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Home , Pseudounipolar neuron, Unipolar neuron

Paper 12: Membrane Biophysics Module 25: Structure of Nerve, , Blood brain barrier generating nerve impulse

Nervous system contains millions of (nerve cells). Neurons are highly specialized to transmit messages from (nerve impulses) one part of your body to another. All neurons have a cell body and one or more fibres. The length of these fibres varies from microscopic to over one metre. There are two different kinds of nerve fibres: a. fibres that carry information towards the cell body, called , and b. fibres that carry information away from it, called . Nerves are tight bundles of nerve fibres. Neurons are sensitive to various types of stimuli such as heat, cold, light, dark and pressure.

Objectives:

 Structure of nerve  Types of neurons  Conduction of nerve signals  Blood-brain barrier  Types of

Introduction

The coordinates voluntary and involuntary actions of the body and transmits signals to and from different parts of its body. In it consists of two main parts, the (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and . The PNS consists mainly of nerves. Nerves that convey signals from the brain are called motor or efferent nerves, while those nerves that send out information from the body to the CNS are called sensory or afferent. Most nerves serve both functions and are called mixed nerves.

‘Neuron’ is a common term for a nerve cell. ‘Nerve’ commonly refers to bundles of nerve fibres from different neurons. Neurons originate process, transmit, and receive nerve impulses. They are connected to other neurons or to cells in muscles, organs, or glands. Nerve impulses travel electrically along the neuron and are transmitted by chemical messengers (neurotransmitters) to the next neuron across a tiny gap, called a , between the neuron and the neighboring cell, which is known as the target cell. In addition to neurons, the nervous system contains large numbers of other cell types, known as neuroglia. The functions of neuroglia are to protect, nourish, and support neurons. Although neurons come in different shapes and sizes they have some common characteristics.

1. Structure of Nerve

Neuron: a Nerve cell with all its processes. Neuron is the structural and functional unit of the nervous system. A typical neuron consists of a cell body or and two types of processes, axons and dendrites (fig.1).

1.1 Parts of nerve cell and their functions

 Dendrites, Cell body, Neuronal membrane, , Nerve ending

1.1.a. Dendrites: The short cytoplasmic processes of cell body are called dendrites. Dendrites receive stimulus/impulse from the axon of another neuron through synapse and conduct nerve impulses induced by stimuli towards the cell body. The dendrites at their origin from cell body are 5-10 µm in thickness but gradually their thickness decreases by profuse branching they are also called receptive organ. Dendrites branch out in treelike fashion and serve as the main apparatus for receiving signals from other nerve cells. The dendritic membrane under the synapse (the post-synaptic membrane) has many 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.

1.1. b. Cell body: The cell body (soma) is nucleated cytoplasmic portion of a neuron. The size of each cell body which varies from 4 to 100µm in diameter and it may be pyramidal, fusiform, pyriform or irregular stellate in shape. The cell body have a large spherical central nucleus along with large number of Nissl's Bodies or granules (groups of ribosomes used for protein synthesis) within the cytoplasm (neuroplasm). It makes all the proteins for the other neuronal parts; dendrites, axons and synaptic terminals and contains specialized organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and polysomes (fig. 2). The amount of the cell organelles vary with the functional activity of the cell. Delicate cytoplasmic threads called neurofibrils are there throughout the entire length of axon and dendrites arising from cell body. The cell body and its processes are surrounded externally by a thin membrane called the neuron membrane. The cell body is present in of the central nervous system-brain and spinal cord.

1.1.c. Neuronal membrane: The neuronal membrane serves as a barrier to surround the neoplasm inside the neuron, and to exclude various substances. The membrane is made of lipids and proteins - fats and chains of amino acids. The basic structure of neuronal membrane is a bilayer or sandwich of phospholipids. The external side of the membrane have the receptors for some molecules. Whenever a molecule attaches to receptors; some changes of the membrane and in the interior of the cell ensue, such as the alteration of permeability to some ions.

The neuronal membrane performs many important functions:

 It allows entry of some ions and small molecules into the cell while keeping others out of the cell,  Establishing an electrical potential inside the cell,

Conducting an impulse being responsive to particular neurotransmitters and modulators.

Figure 1. Structure of a typical neuron.

Figure 2. Anatomy of .

1.1.d. Axon: The long cytoplasmic process of cell body which transmits impulse from soma to other neuron is called axon. Axon is much longer than dendrites. The axon arises from the cell body in a conical elevation called . The length of axon is variable and depends on the functional relationship of the neuron. The cytoplasm of axon known as . The membrane covering axon is called . Axon is present in of central nervous system and peripheral nervous system. The axon, with its surrounded sheath, is called a nerve fibre. The nerve fibres or axon are wrapped in sheath. The myelin sheath is formed by Schwann cells (because they were first described by Theodor Schwann) and each covers a part of the nerve fibre. The region where axon is not covered by myelin sheath is the joint of neighbouring myelinated parts is known as . The cells that surround peripheral nerve fibres (outside brain and spinal cord) are

called Schwann cells. The cells that surround axons within the CNS (brain and spinal cord) are called . The axon is the main conducting unit of the neuron, capable of conveying electrical signals a long distances that range from as short as 0.1 mm to as long as 2 m. Many axon split into numerous branches, thereby transmitting information to diverse targets. Many neurons do not have axons. In these so-called amacrine neurons, all the neuronal processes are dendrites. Neurons with very short axons are also found.

1.1.e. Nerve Ending (Presynaptic Terminals): Axon can give of branches, termed collaters along its course and near the end it undergoes substantial branching into axon terminals or end brush, the last part of which is enlarged to form end bulb.

2. Types of neurons

2.1 Most neurons can be anatomically characterized as: a. or peudounipolar: and axon emerging from same process. Examples: touch and pain sensory neurons (fig. 3). b. : axon and single dendrite on opposite ends of the soma. Examples:.retinal, olfactory c. Multipolar neuron: two or more dendrites, separate from the axon. Examples: motor, pyramidal cells, Purkinje cells, and anterior horn cells, . d. Anaxonic neuron: where axon cannot be distinguished from dendrites. Amacrice cells.

Unipolar neurons are unusual as they do not have dendrites. However they still relay a signal from one cell to another. Although there is always only one axon but it can branch out before it reaches its target

Figure 3. Different kinds of neurons: 1 Unipolar neuron, 2 Bipolar neuron, 3 Multipolar neuron, 4 .

2.2. Conduction of nerve signals

Nerve signals or impulses pass through neurons in the form of electrical signals. These signals cross the (small gaps) between one neuron and the following neuron in chemical form before being transmitted again in electrical form. Signals are also chemically passed on to other target cells, like muscles, which make proper responses.

2.2.a. Electrical and chemical signals

As an electrical signal arrives at the end of a nerve fibre, it activates the discharge of neurotransmitter, which then sends the signal in chemical form to the next neuron. Neurotransmitters are the brain chemicals that pass the information throughout brain and body and also relay signals between neuron. There are two types of neurotrans mitters; excitatory and inhibitory. They are made in the neuronal cell body and then transferred to the or nerve ending. Each nerve ending is coupled to other neurons across a synapse. The physical and chemical nature of each synapse determines the strength and polarity of the new input signal. At this place the brain is the most flexible, and the mainly vulnerable. Altering the constitution of different neurotransmitter can amplify or reduce the amount of stimulation that the firing axon passes on to the adjacent dendrite. Changing the neurotransmitters can also modify whether the stimulation is excitatory or inhibitory.

2.2.b. Synapses The synapse contains a tiny gap separating neurons. Synapses are the junctions formed with other nerve cells where the presynaptic terminal of one cell comes in contact with the postsynaptic membrane of another. At these synapses neurons are excited, inhibited, or modulated. Two types of synapses are there, electrical and chemical (fig. 4).

2.2.b.i. An is a mechanical and electrically conductive link between two adjacent neurons that is formed at a tiny gap between the pre- and postsynaptic neurons known as . Electrical synapses occur where the presynaptic terminal is in electrical continuity with the postsynaptic neuron. Electrical synapses conduct nerve impulses faster compared to chemical synapses and are often found in neural systems that require the fastest possible response.

Figure 4. Structure of Synapse

2.2.b.ii. Chemical synaptic junction is more complex. The gap between the post- and presynaptic terminals is big, and the mode of transmission is not electrical, but carried by neurotransmitters discharged at the presynaptic area of the junction. There are two types of chemical synapses. A. It is an excitatory synapse, commonly found on dendrites. B. It 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 typically one way at these junctions (fig.5).

Figure 5. Structure of a typical

2.3. Neuronal communication

Neurons communicate by sending electrical signals called Action Potentials. Action potentials are produced by the movement of ions across the neuronal membrane (fig 6 and7).

Neurons carry information or messages in the form of electrical signals called nerve impulses. Neurons have to be excited to create a nerve impulse. Different types of stimuli such as sound, light or pressure all excite neurons, but in most cases, neurotransmitters released by other neurons will generate a nerve impulse.

Although millions of neurons that are tightly packed within nervous system, they never really get in touch. So when a nerve impulse reaches the end of one neuron, a neurotransmitter is released and it diffuses from this neuron across a junction and excites the next neuron. The activation of a membrane receptor may either result in depolarization (an excitatory postsynaptic potential) (makes it more likely that an action potential will fire) or hyperpolarization (an inhibitory postsynaptic potential) (makes it less likely that an action potential will fire).

Figure 6. Typical mammalian neurons. Arrows indicate the direction of conduction of action potentials in axons (red). (a) Multipolar (b) A (c) A .

Figure 7. Essentials in chemical synaptic transmission. Action potential travels the length of the axon of a neuron. When the action potential reaches at the presynaptic terminal, it stimulates the release of a small quantity of neurotransmitter, which binds to receptor situated in the membrane of another postynaptic neuron on the opposite side of the synaptic cleft.

How neurotransmitters work

The job of neurotransmitters is to carry nerve impulses across the synapse between neurons and target cells. Neurotransmitters either stimulate or inhibit electrical impulses in target cells. The arrival of a nerve impulse stimulates the discharge of neurotransmitters from synaptic vesicles. They pass across the synapse and open channels in the target cell (post synaptic neuron). Charged particles can then enter and trigger a second impulse.

3. The Blood-Brain Barrier (BBB)

3.1 Structure and function

The BBB is a permeability barrier that separates the brain from the circulatory system. It protects the central nervous system from many substances (viruses, bacteria, and harmful chemicals) while regulating the transport of necessary substances. The BBB is formed by a monolayer of specialized endothelial cells, which are connected by tight junctions and transduce signals from the vascular system and from the brain. are essential to create and contribute to differentiation of the BBB. Brain capillaries form a tight barrier except in specialized areas. The structure and function of the BBB is dependent upon the complex interaction between the different cell types such as the endothelial cells, astrocytes,

and pericytes, and the extracellular matrix of the brain and blood flow in the capillaries (fig.8). The BBB is composed of high-density cells limiting the passage of substances from the bloodstream much more than do the endothelial cells in capillaries at any other place in the body. cell projections called astrocytic feet (glia limitans) enclose the endothelial cells of the BBB, giving biochemical support to those cells.

3.2 Transport across the BBB may be passive or active.

 Passive Transport: no energy is required to pass. Examples: Small uncharged molecules-O2, CO2 and hormones, Molecules that can dissolve in the fats of the capillary walls  Active Transport: energy is required to pass. Examples: Glucose, amino acids, vitamins.

Figure 8. Anatomy of the Blood-Brain-Barrier

4. Glial cells or simply glia are quite different form neurons. They give support and protection to the nerve cells but do not transmit nerve impulses themselves. Glia are non- neuronal cells that form mylen, surround neurons and hold them in position, maintain homeostasis, and provide support and protection for neurons in the central and peripheral nervous system (fig. 9).

4.1 Types of glial cells

4.1 a : these are specialized macrophages capable of phagocytosis that protect neurons of the CNS. These cells are found in all brain areas and spinal cord. Microglial cells are relatively smaller than macroglial cells, with changing shapes and oblong nuclei. They are move within the brain and multiply when the brain is injured.

4.1.b Macroglia: they derive from ectodermal tissue.

Astrocytes (Astroglial Cell) are the most abundant type of macroglial and are found throughout the Central Nervous System. An astrocyte is a star-shaped cell that has many processes extending from its cell body into the surrounding network of nerve fibres. Oligodendrocytes are cells that wrap axons in the CNS with their cell membrane, forming a mylin-sheath. The myelin sheath provides insulation to the axon that permits electrical signals to spread more efficiently. Oligodendrocytes have fewer and thinner processes than astrocytes and no gap junctions. Ependymal cells (ependymocytes) are found in the CNS. These cells are involved in the formation and secretion of cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF and make up the blood-CSF barriers. Radial glia cells arise from neuroepithelial cells after the onset of neurogenesis. Schwann cells are found in the PNS that is around the nerves of the extremeties of the body, e.g. located in the skin. Schwann cells provide myelination to axons in the PNS. Satellite glial cells are small cells that wrap neurons in sensory, sympathetic, and parasympathetic ganglia. Enteric glial cells are found in the intrinsic ganglia of the digestive system.

Figure 9. Different types of neuroglia.

Summary

Nervous tissue consists of two main types of cells: neurons and neuroglia. Neurons transmit nerve impulses that send information around the body. Neurons are nerve cells that originate process, transmit, and receive nerve impulses. They are associated to other neurons or to cells target cells in muscles and organs, Nerve impulses travel electrically along the neuron and are transmitted by chemical messengers called neurotransmitters to the next neuron across a tiny gap, called a synapse, between the neuron and the adjacent cell, which is known as the target cell. In addition to neurons, the nervous system contains large numbers of other types of cell, called neuroglia, which support, protect and nourish neurons. Glia have many functions in support of nerve cells but do not transmit nerve impulses themselves.