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Anatomy and Physiology The Nervous System Functions of the Nervous System 1. Sensory Function – senses certain changes (stimuli) both within the body (internal environment) and outside the body (external environment) Nervous System Functions 2. Integrative Function – analyzes the sensory information, stores some things and makes decisions about behavior Nervous System Functions 3. Motor Functions – may respond to stimuli by causing muscle contractions or glandular secretions Nervous System Divisions Consists of 2 parts: 1. The Central Nervous System (CNS) 2. The Peripheral Nervous System (PNS) The Central Nervous System • Consists of the brain and spinal cord • Here, sensory information is integrated, thought and emotions are generated and memories are formed and stored CNS Continued • Most nerve impulses that cause muscles to contract or glands to make secretions begin here The Peripheral Nervous System • The CNS is connected to sensory receptors, muscles and glands by the PNS • Consists of cranial nerves that arise from the brain and spinal nerves that emerge from the spinal cord PNS Continued • The input components are called sensory or afferent neurons • They send information from sensory receptors to the brain • The output components are called motor or efferent neurons • They send messages from the CNS to muscles or glands PNS Divisions 1. Somatic Nervous System – this part is voluntary, and contains sensory neurons for sensations you are consciously aware of and usually sends messages to skeletal muscles 2. Autonomic Nervous System – receives messages from parts of the body that you are not aware of and sends impulses to smooth muscle, cardiac muscle and glands (involuntary) Nervous System Cell Types 1. Neurons – message sending cells 2. Neuroglia – do not send messages, but support the cells that do Neuroglia • Literally means “nerve glue” • Usually smaller than neurons and 5-50 times more numerous • Unlike neurons, they can multiply and divide in mature nervous tissue Types of Neuroglia - Astrocytes • Star-shaped cells with many processes • Metabolize neurotransmitters, maintain K balance, and maintains a link between neurons and blood vessels Oligodendrocytes • Most common neuroglia in the CNS • Make the myelin sheath Microglia • Small, phagocytic neuroglia • They remove microbes and debris Ependymal Cells • Range from cuboid to columnar, many are ciliated • Make cerebrospinal fluid and circulate it Neurolemmocytes • AKA Schwann cells • Make the myelin sheath in the PNS Satellite Cells • Support neurons in ganglia (clusters on cell bodies in the PNS) Myelination • Myelin – a molecule composed of lipid and protein (appears white) • Myelin Sheath - a layer of myelin surrounding an axon - It insulates the axon and increases the speed of the nerve impulse Neurons • Some send messages a few mm, others are the longest cells in the body • Nerve impulses travels at speeds varying from 1 to 100 m/sec • Synapse – the functional contact area between two neurons or a neuron and muscle (or gland) Parts of a Neuron • Most have 3 parts 1. Cell body 2. Dendrites 3. Axons Cell Body • Usually contain most normal organelles • Chromatophilic Substance – orderly arrangement of rough ER, makes protein to repair damaged axons • Neurofibrils – form the cytoskeleton Dendrites • Input portion of a neuron • Usually they’re processes with many short branches • Usually they are not myelinated Axon • Sends messages to another neuron, muscle or gland • Joins the cell body at a cone-shaped area called the axon hillock • Nerve impulses begin at the first segment, known as the Trigger Zone More About Axons • • • • Cytoplasm is called axoplasm Cell membrane is the axolemma Branches are called axon collaterals Axon terminals, synaptic end bulbs (or varicosities), synaptic vesicles and neurotransmitters Nerve Fibers • A general term for any neuronal process (dendrite or axon) • Nerve – a bundle of many nerve fibers that follow the same path in the PNS • Most include bundles of both sensory and motor fibers • In the PNS, cell bodies usually cluster together to form ganglia Axonal Transport • Things must be transported from the cell body to the axon • Slow Axonal Transport – moves materials 15mm per day - Moves things only from the cell body toward the axon terminals (supplies axon with new axoplasm) - Fast Axonal Transport – moves materials 200400 mm per day - Moves various organelles and materials in both directions Structural Classification of Neurons • Multipolar Neurons – usually have several dendrites and one main axon • Most neurons in the CNS are this type Bipolar Neurons • Have one main axon and one main dendrite • Found in retina of eye, inner ear, and olfactory area of the brain Unipolar Neurons • Have just one process and are always sensory neurons • They begin in the embryo as a bipolar neuron, but the axon and dendrite fuse into a single process • Trigger zone is where dendrites meet the axon Functional Classification • Look at #1-7 on page 339 Gray and White Matter • White Matter – refers to collections of myelinated processes from many neurons - Myelin appears whitish in appearance • Gray Matter - contains neuron cell bodies, dendrites, axon terminals, or bundles of unmyelinated axons and neuroglia - Appears gray because of a lack of myelin Gray Matter in the Spinal Cord Gray and White Matter in the Brain Neurophysiology • Communication among neurons and from neurons to muscle and glands depends on two basic properties of the plasma membrane of excitable cells (neurons an muscle fibers) 1. Membrane Potential 2. Ion Channels Membrane Potential • Like most cells of the body, excitable cells have a Membrane Potential – an electrical voltage difference across the membrane • That difference can change suddenly, causing graded and action potentials (nerve impulses) • Current – the flow of charge Ion Channels • Graded and action potentials occur because the plasma membranes contain a variety of ion channels • Ion Channels – openings in the cell membrane that open and close in response to specific stimuli • The phospholipid bilayer is a good insulator, so the main path for current to flow is through these channels Action and Graded Potentials In a Nutshell • Action Potentials – Provide communication over both long and short distances • Graded Potentials – Provide communication of short distances only Ion Channel Types • See Page 341-342 Resting Membrane Potential • Occurs because of a small buildup of negative charges just inside the cytoplasm and an equal build up of positive charges in the ICF just outside the surface of the membrane • The separated charges form potential energy, measured in millivolts (mV) • The greater the difference in charge across the membrane, the larger the membrane potential(voltage) Resting Membrane Potential • Resting membrane potentials usually range from -90mV to -40mV • The “-” indicates that the negative charge is on the inside of the cell • The normal accepted value for the resting membrane potential is -70mV • A cell that exhibits a membrane potential is said to be polarized Factors that Contribute to the Resting Membrane Potential 1. Unequal distribution of ions across the plasma membrane. (ECF is rich in Na+ and Cl- ions the cytoplasm is high in K+) 2. Relative permeability of the plasma membrane to Na+ and K+. - In a resting neuron or muscle fiber, the permeability of the plasma membrane is 50 to 100 times greater to K+ than Na+. Why are these things important? • See page 342-343 and Figure 12.8 Graded Potentials • Uses chemically, mechanically or light gated ion channels • Graded Potentials are small deviations from the resting membrane potential caused by an appropriate stimulus • These potentials are “graded,” which means that it can vary in amplitude • Useful only for short distance communication Action Potentials • An action potential is a series of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization), and then restore it to the normal resting state (repolarization) • Good for long distance communication Ion Channels • AP’s use voltage gated ion channels (2 kinds) • The first kind opens and allows Na+ to enter the cell, causing depolarization • The K+ channels then open, allowing K+ to flow out and repolarization to occur • Threshold level – the level of depolarization that brings about an action potential (-55mV) Action Potentials • See page 345 Refractory Period • The period of time in which an excitable cell can not generate another action potential • Must repolarize before it can respond again • Only found in action potentials The All-or-None Principle • If depolarization reaches threshold (-55mV), then an action potential arises, if -55mV is not reached, nothing happens • Each time an action potential occurs, it is the same strength Action vs Grades Potentials 1. Amplitude – graded potentials can vary in amplitude, action potentials are all-or-none 2. Duration – graded potentials are much longer (several milliseconds to several minutes) than action potentials (1/2 to 2 milliseconds) 3. Channels – graded use chemically, mechanically and light gated ion channels, action potentials use voltage gated channels Action vs Grades Potentials Cont’d 4. Location – GP’s arise mainly from dendrites and the cell body (a few from axons) while AP’s always begin at the trigger zone of an axon 5. Propagation – GP’s travel short distances, AP’s travel long or short distances 6. Refractory Period – AP’s have a refractory period, GP’s do not Transmission at Synapses • Presynaptic Neuron – the neuron sending the signal • Postsynaptic Neuron – the neuron receiving the message • There are two types of synapses; electrical and chemical Electrical Synapses • At an electrical synapse, ionic current spreads directly from one cell to another through gap junctions • Each gap junction contains a hundred or so tubular protein structures called connexons that form tunnels to connect the cytosol of the two cells • Common in smooth muscle, cardiac muscle, and the developing embryo Advantages of Electrical Synapses 1. Faster communication (connexons) 2. Can synchronize activity (coordinated movements or contractions) 3. Can have two-way communication unlike chemical synapses Chemical Synapses • Separated by a synaptic cleft and no connexons • Presynaptic neurons release neurotransmitters into the synaptic cleft, which diffuse to receptors on the postsynaptic neuron • Postsynaptic Delay – ½ msec; time required for the processes at a chemical synapse Excitatory and Inhibitory Postsynaptic Potentials • Neurotransmitters act on the postsynaptic neuron to create a postsynaptic graded potential • If it depolarizes the postsynaptic membrane, it is excitatory because it moves closer to threshold • If it causes hyperpolarization, it moves away from threshold and is called inhibitory Removal of Neurotransmitters 1. Diffusion out of synaptic cleft 2. Enzymatic degradation (think acetylcholinesterase) 3. Uptake into cells (active transport) Neurotransmitters • Excitatory and inhibitory neurotransmitters are present in both the CNS and PNS • Some neurotransmitters are excitatory in one location but inhibitory in others • Agonist – a substance that enhances that effect of a neurotransmitter • Antagonist – a substance that inhibits that effect of a neurotransmitter Neuronal Circuits • Neuronal Pools – billions of CNS neurons organized into complicated patterns • Neuronal pools are organized into patterns called circuits • Simple Series Circuit – a presynaptic neuron stimulates only one neuron in a pool • Diverging Circuit – a single presynaptic neuron stimulates an increasing number of postsynaptic neurons Neuronal Pools Cont’d • Converging Circuit – the post synaptic neuron receives impulses from several different presynaptic neurons • Reverberating Circuit – one neuron, stimulate another, it stimulates another, then another and eventually an earlier neuron in the cycle is stimulated again. • Parallel After-Discharge Circuit – a single presynaptic neuron stimulates several postysynaptic neurons which stimulate a single neruon