Download Chapter 48: Nervous System

Biology Olympiad
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Chapter 48: Nervous Systems
Purpose: network of cells/tissues that controls body’s responses to different circumstances
o Input: environment changes  sensory receptors receive stimulus and send a signal
o Integration: signal is interpreted  appropriate response signal is conducted
o Motor output: signal is conducted from the CNS to muscular or endocrine cells
Two classes of cells
o Neuron: the functional unit of the nervous system; transmits electrochemical signals
 Types
 Sensory neurons: communicate information about external and internal environments from sensory
receptors to the CNS (input)
 Interneurons: integrate sensory input and motor output (integration)
 Motor neurons: convey impulses from CNS to effector cells (output)
 Synapse: the point of connection between one neuron and another
 Nerves: bundles of neurons wrapped in connective tissue
 Central nervous system (CNS): nerves that integrate signals; brain and spinal cord
 Peripheral nervous system (PNS): nerves that carry signals btwn CNS and rest of body
o Supporting Cells/Glia: structuralize nervous system and aid neuron function (more glia than neurons)
 Radial glia: found in developing embryos; Astrocytes: structural and metabolic support for neurons
 Schwann cells/Oligodencrocytes: grow around and myelinate the axons – electrical insulation
(composed of lipids – poor conductors); speed up signal propagation down the axon
Dendrites: convey
signals from tip to
rest of neuron
Cell body
Axon: conducts
signal towards
tips
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November 14, 2008
Myelin sheaths*:
insulating layers
formed by
supporting cells
*In PNS, the supporting
cells are called Schwann
cells. In the CNS, the
supporting cells are
called oligodendrocytes
Synaptic terminals:
specialized endings that
relay signals to other cells
by releasing chemical
messengers called
neurotransmitters
Basic pathways
o Typical signal pathway
Sensory receptor  sensory neuron  interneuron  motor neuron  effector
o
Reflex: automatic response; no interneuron, no integration; 1 synapse in the spinal cord
Sensory receptor  sensory neuron  motor neuron  effector  reflex
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Ganglion: cluster of nerve cell bodies of similar functions in PNS
Nucleus: cluster of nerve cells of similar function in the brain (NOT THE NUCLEUS OF THE CELL)
Ganglia and nuclei are important because they allow parts of the nervous system to function without
involving the entire system, such as in reflexes
Neural signals
o Membrane potential: the measured voltage across the cell membrane (all cells have electrical gradients)
 Inside: cations – mainly K+, some Na+; many large (some organic) anions – proteins, amino acids, etc.
 Outside: cations – mainly Na+, some K+; anions – mainly Cl–
o Resting potential: the membrane potential for an excitable cell in a resting state; -70 mV for neuron
Biology Olympiad
Chapter 48: Nervous Systems
November 14, 2008
o Gated ion channels: ion channels that allow neurons to change membrane potential in response to stimuli
o Hyperpolarization: increase in the electrical gradient (i.e., membrane potential becomes more negative)
 K+ channels release potassium from inside the cell, making it more negative
o Depolarization: a decrease in the electrical gradient (i.e., membrane potential increases toward 0 mV)
 Na+ channels allow sodium to flow into the cell, making it more positive
o Action potential: nerve impulse that is triggered if the depolarization reaches a certain level,
known as the threshold potential (usually –50 to –55 mV)
 “All-or-none events” – the magnitude of the action potential is independent of the strength of the
signal. As long as the strength of the signal reaches the threshold, the action potential is carried out.
Action potential
Threshold
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Voltage-gated ion channels: channels that open and close in response to membrane potential changes.
These ion channels are responsible the action potential
 Potassium channels: closed when resting, opens slowly in response to depolarization
 Sodium channels: (two types of gates)
 Activation: closed when resting, opens rapidly in response to depolarization
 Inactivation: open when resting, closes slowly in response to depolarization
o Refractory period: neuron is insensitive to depolarization because the channels are not responsive
Action potentials: self-propagating
o The depolarization of one area of the membrane (sodium ions flow inward) spreads to the neighboring
regions of the membrane, thus carrying the action potential along the entire length of the axon.
o In vertebrates, voltage-gated ion channels are concentrated in nodes of Ranvier (gaps btwn Schwann cells)
“Jumping” of action
potential, Saltatory
Conduction
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Synapses
o Presynaptic cell transmits signal across synapse to postsynaptic cell
o Electrical synapses: allow action potential to spread directly from presynaptic to postsynaptic via gap
junctions (channels that allow ion currents of action potential to flow)
o Chemical synapses
 Synaptic cleft: narrow gap separating the presynaptic cell from postsynaptic
Biology Olympiad
Chapter 48: Nervous Systems
November 14, 2008
 Synaptic vesicles: contain neurotransmitters (chemical messengers) and are located in the synaptic cleft
1) Action potential triggers influx of Ca2+ when it reaches the
synaptic terminal
2) Synaptic vesicles fuse with membrane of presynaptic neuron
3) Neurotransmitters are released
4) The neurotransmitter molecules bind with receptors in the
postsynaptic membrane, causing ion channels to open.
5) Neurotransmitters are degraded by enzymes or taken by
another neuron
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Neurotransmitters
o Acetylcholine: Excitatory at vertebrate skeletal muscles (between motor neuron and skeletal muscle cell)
o Biogenic Amines: neurotransmitters derived from amino acids
 Norepinephrine/Epinephrine: excitatory or inhibitory
 Dopamine: generally excitatory
 Serotonin: generally inhibitory
o Amino Acids
 Gamma aminobutyric acid (GABA): inhibitory
 Glycine: inhibitory
 Glutamate: excitatory
 Aspartate: excitatory
o Neuropeptides: short chains of amino acids
 Substance P (oooh): excitatory – mediates our perception of pain
 Endorphins: generally inhibitory – decreases our perception of pain by the CNS
Nervous system organizational types
o Cnidarians (hydra) – Nerve nets: system of nerves that branch throughout the body; simplest form
o Echinoderms (sea star) – Radial nerves that branch from a central nerve ring – similar to nerve net
o Flatworms (planarian)
 Some cephalization: concentration of feeding organs, sensors, and neural structures at the head
 Nerve cord: thick bundle of nerves extending longitudinally
 Transverse cords that spread throughout the body
o Annelids (leech) – Ventral nerve cord with segmental ganglia
o Mollusks (clam, chiton, snail, squid, etc.)
 Varies based on niche
 Some that are slow moving have little cephalization, with anterior nerve rings and ganglia (clam), while
others exhibit cephalization and giant axons (squids)
o Vertebrates
Biology Olympiad
Chapter 48: Nervous Systems
November 14, 2008
 Highly cephalized
 CNS: brain and spinal cord (integration)
 Axons in CNS are bundled very distinctly
 Myelin sheath gives a whitish appearance – white matter
 Gray matter – other parts of neurons; cell bodies, dendrites, etc.
 PNS: cranial and spinal nerves and associated ganglia
 Cranial nerves: originate in brain and spread to organs of upper body and head
 Spinal nerves: originate in spinal cord and spread throughout the body
 Sensory division: sensory neurons that convey info to the CNS from sensory receptors that monitor
external and internal environments (input)
 Motor division: efferent neurons that convey signals from CNS to effector cells (output)
 Somatic nervous system: carries signals to skeletal muscles in response to external stimuli –
generally considered voluntary
 Autonomic nervous system: regulate internal environment – involuntary
 Parasympathetic division: enhance activities that gain and conserve energy
 Sympathetic division: increase energy consumption
 Vertebrate brain
o Brainstem (important for data conduction and large-scale body movement)
 Medulla oblongata: control autonomic and homeostatic function (breathing, swallowing, etc.)
 Pons: fine tunes some same activities as medulla oblongata
 Midbrain: integration of sensory info
 Inferior colliculi: auditory system
 Superior colliculi: visual system
o Cerebellum (coordination of movement)
o Thalamus and Hypothalamus
 Thalamus: main input center for sensory info going to the cerebrum and main output center for motor
information leaving the
cerebrum
 Hypothalamus
 Homeostatic
regulation (!!)
 Sexual response and
mating behaviors,
fight-or-flight,
pleasure
 Circadian rhythms
 Suprachiasmatic nuclei
(SCN): function as
biological clock
o Cerebrum
 Cerebral cortex
(sophisticated behavior;
four lobes)
 Sleep and arousal
 Lateralization, Language, and Speech
 Emotions – Limbic system: parts of thalamus and hypothalamus connected to parts of cerebral
cortex (complex learning, reasoning, personality); amygdala: nucleus of cortex, organizing center
 Memory and Learning – two limbic nuclei, hippocampus (memory storage) and amygdala (memory
filter)
 Basal nuclei (motor coordination)
 Corpus callosum: connects left and right sides of cerebral cortex