NEURONS / NERVE CELL

Reaction to any stimulus is given by NEURON/NERVE CELL that are specialized to carry “messages” through an electrochemical process.

NEURON is the BASIC structural and functional unit of  the nervous system. It posses electrical excitability, the ability to respond to a a stimulus and convert it into an action potential. Nerve impulses travels at a speed ranging from 0.5-130 meters per second.

Action potential- it is an electrical signal that propagates along the surface of the membrane of the neuron.

Neurons come in many different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide. Some of the biggest neurons have cell bodies that are 100 microns wide. (Remember that 1 micron is equal to one thousandth of a millimeter!).

Nerve Cell Function

  • Communication and coordination,
  • Sensory nerves, which carries the message to brain.
  • Allows us to react to a stimulus.
  • They conduct electrical impulses away from the neuron’s cell body.
  • It carries messages from other neurons to a cell body.

Adaptations of the nerve cells: 

The adaptations and modifications in a normal cell diffrentiate it into a  nerve cell that  perform the special task  like conduction of the impulses , respond to chemical and physical stimuli, release of chemical regulators and many more
The ways in which it is adapted to do this are as follows:

  • long axon so can deliver an action potential a long way.
  • Axon is surrounded by myelin sheith which conducts the action potential by saltatory conduction. (myelin sheith containes nodes of ranvier. These have concentrated sodium channels which allows for the action potential to jump from node to node. This increases the speed of the action potential, thus sending a message faster)
  • Synapses at the ends of the nerve cell ensure that the message can only travel in one direction. The presynaptic cleft produces a chemical neurotransmitter. This stimulates receptors on the post synaptic cleft to start a new action potential. There are only recetors on the post-synaptic cleft so therefore the action potential can only travel in one direction. (unidrectional)

Neurons are similar to other cells in the body because:

  1. Neurons are surrounded by a cell membrane.
  2. Neurons have a nucleus that contains genes.
  3. Neurons contain cytoplasm, mitochondria and other organelles.
  4. Neurons carry out basic cellular processes such as protein synthesis and energy production.

 Neurons differ from other cells in the body because:

  1. Neurons have specialize cell parts called dendrites and axons. Dendrites bring electrical signals to the cell body and axons take information away from the cell body.
  2. Neurons communicate with each other through an electrochemical process.
  3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

    Classificaton of neuron 

    FUNCTIONAL CLASSIFICATION- it is on the basis on the direction in which they conduct impulses. The classification is a sfollows

  • Sensory nerve cell/Afferent cells – The nerve cell that transfer the information about stimuli such as light, heat or chemicals from both inside and outside of our body to our central nervous system.
  • Motor nerve cell / Efferent cells– The nerve cell that passes the instructions from our central nervous system to other parts of our body. For example – to  muscles or to our  glands
  • Association nerve cell/ Interneurons – The nerve cell, which connects to our sensory and motor neurons

           STRUCTURAL CLASSIFICATION-  it is on the basis of the number of processes                         extending from the cell body

  • Multipolar neurons- these have several dendrites and one axon. These are present in the brain and spinal cord.
  • Bipolar neurons- it has one main dendrite and one axon. These are present in the retina of the eye, the inner ear and in the olfactory area of the brain.
  • Unipolar neurons– these are sensory neurons that begin in the embryo as bipolar neurons. during development, the axon and dendrite fuse into a single process that divides into two branches a short distance from the cell body. Both branches have the characteristic structure and function of an axon. 
  • Apolar neurons- No definite dendron/ axon. cell process are either absent or if present are not differentiated in axon and dendrons. Nerve impulses radiates in all directions. the neurons of hydra, amacrine cell of retina
  • Pseudounipolar- nerve cell has only axon but a small process develop from axon which acts as dendron. these are found in the dorsal root ganglia of spinal cord.

 

Parts of neurons

1. Cell body/ soma

  • The cell body (soma) is the factory of the neuron.
  • It contains a uni nucleated cytoplasm. 
  • Centriole is absent or immaturely present in the nerve cell thus cell division is absent.
  • It produces all the proteins for the dendrites, axons and synaptic terminals and contains specialized organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and polysomes to provide energy and make the parts, as well as a production line to assemble the parts into completed products.

2) Cytosol – Is the watery and salty fluid with a potassium-rich solution inside the cell containing enzymes responsible for the metabolism of the cell. 

2.a) Nucleus

2.b) Golgi Apparatus – membrane-bound structure that plays a role in packaging peptides and proteins (including neurotransmitters) into vesicles. 

2.c) Polyribosomes – there are several free ribosomes attached by a thread. The thread is a single strand of mRNA

2.d)  Neuronal membrane

2.e) Mitochondrium – this is the part of the cell responsible for the supply of energy in the form of ATP (adenosine triphosphate)

2.f) Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum

2.g) Nissl Bodies / Tigroid body- Groups of ribosomes used for protein synthesis. 

3. Neuronal Membrane 

The neuronal membrane serves as a barrier to enclose the cytoplasm inside the neuron, and to exclude certain substances that float in the fluid that bathes the neuron. The membrane with its mosaic of proteins is responsible for many important functions:

  • accumulating nutrients, and rejecting harmful substances,
  • catalyzing enzymatic reactions, 
  • establishing an electrical potential inside the cell,
  • conducting an impulse
  • being sensitive to particular neurotransmitters and modulators

The membrane is made of lipids and proteins – fats and chains of aminoacids. The basic structure of this membrane is a bilayer or sandwich of phospholipids, organized in such a way that the polar (charged) regions face outward and the non polar regions face inward.

The external face of the membrane contains the receptors, small specialized molecular regions which provide a kind of “attachment port” for other external molecules, in a scheme analogous to a a key and a keyhole. For each external molecule there is a corresponding receptor. Whenever receptors become attached to a molecule, some alterations of the membrane and in the interior of the cell ensue, such as the modification of permeability to some ions. 

4. Dendrites

These structures branch out in treelike fashion and serve as the main apparatus for receiving signals from other nerve cells. They function as an “antennae” of the neuron and are covered by thousands of synapses. The dendritic membrane under the synapse (the post-synaptic membrane) has many specialized protein molecules called receptors that detect the neurotransmitters in the synaptic cleft. A nerve cell can have many dendrites which branch many times, their surface is irregular and covered in dendritic spines which are where the synaptic input connections are made.

 

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4. Axon

Usually a long process which often projects to distant regions of the nervous system. The axon is the main conducting unit of the neuron, capable of conveying electrical signals along distances that range from as short as 0.1 mm to as long as 2 m. Many axon split into several branches, thereby conveying information to different targets. Many neurons do not have axons. In these so-called amacrine neurons, all the neuronal processess are dendrites. Neurons with very short axons are also found. The axons of many neurons are wrapped in a myelin sheat, which is composed of the membranes of intersticial cells and is wrapped around the axons to form several concentric layers. The myelin sheath is broken at various points by the nodes of Ranvier, so that in cross section it looks like a string of sausages. The myelin protects the axon, and prevents interference between axons as they pass along in bundles, sometimes thousands at time.

The cells that wrap around peripheral nerve fibers – that is, nerve fibers outside of the brain and spinal cord – are called Schwann cells (because they were first described by Theodor Schwann). The cells that wrap around axons within the central nervous system (brain and spinal cord) are called oligodendrocytes. The axon, with its surrounded sheath, is called a nerve fiber. Between each pair of sucessive Schwann cells is a gap of a node of Ranvier.

The Axon Hillock

The axon hillock is where the axon is joined to the cell. It is from here that the electrical firing known as an action potential usually occurs.

 

5. Nerve Ending (Presynaptic Terminals)

Synapses are the junctions formed with other nerve cells where the presynaptic terminal of one cell comes into ‘contact‘ with the postsynaptic membrane of another. It is at these junctions that neurons are excited, inhibited, or modulated. There are two types of synapse, electrical and chemical. Electrical synapses occur where the presynaptic terminal is in electrical continuity with the postsynaptic. Ions and small molecules passing through, thus connecting channels from one cell to the next, so that electrical changes in one cell are transmitted almost instantaneously to the next. Ions can generally flow both ways at these junctions i.e. they tend to be bi-directional, although there are electrical junctions where the ions can only flow one way, these are know as rectifying junctions. Rectifying junctions are used to synchronise the firing of nerve cells.

Chemical synaptic junction is more complicated. The gap between the post- and presynaptic terminals is larger, and the mode of transmission is not electrical, but carried by neurotransmitters, neuroactive substances released at the presynaptic side of the junction. There are two types of chemical junctions. Type I is an excitatory synapse, generally found on dendrites, type II is an inhibitory synapse, generally found on cell bodies. Different substances are released at these two types of synapse. The direction of flow of information is usually one way at these junctions.

 

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Each terminal button is connected to other neurons across a small gap called a synapse. The physical and neurochemical characteristics of each synapse determines the strength and polarity of the new input signal. This is where the brain is the most flexible, and the most vulnerable. Changing the constitution of various neurotransmitter chemicals can increase or decrease the amount of stimulation that the firing axon imparts on the neighbouring dendrite. Altering the neurotransmitters can also change whether the stimulation is excitatory or inhibitory.

Axons Dendrites
  • Take information away from the cell body
  • Smooth Surface
  • Generally only 1 axon per cell
  • No ribosomes
  • Can have myelin
  • Branch further from the cell body
  • Bring information to the cell body
  • Rough Surface (dendritic spines)
  • Usually many dendrites per cell
  • Have ribosomes
  • No myelin insulation
  • Branch near the cell body
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