Paper 12: Membrane Biophysics Module 25: Structure of Nerve, Neuron, Blood brain barrier generating nerve impulse Nervous system contains millions of neurons (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 dendrites, and b. fibres that carry information away from it, called axons. 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 glia Introduction The nervous system coordinates voluntary and involuntary actions of the body and transmits signals to and from different parts of its body. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. 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 synapse, 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 soma and two types of processes, axons and dendrites (fig.1). 1.1 Parts of nerve cell and their functions Dendrites, Cell body, Neuronal membrane, Axon, 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 grey matter 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 multipolar neuron. 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 axon hillock. The length of axon is variable and depends on the functional relationship of the neuron. The cytoplasm of axon known as axoplasm. The membrane covering axon is called axolemma. Axon is present in white matter 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 myelin sheath. The myelin sheath is formed by Schwann cells (because they were first described by Theodor Schwann) and each Schwann cell 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 node of Ranvier. 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 oligodendrocytes. 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. Unipolar neuron or peudounipolar: dendrite and axon emerging from same process. Examples: touch and pain sensory neurons (fig. 3). b. Bipolar neuron: 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, granule cell. 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 Pseudounipolar neuron. 2.2. Conduction of nerve signals Nerve signals or impulses pass through neurons in the form of electrical signals. These signals cross the synapses (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 axon terminal 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 electrical synapse 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 gap junction.