NERVES AND NERVE IMPULSE A project by Shobha and Sagarika XI A

1. NERVES AND NERVE IMPULSEA project by Shobha and SagarikaXI A

3. NEURON A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia. A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.

4. STRUCTURE OF A NEURON • A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are filaments that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree".

5. An axon is a special cellular filament that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 m in humans or even more in other species.. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another.

6. STRUCTURAL CLASSIFICATION Polarity Most neurons can be anatomically characterized as: • Unipolar or pseudounipolar: dendrite and axon emerging from same process. • Bipolar: axon and single dendrite on opposite ends of the soma. • Multipolar: more than two dendrites: • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells. • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.

7. FUNCTION OF NEURONS • Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons. • Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons. • Interneurons connect neurons within specific regions of the central nervous system. • Afferent and efferent can also refer generally to neurons which, respectively, bring information to or send information from the brain region

8. CONNECTIVITY SYNAPSE A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft. There are two types of synapses, namely, electrical synapses and chemical synapses.

9. ELECTRICAL SYNAPSE At an electrical synapse, ionic current spreads directly from one cell to another through gapjunctions. Each gap junction contains 100 or so tubular protein structures called connexons that form tunnel to connect the cytosol of the two cells. For eg:- gap junctions are common in cardiac muscle and visceral (single-unit) smooth muscle. They also occur in the CNS.

10. CHEMICAL SYNAPSE At a chemical synapse, the membranes of the pre and post synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at the synapses. The axon terminals contain vesicles filled with these neurotransmitters.

11. HOW DOES NERVE IMPULSE START? We and other animals have several types of receptors of mechanical stimuli. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force such as: • touch • pressure • stretching • sound waves • motion

12. Mechanoreceptors enable us to • detect touch • monitor the position of our muscles, bones, and joints - the sense of proprioception • detect sounds and the motion of the body. Proprioception • Proprioception is our "body sense". • It enables us to unconsciously monitor the position of our body. • It depends on receptors in the muscles, tendons, and joints. • If you have ever tried to walk after one of your legs has "gone to sleep", you will have some appreciation of how difficult coordinated muscular activity would be without proprioception

14. The Resting Membrane Potential When a neuron is not sending a signal, it is at ‘rest’. The membrane is responsible for the different events that occur in a neuron. All animal cell membranes contain a protein pump called the sodium-potassium pump (Na+K+ATPase). This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. 

15. If the pump was to continue unchecked there would be no sodium or potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they “leak”, allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

16. The Action Potential  • The resting potential tells us about what happens when a neurone is at rest. An action potential occurs when a neurone sends information down an axon. This involves an explosion of electrical activity, where the nerve and muscle cells resting membrane potential changes. • In nerve and muscle cells the membranes are electrically excitable, which means they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells are voltage-gated, which means that they can open and close depending on the voltage across the membrane. • The normal membrane potential inside the axon of nerve cells is –70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called the action potential: • An action potential has 2 main phases called depolarisation and repolarisation:

17. The normal membrane potential inside the axon of nerve cells is –70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called the action potential. • An action potential has 2 main phases called depolarisation and repolarisation

18. Depolarization. A stimulus can cause the membrane potential to change a little. The voltage-gated ion channels can detect this change, and when the potential reaches –30mV the sodium channels open for 0.5ms. The causes sodium ions to rush in, making the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside).

19. Repolarization. At a certain point, the depolarization of the membrane causes the sodium channels to close. As a result the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called depolarization. As the polarity becomes restored, there is a slight ‘overshoot’ in the movement of potassium ions (called hyperpolarisation). The resting membrane potential is restored by the Na+K+ATPase pump.

20. TRANSMISSION OF NERVE IMPULSE At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron. 

21. When stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. This is called depolarisation 

22. The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential. This is called repolarisation. 

23. This process continues as a chain-reaction along the axon.  The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon.

24. The sodium/potassium pump restores the resting concentrations of sodium and potassium ions