Lecture 8 Lecture Outline:I Nervous Systema. Basic informationb. Structure and functionII Physiology of neuronsa. Ionic mechanismsb. Electrical signalsNervous System- 3 main functions:o Sensory receptors: monitor changes occurring inside and outside of the bodyo Integrating center: processes and interprets sensory info and decides what should be done at each momento Effectors: receive signals carried from integrating center- Organization:o CNS: brain and spinal cordo PNS: afferent sensory division (sensory) and efferent division (motor) Somatic nervous system: motor neurons, skeletal muscle Autonomic nervous system: sympathetic and parasympathetic nervous system- Smooth muscle, cardiac muscle, glands- Components of Nervous Tissueo Highly cellularo Composed of two principle types of cells: Neurons: functional unit of NS, possess properties of excitability and conductivity Support cells: (glial cells) not excitable, provide metabolic and physical support to neurons- Unlike most neurons, these are capable of mitosis; primary brain tumors caused by uncontrolled multiplication of glial cells- Astrocyte: most common type of glial cell, found in brain, several functions, help regulate chemical composition of interstitial fluid- Microglial cells: used in defense of pathogens- Ependymal cells: secrete cerebrospinal fluid- Oligodendrocyte: form myelin sheath for axons in CNS- Schwann cells: form myelin sheath for axons in PNS- Satellite cells: thought to give rise to skeletal muscle cellsNeurons:- Structure and functions: functional units of NS because they possess properties of excitability and conductivityo Responsible for reflexes, thought, emotion, personality, behavioro Amitotic (vast majority) but long longevityo High metabolic rate; totally dependent on aerobic pathway to make ATP (need O2 and glucose)o Structural Classification: Unipolar Neuron: short process that splits into two longer axons- Synaptic terminals: one of two processes that connects to brain or spinal cord- Make up afferent division of PNSFoundonly in CNSBio 312 Multipolar neurons: multiple processes- Make up efferent division of PNSo Structure: Soma: the cell body which houses the nucleus Dendrites: antennas to receive signals from other neutrons “graded potentials formed here Axon hillock: action potentials initiated Axon: (nerve fiber) conduct action potentials; most are myelinated Axon terminals: release neurotransmitter onto other neurons or effector cells Schwann cells: secrete myelin sheath- Nodes of ranvier: spaces left between myelinated areas of axon Nerve: bundle of many axons PNS- Majority carry signals both ways at same time- Some are only afferent or only efferent Tract: bundle of many axons in CNS Neuron: refers to entire nerve cell Nerve fiber: only refers to the axonPhysiology of Neurons: all living cells have separation of unlike charge across their membrane- Separation of positive and negative charges across the membranes of two cells is called membrane potential- Membrane potential is much larger in nerve cells which gives properties of excitability and conductivity- In ICF and ECF positive and negative charges exists in form of ions or electrolytes- When oppositely charged particles are separated they have the potential of coming together- The magnitude of charge separation is called the electrical potential difference- In cells the separation of charge is small, units are millivolts (mV)- In cells, the potential difference is called the membrane potential (Em)- Movement of charge is called current- Hindrance to movement of charge is resistance- Amount of charge that moves across a membrane depends on magnitude of membrane potential and properties of the membrane (how much resistance)- Neuron at rest has membrane potential is -70mV- Em is always expressed as inside compared to outside- Electrical and chemical gradients both have influence on diffusion of ions across cello Under normal conditions, chemical gradients are generally stronger than electrical gradients- Ionic mechanism of the resting membrane potential:o Created by the combination of : Direction of sodium and potassium electrochemical gradients across membrane Differences in permeability of resting membrane to sodium and potassium- Potassium is far more permeable than sodium- Both ions use leak channels Resting membrane potential is cause by diffusion potential, potassium leaks out faster than sodium leaks in Gradient is maintained by sodium and potassium pumps that act against the natural gradient- Electrical Signals in Neuronso Neurons use transient changes in the value of their membrane potential as signals for receiving, processing and transmitting informationo Changes in membrane potential is produced by any stimulus that changes permeability of membrane to Na+, K+, Cl-, or Ca 2+o Terminology for electrical signals in comparison to -70mV resting potential Depolarization: occurs when the membrane potential becomes less negative or more positive Overshoot: inside of cell becomes positive Repolarization: movement back toward the resting potential Hyperpolarization: development of more negative charge than usualo Electrical signals exist in two forms: Graded or local potentials: signal only over short distances Action potentials: utilized for long distance
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