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“ Nervous coordination is brought about by means Stimuli Receptors of nervous system which is the quickest way of communication CNS Effectors in the body of an animal by electro-chemical messages called nerve Response impulse.”
Nervous system consists of two types of tissues: Neurons Neuroglia (glial cells)Neuroglia are smaller cells which1. Separate neurons from one another2. Form myelin sheath3. Involved in phagocytic functions
“A Neuron is a special kind of animal cell which can generate and conduct electric current.”Structure:A Neuron consists of1. Soma or Cell body2. Dendrites (Receiving end)3. Axon (Conducting end)Function: Receives and integrate various stimuli. Send appropriate instructionsFrom CNS to effectors.
Functionally there are three types of neurons:1. Sensory Neuron which carries sensory information from receptor to other neurons or directly to CNS.2. Motor Neuron which takes commands of the control system to the effecter.3. Inter Neuron found in central nervous system.
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.”
“It is the measure of thecapacity to do electricalwork. It represents a typeof stored energy which ismanifested during theseparation of the chargesacross a barrier.”
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.
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.
•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 Sodium-Potassium Pump every 2 K+ pumped inside the neuron. •The large negative organic ions such as proteins, organic acids etc. are much more inside than outside. Negative •This makes the inside of neuron relatively more negative. Organic 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 Leakage of maintaining the membrane potential.Potassium Ions
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.
Voltage- • Activation Gated GateSodium • InactivationChannels Gate Voltage- • A Single Gated VoltagePotassium SensitiveChannels Gate
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
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.
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.
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.”
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.
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.
A Chemical synapseconsists of threecomponents :1. A Pre-synaptic membrane2. Synaptic cleft3. A Post-synaptic membrane
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 actions are automatic, involuntary responses which occur either due to internal or external stimuli. Example:1. Knee jerk2. Blinking of eyes3. Hand withdrawal on a painful stimulus