Nervous System PDF

1 Nervous System © 2019 McGraw-Hill Education  2 Nervous System Figure 8.1 © 2019 McGraw-Hill Education ...

1 Nervous System © 2019 McGraw-Hill Education  2 Nervous System Figure 8.1 © 2019 McGraw-Hill Education  3 Nervous System Functions 1. Receiving sensory input 2. Integrating information 3. Controlling muscles and glands 4. Maintaining homeostasis 5. Establishing and maintaining mental activity © 2019 McGraw-Hill Education  4 Main Divisions of Nervous System 1 Central nervous system (CNS) brain and spinal cord Peripheral nervous system (PNS) All the nervous tissue outside the CNS Sensory division Conducts action potentials from sensory receptors to the CNS Motor division Conducts action potentials to effector organs, such as muscles and glands © 2019 McGraw-Hill Education  5 Main Divisions of Nervous System 2 Somatic nervous system Transmits action potentials from the CNS to skeletal muscles. Autonomic nervous system Transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands Enteric nervous system A special nervous system found only in the digestive tract. © 2019 McGraw-Hill Education  6 Organization of the Nervous System Figure 8.2 © 2019 McGraw-Hill Education  7 Cells of the Nervous System Neurons receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs. Glial cells supportive cells of the CNS and PNS, meaning these cells do not conduct action potentials. Instead, glial cells carry out different functions that enhance neuron function and maintain normal conditions within nervous tissue. © 2019 McGraw-Hill Education  8 Neurons A neuron (nerve cell) has a: Cell body – which contains a single nucleus Dendrite – which is a cytoplasmic extension from the cell body, that usually receives information from other neurons and transmits the information to the cell body Axon – which is a single long cell process that leaves the cell body at the axon hillock and conducts sensory signals to the CNS and motor signals away from the CNS © 2019 McGraw-Hill Education  9 Typical Neuron Figure 8.3 © 2019 McGraw-Hill Education  10 Structural Types of Neurons 1 Multipolar neurons have many dendrites and a single axon. Most of the neurons within the CNS and nearly all motor neurons are multipolar. Bipolar neurons have two processes: one dendrite and one axon. Bipolar neurons are located in some sensory organs, such as in the retina of the eye and in the nasal cavity. © 2019 McGraw-Hill Education  11 Structural Types of Neurons 2 Pseudo-unipolar neurons have a single process extending from the cell body, which divides into two processes as short distance from the cell body. One process extends to the periphery, and the other extends to the CNS. The two extensions function as a single axon with small, dendrite-like sensory receptors at the periphery. © 2019 McGraw-Hill Education  12 Types of Neurons Figure 8.4 © 2019 McGraw-Hill Education  13 Glial Cells 1 Glial cells are the supportive cells of the CNS and PNS. Astrocytes serve as the major supporting cells in the CNS. Astrocytes can stimulate or inhibit the signaling activity of nearby neurons and form the blood- brain barrier. Ependymal cells line the cavities in the brain that contains cerebrospinal fluid. © 2019 McGraw-Hill Education  14 Glial Cells 2 Microglial cells act in an immune function in the CNS by removing bacteria and cell debris. Oligodendrocytes provide myelin to neurons in the CNS. Schwann cells provide myelin to neurons in the PNS. © 2019 McGraw-Hill Education  15 Types of Glial Cells Figure 8.5 © 2019 McGraw-Hill Education  16 Myelin Sheath 1 Myelin sheaths are specialized layers that wrap around the axons of some neurons, those neurons are termed, myelinated. The sheaths are formed by oligodendrocytes in the CNS and Schwann cells in the PNS. Myelin is an excellent insulator that prevents almost all ion movement across the cell membrane. © 2019 McGraw-Hill Education  17 Myelin Sheath 2 Gaps in the myelin sheath, called nodes of Ranvier, occur about every millimeter. Ion movement can occur at the nodes of Ranvier. Myelination of an axon increases the speed and efficiency of action potential generation along the axon. Multiple sclerosis is a disease of the myelin sheath that causes loss of muscle function. © 2019 McGraw-Hill Education  18 Unmyelinated Neurons Unmyelinated axons lack the myelin sheaths. These axons rest in indentations of the oligodendrocytes in the CNS and the Schwann cells in the PNS. A typical small nerve, which consists of axons of multiple neurons, usually contains more unmyelinated axons than myelinated axons. © 2019 McGraw-Hill Education  19 Myelinated and Unmyelinated Axons Figure 8.6 © 2019 McGraw-Hill Education  20 Organization of Nervous Tissue The nervous tissue varies in color due to the abundance or absence of myelinated axons. Nervous tissue exists as gray matter and white matter. Gray matter consists of groups of neuron cell bodies and their dendrites, where there is very little myelin. White matter consists of bundles of parallel axons with their myelin sheaths, which are whitish in color. © 2019 McGraw-Hill Education  21 Membrane Potentials Resting membrane potentials and action potentials occur in neurons. These potentials are mainly due to differences in concentrations of ions across the membrane, membrane channels, and the sodium-potassium pump. Membrane channels include leak channels and gated channels. Leak channels are always open, whereas gated channels are generally closed, but can be opened due to voltage or chemicals. © 2019 McGraw-Hill Education  22 Leak Membrane Channels Leak channels are always open are and ions can “leak” across the membrane down their concentration gradient. Because there are 50 to 100 times more K+ leak channels than Na+ leak channels, the resting membrane has much greater permeability to K+ than to Na+; therefore, the K+ leak channels have the greatest contribution to the resting membrane potential. © 2019 McGraw-Hill Education  23 Gated Membrane Channels Gated channels are closed until opened by specific signals. Chemically gated channels are opened by neurotransmitters or other chemicals, whereas voltage-gated channels are opened by a change in membrane potential. When opened, the gated channels can change the membrane potential and are thus responsible for the action potential. © 2019 McGraw-Hill Education  24 Sodium-Potassium Pump The sodium-potassium pump compensates for the constant leakage of ions through leak channels. The sodium-potassium pump is required to maintain the greater concentration of Na+ outside the cell membrane and K+ inside. The pump actively transports K+ into the cell and Na+ out of the cell. It is estimated that the sodium-potassium pump consumes 25% of all the ATP in a typical cell and 70% of the ATP in a neuron. © 2019 McGraw-Hill Education  25 Resting Membrane Potential 1 The resting membrane potential exists because of: The concentration of K+ being higher on the inside of the cell membrane and the concentration of Na+ being higher on the outside The presence of many negatively charged molecules, such as proteins, inside the cell that are too large to exit the cell The presence of leak protein channels in the membrane that are more permeable to K+ than it is to Na+ © 2019 McGraw-Hill Education  26 Resting Membrane Potential 2 Na+ tends to diffuse into the cell and K+ tends to diffuse out. In order to maintain the resting membrane potential, the sodium-potassium pump recreates the Na+ and K+ ion gradient by pumping Na+ out of the cell and K+ into the cell. © 2019 McGraw-Hill Education  27 Resting Membrane Potential 3 Figure 8.7(1) © 2019 McGraw-Hill Education  28 Resting Membrane Potential 4 Figure 8.7(2) © 2019 McGraw-Hill Education  29 Resting Membrane Potential 5 Figure 8.7(3) © 2019 McGraw-Hill Education  30 Action Potential 1 Action potentials allow conductivity along nerve or muscle membrane, similar to electricity going along an electrical wire. The channels responsible for the action potential are voltage-gated Na+ and K+ channels, which are closed during rest (resting membrane potential). When a stimulus is applied to the nerve cell, following neurotransmitter activation of chemically gated channels, Na+ channels open very briefly, and Na+ diffuses quickly into the cell. © 2019 McGraw-Hill Education  31 Action Potential 2 This movement of Na+, which is called a local current, causes the inside of the cell membrane to become positive, a change called depolarization. If depolarization is not strong enough, the Na+ channels close again, and the local potential disappears without being conducted along the nerve cell membrane. If depolarization is large enough, Na+ enters the cell so that the local potential reaches a threshold value. This threshold depolarization causes voltage-gated Na+ channels to open, generally at the axon hillock. © 2019 McGraw-Hill Education  32 Action Potential 3 The opening of these channels causes a massive, 600- fold increase in membrane permeability to Na+. Voltage-gated K+ channels also begin to open. As more Na+ enters the cell, depolarization continues at a much faster pace, causing a brief reversal of charge – the inside of the cell membrane becomes positive relative to the outside of the cell membrane. The charge reversal causes Na+ channels to close and Na+ then stops entering the cell. During this time, more K+ channels are opening and K+ leaves the cell, resulting in repolarization. © 2019 McGraw-Hill Education  33 Action Potential 4 At the end of repolarization, the charge on the cell membrane briefly becomes more negative than the resting membrane potential; this condition is called hyperpolarization and occurs briefly. Action potentials occur in an all-or-none fashion All-or-none refers to the fact that if threshold is reached, an action potential occurs; if the threshold is not reached, no action potential occurs. The sodium-potassium pump assists in restoring the resting membrane potential. © 2019 McGraw-Hill Education  34 Action Potential 5 Figure 8.9 © 2019 McGraw-Hill Education  35 Action Potential 6 Figure 8.8 (1) © 2019 McGraw-Hill Education  36 Action Potential 7 Figure 8.8 (2) © 2019 McGraw-Hill Education  37 Action Potential 8 Figure 8.8 (3) © 2019 McGraw-Hill Education  38 Unmyelinated and Myelinated Axon Action Potentials Action potentials are conducted slowly in unmyelinated axons and more rapidly in myelinated axons. Action potentials along unmyelinated axons occur along the entire membrane. Action potentials on myelinated axons occur in a jumping pattern at the nodes of Ranvier. This type of action potential conduction is called saltatory conduction. © 2019 McGraw-Hill Education  39 Unmyelinated Axon Conduction Figure 8.10 © 2019 McGraw-Hill Education  40 Myelinated Axon Conduction Figure 8.11 © 2019 McGraw-Hill Education  41 Axon Conduction Speed The speed of action potential conduction varies widely, even among myelinated axons; it is based on the diameter of axon fibers. Medium-diameter, lightly myelinated axons, characteristic of autonomic neurons, conduct action potentials at the rate of about 3 to 15 meters per second (m/s). Large-diameter, heavily myelinated axons conduct action potentials at the rate of 15 to 120 m/s. © 2019 McGraw-Hill Education  42 Synapse 1 A neuroneuronal synapse is a junction where the axon of one neuron interacts with another neuron. The end of the axon forms a presynaptic terminal and the membrane of the next neuron forms the postsynaptic membrane, with a synaptic cleft between the two membranes. Chemical substances called neurotransmitters are stored in synaptic vesicles in the presynaptic terminal. © 2019 McGraw-Hill Education  43 Synapse 2 An action potential reaching the presynaptic terminal causes voltage-gated Ca2+ channels to open, and Ca2+ moves into the cell. This influx of Ca2+ causes the release of neurotransmitters by exocytosis from the presynaptic terminal. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptor molecules on the postsynaptic membrane. © 2019 McGraw-Hill Education  44 Synapse 3 The binding of neurotransmitters to these membrane receptors causes chemically gated channels for Na+, K+, or Cl− to open or close in the postsynaptic membrane. The specific channel type and whether or not the channel opens or closes depend on the type of neurotransmitter in the presynaptic terminal and the type of receptors on the postsynaptic membrane. The response may be either stimulation or inhibition of an action potential in the postsynaptic cell. © 2019 McGraw-Hill Education  45 Synapse 4 If Na+ channels open, the postsynaptic cell becomes depolarized, and an action potential will result if threshold is reached. If K+ or Cl− channels open, the inside of the postsynaptic cell tends to become more negative, or hyperpolarized, and an action potential is inhibited from occurring. There are many neurotransmitters, with the best known being acetylcholine and norepinephrine. © 2019 McGraw-Hill Education  46 Synapse 5 Neurotransmitters do not normally remain in the synaptic cleft indefinitely, thus their effects are short duration. These substances become reduced in concentration when they are either rapidly broken down by enzymes within the synaptic cleft or are transported back into the presynaptic terminal. An enzyme called acetylcholinesterase breaks down the acetylcholine. Norepinephrine is either actively transported back into the presynaptic terminal or broken down by enzymes. © 2019 McGraw-Hill Education  47 The Synapse Figure 8.12 © 2019 McGraw-Hill Education  48 Reflex A reflex is an involuntary reaction in response to a stimulus applied to the periphery and transmitted to the CNS. Reflexes allow a person to react to stimuli more quickly than is possible if conscious thought is involved. Most reflexes occur in the spinal cord or brainstem rather than in the higher brain centers. A reflex arc is the neuronal pathway by which a reflex occurs and has five basic components. © 2019 McGraw-Hill Education  49 Reflex Arc Components 1. A sensory receptor 2. A sensory neuron 3. Interneurons, which are neurons located between and communicating with two other neurons 4. A motor neuron 5. An effector organ (muscles or glands). Note: The simplest reflex arcs do not involve interneurons. © 2019 McGraw-Hill Education  50 Reflex Arc Figure 8.13 © 2019 McGraw-Hill Education  51 Neuronal Pathway (Converging) The CNS has simple to complex neuronal pathways. A converging pathway is a simple pathway in which two or more neurons synapse with the same postsynaptic neuron. This allows information transmitted in more than one neuronal pathway to converge into a single pathway. © 2019 McGraw-Hill Education  52 Neuronal Pathway (Diverging) A diverging pathway is a simple pathway in which an axon from one neuron divides and synapses with more than one other postsynaptic neuron. This allows information transmitted in one neuronal pathway to diverge into two or more pathways. © 2019 McGraw-Hill Education  53 Neuronal Pathways Figure 8.14 © 2019 McGraw-Hill Education  54 Summation 1 A single presynaptic action potential usually does not cause a sufficiently large postsynaptic local potential to reach threshold and produce an action potential in the target cell. Many presynaptic action potentials are needed in a process called summation. Summation of signals in neuronal pathways allows integration of multiple subthreshold local potentials. Summation of the local potentials can bring the membrane potential to threshold and trigger an action potential. © 2019 McGraw-Hill Education  55 Summation 2 Spatial summation occurs when the local potentials originate from different locations on the postsynaptic neuron—for example, from converging pathways. Temporal summation occurs when local potentials overlap in time. This can occur from a single input that fires rapidly, which allows the resulting local potentials to overlap briefly. Spatial and temporal summation can lead to stimulation or inhibition, depending on the type of signal. © 2019 McGraw-Hill Education