Lecture - Nervous System Audio

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Nervous System

Nervous System Functions:

Nervous System Functions 1) Sensory input – gathering information To monitor changes occurring inside and outside the body Changes = stimuli 2) Integration To process and interpret sensory input and decide if action is needed 3) Motor output A response to integrated stimuli The response activates muscles or glands Figure 11.1

Organization of the Nervous System:

Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Spinal and cranial nerves Carries messages to and from the spinal cord and brain

Peripheral Nervous System (PNS): Two Functional Divisions:

Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Somatic afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent fibers – transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts:

Motor Division: Two Main Parts Somatic nervous system Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands Divisions – sympathetic and parasympathetic

Histology of Nerve Tissue:

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons (glial cells) Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth

Neurons (Nerve Cells):

Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane function in: Electrical signaling Cell-to-cell signaling during development

Neurons - cell body (soma):

Neurons - cell body (soma) Figure 11.4b Contains the nucleus Is the focal point for the outgrowth of neuronal processes Has no centrioles (amitotic) Contains an axon hillock – cone-shaped area from which axons arise

Processes:

Processes Armlike extensions from the soma; two types Called tracts in the CNS and nerves in the PNS Dendrites They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials) Axon Arise from the hillock; one per neuron (two axons = axon collaterals; rare) Long axons are called nerve fibers Axonal terminal – branched terminus of an axon

Axons: Function:

Axons: Function Generate and transmit action potentials; secrete neurotransmitters from the axon terminals Myelin sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

Nodes of Ranvier (Neurofibral Nodes):

Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge

Neuron Classification:

Neuron Classification Structural: Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (association neurons) — shuttle signals through CNS pathways

Electrical Current and the Body:

Electrical Current and the Body Reflects the flow of ions rather than electrons There is a potential on either side of membranes when: The number of ions is different across the membrane The membrane provides a resistance to ion flow

Role of Ion Channels:

Role of Ion Channels Types of plasma membrane ion channels: Passive, or leakage, channels – always open Chemically gated channels – open with binding of a specific neurotransmitter Voltage-gated channels – open and close in response to membrane potential Mechanically gated channels – open and close in response to physical deformation of receptors

Operation of a Chemically Gated Channel:

Operation of a Chemically Gated Channel Example: Na + -K + gated channel Closed when a neurotransmitter is not bound to receptor Na + cannot enter the cell and K + cannot exit the cell Open when a neurotransmitter is attached to receptor Na + enters the cell and K + exits the cell

Operation of a Voltage-Gated Channel:

Operation of a Voltage-Gated Channel Example: Na + channel Closed when the intracellular environment is negative Na + cannot enter the cell Open when the intracellular environment is positive Na + can enter the cell

Gated Channels:

Gated Channels When gated channels are open: Ions move quickly across the membrane Movement is along their electrochemical gradients An electrical current is created Voltage changes across the membrane Electrochemical gradient – the electrical and chemical gradients taken together Ions flow along their chemical gradient when they move from high concentration to low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge

Resting Membrane Potential (Vr):

Resting Membrane Potential (V r ) The potential difference (–70 mV) across the membrane of a resting neuron It is generated by different concentrations of Na + , K + , Cl  , and protein anions (A  ) Ionic differences are the consequence of: Differential permeability of the membrane to Na + and K + Operation of the sodium-potassium pump

Resting Membrane Potential (Vr):

Resting Membrane Potential (V r ) Figure 11.8

Membrane Potentials: Signals:

Membrane Potentials: Signals Used to integrate, send, and receive information Membrane potential changes are produced by: Changes in membrane permeability to ions Alterations of ion concentrations across the membrane Types of signals – graded potentials and action potentials

Changes in Membrane Potential:

Changes in Membrane Potential Depolarization – the inside of the membrane becomes less negative Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

Graded Potentials:

Graded Potentials Short-lived, local changes in membrane potential Decrease in intensity with distance Magnitude varies directly with the strength of the stimulus Sufficiently strong graded potentials can initiate action potentials

Action Potentials (APs):

Action Potentials (APs) A brief reversal of membrane potential with a total amplitude of 100 mV Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance They are the principal means of neural communication

Phases of the Action Potential:

Phases of the Action Potential 1 – resting state 2 – depolarization phase 3 – repolarization phase 4 – hyperpolarization Figure 11.12

Action Potential: Resting State:

Action Potential: Resting State Na + and K + channels are closed Leakage accounts for small movements of Na + and K + Each Na + channel has two voltage-regulated gates Activation gates – closed in the resting state Inactivation gates – open in the resting state Figure 11.12.1

Action Potential: Depolarization Phase:

Action Potential: Depolarization Phase Na + permeability increases; membrane potential reverses Na + gates are opened; K + gates are closed Threshold – a critical level of depolarization (-55 to -50 mV) At threshold, depolarization becomes self-generating Figure 11.12.2

Action Potential: Repolarization Phase:

Action Potential: Repolarization Phase Sodium inactivation gates close  m embrane permeability to Na + declines to resting levels As sodium gates close, voltage-sensitive K + gates open  K + exits the cell and internal negativity of the resting neuron is restored Sodium potassium pump ultimately restores the resting ionic concentrations Figure 11.12.3

Action Potential: Hyperpolarization:

Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K + This efflux causes hyperpolarization of the membrane (undershoot) The neuron is insensitive to stimulus and depolarization during this time Figure 11.12.4

Threshold and Action Potentials:

Threshold and Action Potentials Threshold – membrane is depolarized by 15 to 20 mV; started by amount of current flowing through the membrane Weak (subthreshold) stimuli are not relayed into action potentials Strong (threshold) stimuli are relayed into action potentials All-or-none phenomenon – action potentials either happen completely, or not at all

Coding for Stimulus Intensity:

Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity Strong stimuli can generate an action potential more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulse transmission

Absolute Refractory Period:

Absolute Refractory Period Time from the opening of the Na + activation gates until the closing of inactivation gates Prevents the neuron from generating an action potential Ensures that each action potential is separate Enforces one-way transmission of nerve impulses

Relative Refractory Period:

Relative Refractory Period The interval following the absolute refractory period when: Sodium gates are closed Potassium gates are open Repolarization is occurring The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events

Conduction Velocities of Axons:

Conduction Velocities of Axons Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster the impulse Presence of a myelin sheath – myelination dramatically increases impulse speed

Synapses:

Synapses A junction that mediates information transfer from one neuron either to another neuron or to an effector cell Presynaptic neuron – conducts impulses toward the synapse Postsynaptic neuron – transmits impulses away from the synapse

Types of Synapses:

Types of Synapses Axodendritic – synapses between the axon of one neuron and the dendrite of another Axosomatic – synapses between the axon of one neuron and the soma of another Other types of synapses include: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma)

Electrical Synapses:

Electrical Synapses Electrical synapses: Are less common than chemical synapses Correspond to gap junctions found in other cell types Are important in the CNS in: Arousal from sleep Mental attention Emotions and memory Ion and water homeostasis

Chemical Synapses:

Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts: Axonal terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the dendrite(s) or soma of the postsynaptic neuron

Synaptic Cleft: Information Transfer:

Synaptic vesicles containing neurotransmitter molecules Axon of presynaptic neuron Synaptic cleft Ion channel (closed) Ion channel (open) Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Ion channel closed Ion channel open Neurotransmitter Receptor Postsynaptic membrane Degraded neurotransmitter Na + Ca 2+ 1 2 3 4 5 Action potential Figure 11.18 Synaptic Cleft: Information Transfer Prevents impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: 1) Is a chemical event (as opposed to an electrical one) 2) Ensures unidirectional communication between neurons

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