Nerves and Nerve Impulse

Transcription

1 Nerves and Nerve Impulse Terms Absolute refractory period: Period following stimulation during which no additional action potential can be evoked. Acetylcholine: Chemical transmitter substance released by some nerve endings. Action potential: A large transient depolarization event, including polarity reversal, that is conducted along the membrane of a muscle cell or a nerve fiber. Adrenaline: see Epinephrine Afferent neuron: Nerve cell that carries impulses towards the Central Nervous System (CNS). Autonomic Nervous System: Efferent division of the peripheral nervous system that innovates cardiac and smooth muscles and glands; also called the involuntary or visceral motor system. Axon: Neuron process that carries impulses away from the nerve cell body; efferent process; the conducting portion of a nerve cell. Bipolar neuron: Neuron with axon and dendrite that extend from opposite sides of the cell body. Catecholamines: Neurotransmitters i.e. epinephrine and norepinephrine. Central nervous system: The brain and the spinal cord Cholinergic fibers: Nerve ending that upon stimulation releases Acetylcholine. Conductivity: That ability to transmit an electrical impulse Dendrite: Branching neuron process that serves as a receptive, or input, region; transmits the nerve impulse to the cell body. Depolarization: The loss of a state of polarity; loss of a negative charge inside the cells. Diffusion: The spreading of particles in a gas or solution with a movement towards uniform distribution of particles. Effector: Organ, gland, or muscle capable of being activated by nerve ending. Efferent: Carrying away or away from, especially a nerve fiber that carries impulses away from CNS. Mixed nerves: Nerves containing the processes of motor and sensory neurons; their impulses travel to and from CNS. Motor nerves: Nerves that carry impulses leaving the brain and the spinal cord. Motor unit: A motor neuron and all the muscle cells it stimulates.

2 Multipolar neuron: Neuron that has one long axon and numerous dendrites. Myelin sheath: Fatty insulating sheath that surrounds all but the smallest nerve. Neuralgia: Nonexcitable cells of neural tissue that supports, protects, and insulates the neurons. Neuron: Cell of the nervous system specialized to generate and transmit nerve impulses. Neurotransmitters: Chemical released by nerve that may upon binding to receptors of neurons or Effectors cells, stimulate or inhibit them. Parasympathetic nervous system: The division of the autonomic nervous system that over-sees digestion, elimination, and glandular function; the resting and digestion subdivision. Peripheral nervous system (PNS): Portion of the nervous system consisting of nerves and ganglia that lie outside the brain and spinal cord. Polarized: State of the plasma membrane of an unstimulated neuron or muscle cell in which the inside of the cell is relatively negative in comparison to the outside; the resting state. Receptor: Peripheral nerve ending specialized for response to particular types of stimuli. Relative refractory period: Period following stimulation during which only a stronger than usual stimulus can evoke an action potential. Sensory nerve: Nerve that contains processes of sensory neurons and carries nerve impulse to the CNS. Sensory neuron: Neuron that initiates nerve impulses following receptor stimulation. Sensory receptor: Dendrites and organs, or parts of other cell types, specialized to respond to a stimulus. Sensory transduction: Conversion of stimulus energy into a nerve impulse. Somatic nervous system: Division of the peripheral nervous system that provides the motor innovation of skeletal muscles. Stimulus: An excitant or irritant; a change in the environment that evokes a response. Synapse: Functional junction or point of close contact between two neurons or between a neuron and an Effector cell. Synaptic cleft: Fluid- filled space at a synapse between neurons and muscle. Threshold: Weakest stimulus capable of producing a response in an irritable tissue. Neurons Neurons are also called nerve cells. They are highly specialized cells that conduct messages in the form of nerve impulses. Neurons are the structural unit of the nervous system. Neurons are made up of dendrites, the cell body, axon, and the axon terminals. Some neurons are Myelinated and others are Unmyelinated Myelinated neurons are called white fiber and are found in the white matter of the brain.

3 Unmyelinated neurons are called gray fibers and make up the gray matter. Neurilemma aids in the repair of damaged nerve fibers. Classification of Neurons A. Structural Classification: Neurons are grouped structurally according to the numbers of processes that extend from their cell body. Three major groups: 1. Multipolar neurons - numerous branching dendrites and one axon - most common neuron type in humans - major neuron type in the central nervous system 2. Bipolar neurons - have two processes - an axon and dendrite - that extend from opposite sides of the cell body - rare in the adult body - found in special sense organs, where they act as receptor cells (e.g.) eyes, and nose 3. Unipolar neurons

4 - have single process that emerges from the cell body, which is very short and divides into axon and dendrite. - sometimes called pseudounpolar neurons B. Functional Classification: Neurons are divided according to the direction the nerve impulse is traveling relative to the central nervous system Three major types: 1. Sensory or afferent neurons o most are unpolar neurons, except for the bipolar neurons that are found in the special senses organs o typically heavily Myelinated o function as sensory receptors 2. Motor or efferent neurons o carry impulses away from the central nervous system (CNS) to the effector organs (muscles and glands) of the body o multipolar, except for some neurons of the autonomic nervous system, their cell bodies are in the CNS. 3. Association neurons or interneurons o lie between motor and sensory neurons in the neural pathways, where integration takes place o typically multipolar o confined entirely within the CNS Nerves Endoneurium - sheath of connective tissue that surrounds each nerve fiber. Perineurium - connective tissue that bounds groups of nerve fibers together, to form bundles of fibers called fascicles. Epineurium - Tough fibrous sheath that encloses all the fascicles to form a nerve

5 Neurophysiology Neurons are highly sensitive to stimuli. When a neuron is adequately stimulated, an electrical impulse is conducted along the length of the axon. The response is always the same, regardless of the source or type of stimulus. A.) Basic Principles of Electricity In the body, electrical currents reflect the flow of ions across the cell membrane. Plasma membranes contain a variety of ion channels constructed by membrane proteins. Some of these are passive, or leakage, these channels are always open. Other gates are active, or gated, these channels are characterized by a molecular "gate". There are two types of gated gates: o Chemically-gated channels open when the appropriate neurotransmitters bind to receptors. o Voltage-gated channels open and close in response to changes in the membrane potential, or voltage. When the "gates" open, the ions diffuse across the membrane creating electrical current. Ions move along their chemical gradients when they diffuse passively from an area of higher concentration to an area of lower concentration, and along their electrical gradients when they move towards an area of opposite electrical charge. B.) The Resting Membrane Potential: The Polarized State neurons at rest are said to be polarized, with an excess of negative charge inside the neuron and corresponding positive charge outside. This potential difference at rest is called the Resting Membrane potential

6 at resting state, the cell is impermeable to Na + ions and low permeability to K + ions, and the Na + /K + pump maintain the ionic concentration gradients that lead to the resting membrane potential Sodium (Na + ) are in a higher concentration in the extracelluar fluid (outside) Potassium (K + )are in a higher concentration in the intracelluar fluid (Inside) membrane has low permeability to any charged species of ions, but there are protein channels or gates selective for certain ions embedded within the membrane Na + /K + pump requires energy which it gets from ATP hydrolysis, also known as the Na + /K + ATPase the resting membrane potential is -75 mv to -60 mv these ionic differences are the consequence of an action potential: 1. the differential permeability of the plasma membrane to Na + and K + (the membrane impartiality to Na + ) 2. the operation of the sodium-potassium pump, which actively transports Na + out of the cell and K + into the cell. For every 2 Na + that are pumped out 1K + is pumped in. Therefore the sodium-potassium pump stabilizes the resting membrane potential by maintaining the diffusion gradients for sodium and potassium C). Action Potentials An action potential is a brief, but large, depolarization with a total amplitude (change in voltage) of about 100 mv (from -70 mv to +30 mv) action potential is initiated when the stimulus reaches the threshold level. The threshold level works on the all or none principle. In other words, the stimulus must reach a "certain" voltage in order for a impulse to be generated. Once the threshold level is reached, it will always result in a action potential. depolarizing phase of the action potential is due to the increased permeability of the membrane to Na + due to the Na + gates opening. This causes the Na + to rush into the neuron; hence changing the resting membrane potential from -70 mv to +30 mv. repolarizing phase of the action potential: the membrane permeability to Na + decreases and K + increases as the Na + gates close and the K + gates open. The

7 sodium- potassium (Na + /K + ) pump, also known as the ATPase, becomes active to restore the resting membrane potential. The Na + /K + pump gets its energy from ATP hydrolysis. initial depolarization of the membrane is caused by the opening of the voltage sensitive Na + channels. A current then moves across the membrane carried by Na + ions. action potential moves down the axon in a unidirectional fashion, because as it moves forward the refectory period begins behind it. Action potential propagation in a Myelinated axon myelin sheath covers the axon and acts as an insulator, which reduces the leakage of K + ions in advance of the inward Na + currents which would otherwise occur during the depolarization phase of the action potential so that the depolarization can spread further away along the axon. the myelin sheath is interrupted by Nodes of Ranvier, where the depolarization occurs the nerves with myelin sheath have faster conducting nerve fibers, due to the nerve impulse traveling from node to node Resting Membrane Potential higher concentration of Na + on the outside of the neuron membrane, and higher concentration of K + in the inside of the neuron Na + gates are closed the Na + - K + pump active Depolarization / Action Potential

8 Events occurring during action potential / depolarization 1. Increase membrane permeability to Na + due to the opening of the Na+ gates 2. Diffusion of Na + to the inside and the K + to the outside of the neuron, thereby reversing the charge of the inside from negative to positive. Repolarization The Synapse each neuron is a separate unit, there is no anatomic connection between neurons or their effector the space between the axon terminal of one neuron and the dendrites or an effector is called a synapse the space between an axon terminal and a muscle is called neuromuscular junction in transmitting the action potential to the next neuron or to the effector, the axon terminal releases neurotransmitters into the synapse. These neurotransmitters diffuse across the synapse to their receptor sites located on the dendrites of the postneuron or the effector (receptor sites are specific to certain neurotransmitters) once enough of the receptor sites are "filled" they initiate an action potential in the post synapic neuron. The remaining of the neurotransmitter is metabolized by a specific enzyme. examples of neurotransmitters: epinephrine / adrenaline norepinephrine/ noradreadin

9 1. Label the following diagram. Review Questions

10 2. carries impulses away from the cell body. 3. carries impulses towards the cell body. 4. The space between the axon terminal and the dendrites of the cell body of the next neuron is called the. 5. State the function of the myelin sheath. 6. At synapses, the nerve impulse is carried by a that is released by the synaptic knobs on the of a neuron. 7. When a neuron is not carrying an impulse, its cell membrane has a charge on the outside and a charge on the inside of the membrane. 8. Depolarization is brought about by a stimulus which makes the neuron membrane very permeable to ions, which rush the cell. As a result, the membrane now has a charge outside and charge inside the neuron. 9. Explain in a point form the events that occur during each of the following stages of nerve impulse: (on a separate sheet of paper) a. depolarization b. repolarization c. resting membrane potential

11 10. are called white fibers which make up the. Unmyelinated neurons appear and make up the. 11. In order for an action potential to be initiated the must be reached. 12. Myelinated neurons are faster/slower than Unmyelinated neurons. 13. Explain why the action potential moves only in one direction.