Chapter 45 Nervous Regulation

Nervous Systems Contains Contains : Subdivided into Brain Spinal Cord Peripheral Nervous System (PNS) Central Nervous System (CNS) Contains : Sensory Receptors Sensory Neurons Stimulus detection Effectors Muscles and Glands Motor Neurons Respond to stimuli Subdivided into Autonomic System Involuntary regulation Somatic System Voluntary regulation Contains Brain Control and coordination of responses to stimuli Spinal Cord Interneurons – relay information Interneurons are located in both the brain and spinal cord Nervous Systems

I) Impulse Pathways Impulses move in one-way pathways along neurons Receptor (sensory structures) Sensory Neurons Interneurons (brain and/or spinal cord) Motor Neurons Effectors (response structures) ReceptorSensory NeuronInterneuronMotor NeuronEffector

II) Neurons Neurons are nerve cells with specialized structures Cell body (cyton) – contains nucleus and organelles to regulate cell processes Dendrites – cytoplasmic extensions that contain receptors for sensory stimuli Temperature, touch, pressure, chemicals Axon – long extension of cytosol that carries impulses Myelin Sheath – lipid layer that insulates axons Myelin is formed by Schwann cells growing on the axon Schwann cells are a support cell known as a glial cell Terminal Branches – end branches of neuron Synaptic Knobs – end of the neuron that contain neurotransmitters

III) Nerve Impulses Neurons are electrically excitable Impulses are the result of depolarizations along the neuron Neurons maintain a resting potential using the sodium- potassium pump Ions move across the membrane through ion channels Voltage gated channels open at a specific charge Sodium – open at -55mV and close at +35 mV Potassium – open at +35 mV and close at -70 mV Calcium – located at synaptic knobs only - open at +35mV Resting Potential Internal charge is -70mV (lower than the outside) Sodium is in a higher concentration outside the cell Polarized membrane has potential for activity to occur Depolarization Occurs when ions move across a membrane and the charge difference changes Becomes less polarized

Threshold Potential (-55mV) Local Potential – some Na+ ions enter the cell in response to a stimulus (-70mv to -56V) Results in a depolarization as more Na+ enters the cell reducing the charge difference across the membrane Interior of neuron gradually increases in electrical charge Ligand-gated Na+ channels open Threshold Potential (-55mV) The point at which an impulse “fires” All-or-nothing event – must achieve -55mV voltage When threshold is reached ALL voltage gated Na+ ion channels open and Na+ ions rush into the cell Action Potential (Impulse) Moment when all Na+ channels open and the cell interior becomes positively charged to +35mV Repolarization – return to resting potential (-70mV) All voltage gated Na+ channels close and voltage-gated K+ channels open to let potassium exit the cell The Na+ / K+ pump moves Na+ out of the cell Remember the Na+ / K+ pump operates the entire time During repolarization Na+ and K+ exit the neuron to return the interior to a negative (-70mV) charge animation

F. Impulse Propagation Nerve impulses travel at very high rates Approximately 100 meters/second Two Factors Affect Impulse Speed Diameter of Axon Large diameter reduces internal resistance Myelination of Neurons Myelin sheath insulates nerves and promotes “jumping” impulses Saltatory Conduction – impulses “jump” from node to node animation Fewer depolarizations are required - Depolarization only occurs at exposed areas on Nodes of Ranvier

IV) Synapses Microscopic gaps between neurons Synaptic cleft (synapse) is 20 nm wide Two components to a synapse Presynaptic Membrane (synaptic knob of terminal branch) Contains synaptic vesicles filled with neurotransmitter Calcium ions enter the presynaptic membrane in response to the influx of sodium from an action potential This causes the vesicles to fuse with cell membrane and release neurotransmitters into synaptic cleft Post Synaptic Membrane (dendrites of another nerve cell) Contains ligand-gated sodium ion channels Ligand-gated channels open only when a neurotransmitter (ligand) binds to the receptor site This causes an influx of sodium ions and starts an action potential in the post-synaptic membrane animation

V) Human Nervous System Central Nervous System Brain 2 hemispheres (sides) separated by a large furrow Connected by nerves called corpus callosum Provides integration between 2 hemispheres 3 Major parts of the brain Cerebrum – thought, senses, memory, conscious activity Cerebellum – coordination of movements Medulla Oblongata – involuntary actions, reflexes, heart rate, breathing, blood pressure Other structures Hypothalamus – body temperature, control of pituitary gland, hunger, thirst Thalamus – integration of sensory impulses to cerebral cortex

Peripheral Nervous System Spinal Cord Relay of impulses from peripheral nervous system Reflex regulation Reduces reaction time Does not require thought and cerebral processing Peripheral Nervous System Contains sensory and motor neurons Two Subdivisions Autonomic System – involuntary actions Heart rate Blood pressure Hormone secretion Secretion of digestive enzymes

Subdivisions of Autonomic System Sympathetic Nervous System Parasympathetic Nervous System Stimulation Increases the following: Heart rate Blood pressure Adrenaline release Glucose released into blood Dilates iris Inhibits digestion Dry mouth Conserve Energy Decreases the following: Heart rate Blood pressure Glucose release Increased salivation Increased digestion “rest and rumination” Subdivisions of Autonomic System

Somatic Nervous System Control of voluntary activities Skeletal muscles Diaphragm Blinking