemical change in visceral organs which gets interpreted as sensations like nausea, hunger, gas, cramping, etc).
3. Interneurons sends messages from one neurone to another.

Neural Circuits

The 1.4 kg of jelly-like material found within our skulls is the most complex machine on Earth and perhaps the universe. Its phenomenal features would not be possible without the hundreds of billions of neurons that make it up, and, importantly, the circuits between those neurons.

• Neurons do not operate independently; they always work in conjunction with one another. They are organized into circuits that process specific kinds of information.
• The synaptic connections that define a circuit are typically made in a dense tangle of dendrites, axons terminals, and glial cell processes that together constitute neuropil (the suffix -pil comes from the Greek word pilos, meaning “felt”) and this is the region where most synaptic connectivity occurs.
• The direction of information flow in any particular circuit is essential to understanding its function. Three classes of neurons are the basic constituents of all neural circuits.
  1. Nerve cells that carry information toward the central nervous system (or farther centrally within the spinal cord and brain) are called afferent neurons
  2. Nerve cells that carry information away from the brain or spinal cord (or away from the circuit in question) are called efferent neurons.
  3. Nerve cells that only participate in the local aspects of a circuit are called interneurons or local circuit neurons.

A simple example is the circuit that facilitates the myotatic (or "knee-jerk") spinal reflex. The above figure illustrates the monosynaptic reflex pathway that stimulates muscle contraction and the di-synaptic reflex pathway that inhibits the contraction of the antagonistic muscle.

Significant progress has been made in understanding how various simple neural networks are involved in information processing and the regulation of behavior. Feedforward excitation and feedforward inhibition play key roles in mediating reflex behaviors. Lateral inhibition is crucial for edge enhancement. Recurrent excitation serves as an important mechanism for memory formation, while recurrent inhibition can be vital for generating locomotor behaviors. Additionally, convergence and divergence are integral components of these microcircuits. Similar network motifs are also found in biochemical and gene networks.

The neuronal networks also mediate more complex, higher-order functions of the brain. Advances in this field are becoming possible due to electrophysiological and optical recording techniques, along with modern imaging methods such as functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI). fMRI enables researchers to identify brain areas engaged in cognitive tasks, while DTI visualizes the pathways connecting different brain regions.

Neuropathy

The peripheral nerves connect the brain and spinal cord to the rest of the body. Peripheral neuropathy is a relatively common neurological disorder that can occur as the result of a wide range of underlying conditions or diseases. Diabetes is perhaps the most common – approximately 50 per cent of diabetics suffer from diabetic neuropathy.

Neuron Wound Healing

Injured nerves should be repaired as early as possible. The current criterion standard technique of repair remains epineural suture approximation of transected nerves with fine monofilament suture with interposition grafting using an autologous nerve if a tensionless repair cannot be achieved.

Factors influencing nerve healing potential following repair include:

• Age: the most important factor. Younger patients have more positive outcomes (compared to adults) and the recovery time is quicker. Studies have shown that, even in the pediatric population, patients < 6 years of age recovery more quickly compared to their adolescent counterparts.
• Level of injury: distal injuries have a better chance of recovery compared to more proximal injuries
• Injury pattern: sharp lacerations/transections do better than crush injuries
• Delay in repair: chronic injuries do poorly
• Secondary tissue damage occurs with excessive handling; Nerve endings should be properly aligned (and not under tension) during a direct repair; Excessive suture material used during the repair risks the development of a neuroma at the site of repair.

Neurodegenerative Disorders

Many conditions can affect neurons and their transmission. Some of these include neurodegenerative diseases e.g. Motor Neurone Disease (MND), some dementias, Huntington disease, spinal and bulbar muscular atrophies, as well as lesions of the motor neurons.

Many neurodegenerative conditions, from glaucoma to Alzheimer's disease, are characterized by injury to axons. Injury to axons often leads to neuronal impairment and cell death.

Adult Neurogenesis

"Adult neurogenesis" refers to the ability of the central nervous system (brain and spinal cord) to generate new neurons in adulthood called adult generated neurons.

Summary

• Cell body of a neuron contains the nucleus and lysosomes and is the site of synthesis and degradation of virtually all neuronal proteins and membranes.
• Axons are long processes specialized for the conduction of action potentials away from the neuronal cell body.
• Action potentials are sudden membrane depolarizations followed by a rapid repolarization. They originate at the axon hillock and move toward axon terminals, where the electric impulse is transmitted to other cells via an electric or chemical synapse.
• Most neurons have multiple dendrites, which receive chemical signals from the axon termini of other neurons.
• When an action potential reaches a chemical synapse, a neurotransmitter is released into the synaptic cleft. Binding of the neurotransmitter to receptors on the postsynaptic cell changes the ion permeability and thus the electric potential of the postsynaptic plasma membrane.
• Neurons are organized into circuits. In a reflex arc, such as the knee-jerk reflex, interneurons connect multiple sensory and motor neurons, allowing one sensory neuron to affect multiple motor neurons. One muscle can be stimulated to contract while another is inhibited from contracting.