2 Overview of the Nervous System Three major functions:Sensory input – sensory receptors receive signal – peripheral nervous system (nerves, eyes, ears, etc.)Integration – signal is interpreted and response started – central nervous system (brain and spinal cord)Motor output – response to stimulus – peripheral nervous system (nerves, muscle or gland cells)6.5.1
5 NeuronsFunction - conduct messages to help communication between parts of nervous system.Neurons are helped by numerous supporting cells, which provide structural support, protection, and insulation of neurons.6.5.2
6 Neuron Structure Cell body – large central part of neuron Contains nucleus and other organelles– Dendrites – receive and move signal from tips to cell body (into neuron)Axons – carry signals away from cell body to tips (out of neuron)6.5.2
7 Neuron StructureSchwann cells – supporting cells that form insulating myelin sheath layer.Increases speed of signalNodes of Ranvier – spaces in between the Schwann cellsSynaptic terminal – end of axon where neurotransmitters are released into synapse6.5.2
9 Types of NeuronsSensory neurons – communicate information about external and internal environments to central nervous system (input)Interneurons – link sensory response to motor output.Motor neurons – communicate response from central nervous system to effector cells (motor output)All combined, these neurons create a reflex arc, which integrates a stimulus and response.6.5.3
11 Membrane PotentialMembrane potential – the difference in electrical charge across the plasma membrane.The inside of the cell is negative with respect to the outside.Neurons have a resting membrane potential of -70mV
12 Membrane Potential Inside the cell: Outside the cell: Cations: potassium (K+) and few sodium (Na+)Anions: proteins, sulfate, phosphate (collectively A-) and few chloride (Cl-)Outside the cell:Cations: Sodium (Na+) and few potassium (K+)Anions: chloride (Cl-)
14 Membrane Potential – How it’s Created The plasma membrane is more permeable (more membrane channels) to K+ than to Na+.Therefore, large amounts of K+ are transferred out of the cell (down the concentration gradient)Small amounts of Na+ are transferred into the cell (down the concentration gradient)The movement of K+ and Na+ across the membrane generate a net negative membrane potential (-70mV)A sodium-potassium pump is used to move K+ back into the cell and Na+ back out of the cell to maintain the constant concentration gradients.
16 Changes in Membrane Potential Neurons are excitable cells – a stimulus can change the neuron’s membrane potentialResting potential – membrane potential of unexcited neuron (-70mV)Neurons become “excited,” when a stimulus opens a gated ion channel and increases the movement of K+ or Na+ across the plasma membrane6.5.4
17 Changes in Membrane Potential Hyperpolarization:A stimulus opens a K+ ion channel and efflux of K+ out of the cell increasesMembrane potential becomes more negative
21 Action PotentialWhen depolarization reaches a certain point, the threshold potential is achieved.When threshold potential is reached, an action potential is triggered.Action potential is a nerve impulse.Action potentials consist of a rapid depolarization, a rapid repolarization, and undershoot (hyperpolarization)6.5.4
23 Action Potential Caused by voltage-gated channels Open and close in response to changes in membrane potentialK+ channels – one gate; closed at resting potential; opens slowly during depolarizationNa+ channels – two gates:Activation gate – closed at resting potential; opens rapidly during depolarizationInactivation gate – open at resting potential; closes slowly during depolarization6.5.5
24 Steps in Action Potential Depolarization: Na+ activation gates open and Na+ enters cell.Repolarization: Na+ inactivation gate closes (prevents Na+ influx) and K+ gate opens and K+ exits cell.Undershoot: K+ gates remain open and K+ continues to leave cellResting state: All gates closed, Na+/K+ pump (active transport) moves Na+ out and K+ in to restore resting potential.6.5.5
27 Propagation of the Action Potential Action potentials are all-or-none eventsThere is no BIG action potential or small action potentialThe nervous system determines the strength of a stimulus by the frequency of action potentialsAction potentials do not travel along the axons of neurons, but are continually regenerated.
28 Synapses Synapse – junction between two neurons Transmitting cell – presynaptic cellReceiving cell – postsynaptic cellNeurons are separated by a gap called the synaptic cleft.Messages are transmitted across the synaptic cleft by chemical neurotransmitters.6.5.6
29 Steps in Synaptic Transmission A nerve impulse reaches end of presynaptic neuron.Presynaptic membrane depolarizes, opening voltage-gated Ca2+ channels.Ca2+ ions diffuse into presynaptic neuronInflux of Ca2+ causes neurotransmitter vesicles to fuse to presynaptic membrane and release neurotransmitters into the synaptic cleft (exocytosis)6.5.6
30 Steps in Synaptic Transmission Neurotransmitter diffuses across synaptic cleft and bind to receptors on postsynaptic membrane.Receptors open gated ion channels in postsynaptic membrane.Specific receptors open specific ion channelsMay open Na+, K+, or Cl- channelsDifferent ions have different responses (excitatory or inhibitory)6.5.6
31 Steps in Synaptic Transmission Enzymes quickly degrade neurotransmitter, ending its activity.E.g. acetylcholine is degraded by cholinesterase.Ca2+ is pumped out of presynaptic cell back into synaptic membrane.6.5.6
Your consent to our cookies if you continue to use this website.