Chapter 42 Smartbook Notes-
42.1 Nervous System Organization
An animal must be able to respond to environmental stimuli. A fly escapes a flyswatter; the antennae of a crayfish detect food and the crayfish moves towards it. To accomplish these actions, animals must have sensory receptors that can detect the stimulus and motor effectors that can respond to it. In most invertebrate phyla and in all vertebrate classes, sensory receptors and motor effectors are linked by way of the nervous system.
The nervous ustem of vertebrates consists of neurons and supporting cells
The central nervous system is the “command center”
In vertebrates, sensory neurons carry impulses from sensory receptors to the central nervous system (CNS), which is composed of the brain and spinal cord. Motor neurons carry impulses from the CNS to the effectors-muscles and glands
A third type of neuron is present in the nervous systems of most invertebrates and all vertebrates: interneurons- These neurons are located in the brain and spinal cord of vertebrates, where they help provide more complex reflexes and higher functions, including learning and memory
The peripheral nervous system collects information and carries out responses
Sensory and motor neurons constitute the peripheral nervous system (PNS) in vertebrates. Motor neurons that stimulate skeletal muscles to contract make up the somatic nervous system: regulates the activity of the smooth muscles, cardiac muscle, and glans compose the autonomic nervous system.
The structure of neurons supports their function
The cell body is an enlarged region containing the nucleus. Motor and association neurons possess a profusion of highly branched dendrites, enabling those cells to receive information from many different sources simultaneously. some neurons have extensions form the dendrites called dendritic spines that increase the surface area available to receive stimuli
glial cells support neurons - neurons are supported both structurally and functionally by glial cells, which are collectively called neuroglia
Two of the most important kinda of neuroglia in vertebrates are Schwann cells and oligodendrocytes, which produce myelin sheaths that surround the axons of many neurons
Schwann cells produce myelin in the PNS, and oligodendrocytes produce myelin in the CNS. During development, these cells wrap themselves around each axon several times to form the myelin sheath- an insulating covering consisting of multiple layers of compacted membrane
-important because they produce myelin sheaths
42.2 The Mechanism of Nerve Impulse Transmission
Neuronal function depends on the ability to create an electric potential across the plasma membrane and then alter this potential to propagate signals . Because cells are aqueous solutions, electric charge is carried by ions, and cells manipulate the concentrations of a number of important ions across the plasma membrane.
When neuron is stimulated, electrical changed in the plasma membrane spread or propagate from one part of the cell to another. this signaling depends on the properties of a variety of specialized membrane transport proteins.
An electrical difference exists across the plasma membrane
Ion channels are specific for different ions, such as K plus or Na plus, and they can be either leakage channels or gated channels.
Membrane channels that are open all the time are leakage channels, while channels that open in response to a stimulus are called gated channels.
Changes in membrane potential are due to changes in membrane permeability
The resting potential is due to the diffusion of K plus through leakage channels. Changes in membrane potential are sue to the action of a different class of ion channels called gated channels.
ligand-gated channels: respond to a chemical signal
voltage-gated channels: respond to changes in membrane potential
ligand-gated channels produce graded potentials that determine whether an axon will fire , and volatage-gated channels produce action potentials
If the membrane potential becomes less negative (more positive), it is a depolarization whereas if the membrane potential becomes more negative, it is a hyperpolarization.
Action potentials result when sodium and potassium ions mvoe across the membrane
The graphic representation of an action potential consists of: a rising phase, a falling phase, and an undershoot phase. When an excitatory (depolarizing) stimulus of enough strength, often called the threshold stimulus, is recieved by a neuron, it opens up chemically gated Na+ channels. When the threshold potential of -55mV is reached, voltage-gated channels open.
Neuronal function most directly depends on the ability to manipulate the concentrations of ions across the cell membrane.
Action potentials are propagated long axons
The movement of an action potential through an aoxn is not generated by ions flowing from the base of the axon to the end. Instead, an action potential originates at the base of the axon, and is the recreated in adjacent stretches of membrane along the axon
An action potential is propagated
There are two ways to increase the velocity of nerve impulses
Animals have evolved two ways to increase the velocity of nerve impulses. The velocity of conduction is greater if the diameter of the axon is large or if the axon is myelinated.
If an axon is increased in diameter of if it is myelinated, then there will be an increase in the speed of nerve impulses
42.3 Synapses: Where Neurons Communicate with Other Cells
An action potential passing down an axon eventually reaches the end of the axon and all of its branches. Neurons communicate with other neurons, with muscle cells, or with gland cells through specialized intercellular junctions called synapses that are found at the end of an axon.
the neurons whose axon transmits action potentials to the synapse is termed the presynaptic cell, and the cell receiving the signal on the other side of the synapse is the postsynaptic cell.
The two types of synapses are electrical and chemical
The nervous systems of animals have two basic types of synapses: electrical and chemical. Electrical synapses involve direct cytoplasmic connections formed by gap junctions between the pre- and postsynaptic neurons. Membrane potential changes, including action potentials, pass directly and rapidly from one cell to the other through the gap junctions
electrical synapses are common in invertebrate nervous systems, but less so in vertebrates
The vast majority of vertebrate synapses are chemical synapses
The type of synapse that involves direct cytoplasmic extensions from the presynaptic cell to the postsynaptic cell is an electrical synapse
The neuromuscular junction is the synapse between a neuron and a mucle fiber.
Many different chemical compound serve as neurotransmitters
The activation of which of the following causes small changes in membrane potential known as graded potentials
Acetylcholine
The neurotransmitter that is released in the synapse between a motor neuron and a muscle fiber is ACh or acetylcholine
Acetylcholine is the neurotransmitter that crosses the synapse between a motor neurons and a muscle fiber.
This synapse is called a neuromuscular junction. Acetylcholine binds to its receptor proteins in the postsynaptic membrane and causes ligandgated ion channels within these proteins to open. As a result, that site on the postsynaptic memebrane produces a a depolarization, called an excitatory postsynaptic potential (EPSP)
The EPSP, if large enough, can open the voltage-gated channels for Na+ and K+ that are responsible for action potentials. Becuase the postsynaptic cell in this case is a skeletal muscle fiber, the action potentials it produces stimulate muscle contraction through mechanisms.
An action potential is propagated along the axon pathway by which of the following?
Recreating the acion potential in adjacent stretches of axon membrane.
Which of the following factors can increase the velocity of a nerve impulse?
Increasing the diameter
Myelination of the axon
The binding of which of the following neurotransmitters to receptor proteins at the postsynaptic membrane cause an EPSP in the postsynaptic neuron? - ACh
The binding of ACh to postsynaptic receptors proteins cause which of the following
Opening of ligand-gated ion channels
An EPSP at the site
Glycine and GABA are neurotransmitters that act in which of the following ways?
They bind to receptor proteins
They open ligand gated channels for CL-
A large enough EPSP can cause the opening of Na+ and K+ voltage gated channels which can result in which of the following
an action potential
Epinephrine, dopamine, and norepinephrine can be categorized as which of the following?
-Biogenic amines
Skeletal muscles are simulated by the somatic motor neurons of the peripheral nervous system
The medulla oblongata, sympathetic division , and parasympathetic devision are components of the autonomic nervous system