The Nervous System Chapter 44 Nervous System Organization All animals must be able to respond to environmental stimuli -Sensory receptors = Detect stimulus -Motor effectors = Respond to it -The nervous system links the two -Consists of neurons and supporting cells 2 Nervous System Organization Vertebrates have three types of neurons -Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) -Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) -Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory) 3 1
Nervous System Organization The CNS consists of the brain and spinal cord The peripheral nervous system (PNS) consists of sensory and motor neurons -Somatic NS stimulates skeletal muscles -Autonomic NS stimulates smooth and cardiac muscles, as well as glands -Sympathetic and parasympathetic NS -Counterbalance each other 4 Nervous System Organization Neurons have the same basic structure -Cell body = Enlarged part containing nucleus -Dendrites = Short, cytoplasmic extensions that receive stimuli -Axon = Single, long extension that conducts impulses away from cell body 5 Nervous System Organization 6 2
Nervous System Organization Neurons are supported both structurally and functionally by cells called neuroglia -Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons -In the CNS, myelinated axons form white matter -Dendrites/cell bodies form gray matter -In the PNS, myelinated axons are bundled to form nerves 7 When a neuron is not being stimulated, it maintains a resting potential -Ranges from -40 to -90 millivolts (mv) -Average about -70 mv 8 The inside of the cell is more negatively charged than the outside because of: 1. Sodium-potassium pump = Brings two K + into cell for every three Na + it pumps out 2. Ion leakage channels = Allow more K + to diffuse out than Na + to diffuse in 9 3
10 11 Depolarization makes the membrane potential more positive, whereas a hyperpolarization makes it more negative -These small changes result in graded potentials -Can reinforce or negate each other Summation is the ability of graded potentials to combine 12 4
Action potentials result when depolarization reaches the threshold potential The action potential is caused by voltagegated ion channels -Two different channels are used: -Voltage-gated Na + channels -Voltage-gated K + channels 13 When the threshold voltage is reached, sodium channels open rapidly -Transient influx of Na + causes the membrane to depolarize In contrast, potassium channel opens slowly -Efflux of K + repolarizes the membrane 14 The action potential has three phases: -Rising, falling and undershoot Action potentials are always separate, all-ornone events with the same amplitude The intensity of a stimulus is coded by the frequency, not amplitude, of action potentials 15 5
Two ways to increase velocity of conduction: 1. Axon has a large diameter -Less resistance to current flow -Found primarily in invertebrates 2. Axon is myelinated -Action potential is only produced at the nodes of Ranvier -Impulse jumps from node to node -Saltatory conduction 16 17 Synapses Synapses are intercellular junctions -Presynaptic cell transmits action potential -Postsynaptic cell receives it Two basic types: electrical and chemical Electrical synapses involve direct cytoplasmic connections between the two cells formed by gap junctions -Relatively rare in vertebrates 18 6
Synapses Chemical synapses have a synaptic cleft between the two cells -End of presynaptic cell contains synaptic vesicles packed with neurotransmitters 19 Synapses Action potential triggers influx of Ca 2+ -Synaptic vesicles fuse with cell membrane -Neurotransmitter is released by exocytosis -Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins -Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake 20 Synapses 21 7
Synaptic Integration There are two ways that the membrane can reach the threshold voltage -Spatial summation -Many different dendrites produce EPSPs -Temporal summation -One dendrite produces repeated EPSPs 22 The Central Nervous System Sponges are only major phylum without nerves Cnidarians have the simplest nervous system -Neurons linked to each other in a nerve net -No associative activity Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity -Two nerve cords run down the body -Permit complex muscle control 23 8