BY Ms ERUM GUL ZOOLOGY DEPARTMENT DA DCW PHASE VII NERVE IMPULSE BY Ms ERUM GUL ZOOLOGY DEPARTMENT DA DCW PHASE VII

LECTURE NO. 02 SUBJECT : ZOOLOGY CLASS : XII TOPIC : NERVE IMPULSE TIME : 40 MINUTES DATE : 26TH JULY, 2011

CONTENTS Definition Of A Nerve Impulse Electrical Potential Membrane Potential Resting Membrane Potential( RMP) Factors Involving RMP Active Membrane Potential (AMP) Factors Involving AMP The Action Potential The Propagation Of Nerve Impulse Synapse And Its Components Reflex Action

WHAT IS A NERVE IMPULSE? DEFINITION “Nerve Impulse is a wave of electrochemical changes which travels along the length of the neuron involving chemical reactions and movement of ions across the cell membrane.”

ELECTRICAL POTENTIAL “It is the measure of the capacity to do electrical work. It represents a type of stored energy which is manifested during the separation of the charges across a barrier.”

MEMBRANE POTENTIAL The electrical potential that exists across a cell membrane is known as membrane potential. In case of a neuron, the charges are negative and positive ions (Na+, K+, Cl- etc.) and the charge separating barrier is the plasma membrane.

RESTING MEMBRANE POTENTIAL A typical neuron at rest is electrically more positive outside than inside the plasma membrane. The net difference in charge between the inner and outer surface of a non-conducting neuron is called the “Resting Membrane Potential or RMP.” No conduction of nerve impulse. Membrane potential is equal to -70 mV/-0.07V.

Resting membrane potential

FACTORS INVOLVING RMP Sodium-Potassium Pump At rest Sodium ions are 10 times higher in concentration than inside. These are very active pumps located in the cell membrane of all the neurons. Driven by the splitting of ATP these pumps actively transport 3Na+ out for every 2 K+ pumped inside the neuron. Negative Organic Ions The large negative organic ions such as proteins, organic acids etc. are much more inside than outside. This makes the inside of neuron relatively more negative. Leakage of Potassium Ions The plasma membrane of neuron is virtually impermeable to all ions except Potassium which leaks out of the cell. The loss of this positive ion from the neuron by diffusion also accounts for maintaining the membrane potential.

THE SODIUM-POTASSIUM PUMP

ACTIVE MEMBRANE POTENTIAL A Nerve Impulse is initiated by an appropriate stimulus called “threshold stimulus.” Such a stimulus results in a remarkable localized change in resting membrane potential which is replaced by a new potential called “Active Membrane Potential (AMP) or Action Potential.” This change(depolarization)is for a brief instant( perhaps for a few milliseconds) due to the reversal of charges at the stimulated site of neurolemma. Conduction of nerve impulse. Membrane potential becomes +40mV/+0.04 V.

FACTORS INVOLVING AMP Voltage-Gated Sodium Channels Activation Gate Inactivation Gate Voltage-Gated Potassium Channels A Single Voltage Sensitive Gate

Response Of Voltage Sensitive Gates Of Ion Channels Sodium channel Activation Gate : Closed at rest but ‘opens rapidly’ upon depolarization. Sodium channel Inactivation Gate : Opened at rest but ‘closes slowly’ upon depolarization. Single Voltage Sensitive Potassium Gate: Closed at rest but ‘opens slowly’ upon depolarization.

THE ACTION POTENTIAL The action potential or AMP is actually the Nerve Impulse. Once an action potential is triggered, the membrane potential goes through a stereotypical sequence of changes which involves the following steps: Depolarization Repolarization Hyper polarization

Depolarization The activation gate of Sodium channels opens rapidly causing an influx of Na+ ions. This influx of Na+ positively feedback to open all the Sodium channels at the stimulated site. Sodium permeability becomes 1000 times greater than at rest. The inner side of neurolemma becomes relatively more positive than the outer side. Membrane potential changes from -70mV to +40mV.

Repolarization Inactivation gate of Sodium channel closes making Sodium permeability comes to its low resting level. Potassium channels opens causing a rapid efflux of K+ ions restoring the internal negativity of the membrane.

Hyper polarization The continuous outflow of K+ ions makes the membrane potential more negative i.e. hyperpolarize it. During this phase, also called as undershoot, both the activation and inactivation gates of Sodium channel are closed. If a second depolarizing stimulus arrives during this phase, it will be unable to trigger an action potential. This period when a neuron is insensitive to depolarization is called “refractory period.”

PROPAGATION OF NERVE IMPULSE (The one-way avenue) The action potential that developed locally spreads along the entire neurolemma is called the propagation of nerve impulse. A neuron is usually stimulated at its dendrites or cell body and resulting action potential is regenerated anew in a sequence along the axon to the other end of the cell until it reaches Synapse.

WHAT IS A SYNAPSE? Synapse is a unique junction that controls communication between neurons. Consecutive neurons are so arranged that the axon endings of one neuron are connected to the dendrites or cell body of the other neuron. There is no cytoplasmic connections in between but there are microscopic gaps at these contact points which are called Synapse. Chemical messengers called Neurotransmitters (Acetylcholine,Dopamine,Serotonin) help in communication between the neurons.

Components of synapse A Chemical synapse consists of three A Pre-synaptic membrane Synaptic cleft A Post-synaptic membrane

Conduction through synapse Action potential reaches the pre-synaptic membrane. Calcium channels open causing an influx of Ca+ ions which in turn causes the synaptic vesicles to fuse with the membrane. Synaptic vesicles release the neurotransmitter molecules into the synaptic cleft which binds to the receptors present on the post-synaptic membrane. This binding opens the specific ion channels of post-synaptic membrane, thus generating an action potential in it.

REFLEX ACTION Reflex actions are automatic, involuntary responses which occur either due to internal or external stimuli. Example: Knee jerk Blinking of eyes Hand withdrawal on a painful stimulus

RESOURCES TEXTBOOK Of BIOLOGY XII (STBB) BIOLOGY For XII (PTBB) BIOLOGY By NEIL A. CAMPBELL Science photo library www.google.com www.youtube.com (Video) www.millerandlevine.com www.dummies.com