Unit II - Lecture 1 Neuron Functioning PDF

UNIT II: THE FUNCTIONING BRAIN: STRUCTURE AND FUNCTIONS OF NEURONS; NEURAL CONDUCTION AND SYNAPTIC TRANSMISSION  Structure and functions of the most impor tant cells of the ner vous system – the NEURONS  A neuron that detects changes in the internal or external environment and sends info...

UNIT II: THE FUNCTIONING BRAIN: STRUCTURE AND FUNCTIONS OF NEURONS; NEURAL CONDUCTION AND SYNAPTIC TRANSMISSION  Structure and functions of the most impor tant cells of the ner vous system – the NEURONS  A neuron that detects changes in the internal or external environment and sends information about these changes to the central ner vous system – SENSORY NEURON  A neuron located within the central ner vous system that controls the contraction of muscles or the secretion of a gland – MOTOR NEURONS  A neuron located entirely within the central ner vous system between the sensor y and the motor neuron –INTER NEURON LOCAL INTERNEURONS RELAY INTERNEURON Form circuits with Connect circuits of local nearby neurons and interneurons in one analyse small piece of region of the brain with information those in other regions  NEURON  The neuron (ner ve cell) is the information processing and information transmitting element of the ner vous system  Neurons come in many shapes and varieties, according to the specialized jobs they per form  Most neurons have, in one form or another, the following four structures or regions:  Cell body or soma  Dendrites  Axon  Terminal buttons STRUCTURE OF A NEURON  SOMA  The soma (cell body) contains the nucleus and much of the machiner y that provides for the life processes of the cell.  Its shape varies considerably in different kinds of neurons  DENDRITES  Dendron is the Greek word for tree  The dendrites of a neuron look like a tree  Neuron “converse” with one another  Dendrite ser ve as important recipients of the message  A branched, treelike structure attached to the soma of a neuron, receives information from the terminal buttons of other neurons  The messages that pass from neuron to neuron are transmitted across the synapse , a junction between the terminal button of an axon and the somatic or dendritic membrane of another neuron  The word synapse derives from the Greek sunaptein, to join together  Communication at a synapse proceeds in one direction: from the terminal button to the membrane of the other neuron  The neuron receives excitation at synapses from the terminal buttons of another neuron  AXON  The axon is a long, slender tube, of ten covered by a myelin sheath  The axon carries information from the cell body to the terminal buttons  The transferred excitation is focussed by the axon hillock on the axon  The basic message it carries is called an action potential, which is an important function  An action potential is a brief electrical/chemical event that starts at the end of an axon next to the cell body and travels to the terminal buttons  It is like a brief pulse, in a given axon the action potential is always of the same size and duration.  When it reaches a point where the axon branches, it splits but does not diminish in size.  Each branch receives a full strength action potential  The axon of a neuron may or may not have a myelin sheath, which is a fatty covering interrupted at the nodes of Ranvier  Nerve cells with myelin sheaths conduct faster than ner ve cells without them.  The neurolemma, a covering for some neurons, is made up of a separate cell known as the Schwann cells  It is believed that this cell secretes the fatty myelin sheath  The main function of the neurolemma seems to be protective – it holds the long, thin axon together.  Like dendrites, axons and their branches come in different shapes,  The three principle type of neurons are classified according to the way in which their axons and dendrites leave the soma  MULTIPOLAR NEURON A neuron in which the somatic membrane gives rise to one axon and many dendrites attached to its soma Most common type found in the CNS  BIPOLAR NEURONS  A neuron with one axon and one dendrite attached to the opposite ends of the soma  Usually sensory, i.e. their dendrite detect events occurring in the environment and communicate information about these events to the CNS  UNIPOLAR NEURONS  A neuron with one axon attached to its soma; the axon divides with one branch receiving the sensor y information and the other sending the information into the CNS  Transmit sensor y information from the environment to the CNS  The treelike branches outside the CNS are dendrites that detect sensor y information.  The branches on the end of the axon within the CNS end in terminal buttons  The dendrites of most unipolar neurons detect touch, temperature changes, and other sensor y events that affect the skin  Other unipolar neurons detect events in our joints, muscles and internal organs  The CNS communicates with the rest of the body through ner ves attached to the brain and the spinal cord.  Ner ves are bundles of many thousands individual fibres all wrapped in a tough, protective membrane  Under a microscope nerves look something like telephone cables, and their bundle of wires  Like the individual wires in a telephone cable, ner ve fibres transmit messages through the nerve, from a sense organ to the brain or from the brain to a muscle or gland.  TERMINAL BUTTONS  Most axons divide and branch many times  At the ends of the twigs are found little knobs called terminal buttons (french word buton).  Special function: when an action potential travelling down the axon reaches them, they secrete a chemical called a neurotransmitter.  This chemical either excites or inhibits the receiving cell and thus helps to determine whether an action potential occurs in its axon.  An individual neuron receives information from the terminal buttons of axons of other neurons, and the terminal buttons of its axons form synapses with other neurons.  A neuron may receive information from dozens or even hundreds of other neurons, each of which can form a large number of synaptic connections with it  INTERNAL STRUCTURE  The membrane defines the boundary of the cell  It contains a double layer of lipid (fatlike) molecules  Embedded in the membrane are a variety of protein molecules that have special functions  Some proteins detect substances outside the cell (hormones) and pass information about the presence of these substances to the interior of the cell.  Other proteins control access to the interior of the cell, permitting some substances to enter but barring others.  Still other proteins act as transporters, actively carrying certain molecules into or out of the cell  The cell is filled with cytoplasm, a jelly -like substance that contains small specialized structures  Among those structures are mitochondria – break down nutrients such as glucose and provide the cell with energy to perform its functions.  Mitochondria produce a chemical called adenosine triphosphate (ATP), which can be used throughout the cell as an energy source.  Mitochondria contain their own genetic information and multiply independently of the cells that they live in.  Deep inside the cell is the nucleus (from the Latin word for “nut”)  The nucleus contains the chromosomes  Chromosomes consist of long strands of deoxyribonucleic acid (DNA)  Function: contain the recipes for making proteins  Genes are the por tions of the chromosome which direct synthesis of one or more proteins.  Proteins are impor tant in cell function  If a neuron grown in a tissue culture is exposed to a detergent, the lipid membrane and much of the interior of the cell dissolve away, leaving a matrix of insoluble strands of protein  This matrix is called the cytoskeleton - gives the neuron its shape  The cytoskeleton is made of various kinds of protein strands, linked to each other and forming a cohesive mass  Besides providing structure, proteins ser ve as enzymes  Enzymes are the cell’s marriage brokers or divorce judges  They cause par ticular molecules to join together or split apar t  Thus enzymes determine what gets made from the raw materials contained in the cell, and they determine which molecules remain intact  Proteins are also involved in transpor ting substances within the cell  Axons can be extremely long (foot to the base of the brain)  Because terminal buttons need some items that can be produced only in the soma, there must be a system that can transpor t these items rapidly and efficiently through the axoplasm (cytoplasm of the axon)  This system, axoplasmic transpor t, is an active process that propels substances from one end of the axon to the other  This process is accomplished by long protein strands called microtubules  Bundles of thir teen filaments arranged around a hollow core  Microtubules ser ve as railroad tracks, guiding the progress of the substances being transpor ted  Movement from the soma to the terminal buttons is called anterograde axoplasmic transpor t (antero – towards the front)  Retrograde axoplasmic transport carries substances from the terminal buttons back to the soma ( retro – toward the back)  Anterograde axoplasmic transpor t is relatively fast; upto 500mm per day  Retrograde axoplasmic transport is about half as fast  Energy for both forms of transport is provided by ATP, produced by the mitochondria