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
• Nervous system links the two– Consists of neurons and supporting cells
2
Nervous System Organization
• Vertebrates have three types of neurons1. Sensory neurons (afferent neurons) carry
impulses to central nervous system (CNS)
2. Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands)
3. Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)
3
Nervous System Organization
• Central nervous system (CNS )– Brain and spinal cord
• Peripheral nervous system (PNS) – 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
5
6
PN
SC
NS
Brain and Spinal Cord
Sympathetic nervoussystem
"fight or flight"
Parasympathetic nervoussystem
"rest and repose"
Somatic nervoussystem
(voluntary)
Sensory neuronsregistering external
stimuli
Autonomic nervoussystem
(involuntary)
Sensory Pathways Motor Pathways
central nervous system (CNS)peripheral nervous system (PNS)
Sensory neuronsregistering external
stimuli
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
7
8
Nervous System Organization
Nervous System Organization
• Neuroglia– Support neurons both structurally and
functionally– 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
9
Nerve Impulse Transmission
• The inside of the cell is more negatively charged than the outside
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
10
11
Nerve Impulse Transmission
• Two major forces act on ions in establishing the resting membrane potential1. Electrical potential produced by unequal
distribution of charges
2. Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other
12
Nerve Impulse Transmission
• Sodium–potassium pump creates significant concentration gradient
• Concentration of K+ is much higher inside the cell• Membrane not permeable to negative ions• Leads to buildup of positive charges outside and
negative charges inside cell• Attractive force to bring K+ back inside cell• Equilibrium potential – balance between
diffusional force and electrical force
13
14
Nerve Impulse Transmission
Nerve Impulse Transmission
• Depolarization makes the membrane potential more positive
• Hyperpolarization makes it more negative• These small changes result in graded potentials• Size depends on either the strength of the
stimulus or the amount of ligand available to bind with their receptors
• Can reinforce or negate each other• Summation is the ability of graded potentials to
combine15
16
Nerve Impulse Transmission
Nerve Impulse Transmission
• Action potentials– Result when depolarization reaches the threshold
potential (–55 mV)– Depolarizations bring a neuron closer to the
threshold– Hyperpolarizations move the neuron further from
the threshold– Caused by voltage-gated ion channels
• Voltage-gated Na+ channels • Voltage-gated K+ channels
17
Nerve Impulse Transmission
• Voltage-gated Na+ channels – Activation gate and inactivation gate– At rest, activation gate closed, inactivation gate open– Transient influx of Na+ causes the membrane to
depolarize
• Voltage-gated K+ channels– Single activation gate that is closed in the resting
state– K+ channel opens slowly– Efflux of K+ repolarizes the membrane
18
Nerve Impulse Transmission
• The action potential has three phases– Rising, falling, and undershoot
• Action potentials are always separate, all-or-none events with the same amplitude
• Do not add up or interfere with each other
• Intensity of a stimulus is coded by the frequency, not amplitude, of action potentials
19
20
Nerve Impulse Transmission
• Propagation of action potentials– Each action potential, in its rising phase,
reflects a reversal in membrane polarity– Positive charges due to influx of Na+ can
depolarize the adjacent region to threshold – And so the next region produces its own action
potential– Meanwhile, the previous region repolarizes
back to the resting membrane potential• Signal does not go back toward cell body
21
22
23
Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Nerve Impulse Transmission
• Two ways to increase velocity of conduction– Axon has a large diameter
• Less resistance to current flow• Found primarily in invertebrates
– Axon is myelinated • Action potential is only produced at the
nodes of Ranvier• Impulse jumps from node to node• Saltatory conduction
24
25
Nerve Impulse Transmission
Synapses
• Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells
• Presynaptic cell transmits action potential
• Postsynaptic cell receives it
• Two basic types: electrical and chemical
26
• Electrical synapses– Involve direct cytoplasmic
connections between the two cells formed by gap junctions
– Relatively rare in vertebrates
• Chemical synapses– Have a synaptic cleft
between the two cells– End of presynaptic cell
contains synaptic vesicles packed with neurotransmitters
27
Synapses
• Chemical synapses– Action potential triggers influx of Ca2+ – 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– Produces graded potentials in the postsynaptic
membrane– Neurotransmitter action is terminated by
enzymatic cleavage or cellular uptake28
29
Synapses
Neurotransmitters
• Acetylcholine (ACh)– Crosses the
synapse between a motor neuron and a muscle fiber
– Neuromuscular junction
30
Neurotransmitters
• Amino acids– Glutamate
• Major excitatory neurotransmitter in the vertebrate CNS
• Glycine and GABA (-aminobutyric acid) are inhibitory neurotransmitters
– Open ligand-gated channels for Cl– – Produce a hyperpolarization called an inhibitory
postsynaptic potential (IPSP)
31
32
Neurotransmitters
• Biogenic amines– Epinephrine (adrenaline) and norepinephrine
are responsible for the “fight or flight” response
– Dopamine is used in some areas of the brain that control body movements
– Serotonin is involved in the regulation of sleep
33
Neurotransmitters
• Neuropeptides– Substance P is released from sensory
neurons activated by painful stimuli– Intensity of pain perception depends on
enkephalins and endorphins– Nitric oxide (NO)
• A gas – produced as needed from arginine• Causes smooth muscle relaxation
34
Drug Addiction
• Habituation– Prolonged exposure to a stimulus may cause
cells to lose the ability to respond to it– Cell decreases the number of receptors
because there is an abundance of neurotransmitters
– In long-term drug use, means that more of the drug is needed to obtain the same effect
35
Drug Addiction
• Cocaine – Affects neurons in the brain’s “pleasure
pathways” (limbic system) – Binds dopamine transporters and prevents
the reuptake of dopamine– Dopamine survives longer in the synapse and
fires pleasure pathways more and more
36
37
Drug Addiction
• Nicotine– Binds directly to a specific receptor on
postsynaptic neurons of the brain– Binds to a receptor for acetylcholine– Brain adjusts to prolonged exposure by
“turning down the volume” by• Making fewer receptors to which nicotine binds• Altering the pattern of activation of the nicotine
receptors
38
Cerebrum
• The increase in brain size in mammals reflects the great enlargement of the cerebrum
• Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum
• Each hemisphere receives sensory input from the opposite side
• Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes
39
40
Cerebrum
Cerebrum
• Cerebral cortex– Outer layer of the cerebrum– Contains about 10% of all neurons in brain– Highly convoluted surface
• Increases threefold the surface area of the human brain
– Divided into three regions, each with a specific function
41
Cerebrum
• Cerebral cortex• Primary motor cortex – movement control• Primary somatosensory cortex – sensory
control • Association cortex – higher mental functions • Basal ganglia
• Aggregates of neuron cell bodies – gray matter• Participate in the control of body movements
42
43
44
Each of these regions of the cerebral cortex is associated with a different region of the body
Other Brain Structures
• Thalamus– Integrates visual, auditory, and somatosensory
information
• Hypothalamus– Integrates visceral activities – Controls pituitary gland
• Limbic system– Hypothalamus, hippocampus, and amygdala– Responsible for emotional responses
45
Complex Functions of the Brain
• Sleep and arousal– One section of reticular formation is the
reticular-activating system • Controls consciousness and alertness
– Brain state can be monitored by means of an electroencephalogram (EEG)
• Records electrical activity
46
Complex Functions of the Brain
• Language– Left hemisphere is “dominant” hemisphere
• Different regions control various language activities• Adept at sequential reasoning
– Right hemisphere is adept at spatial reasoning
• Primarily involved in musical ability• Nondominant hemisphere is also important for the
consolidation of memories of nonverbal experiences
47
48
Complex Functions of the Brain
• Memory– Appears dispersed across the brain– Short-term memory is stored in the form of
transient neural excitations– Long-term memory appears to involve
structural changes in neural connections– Two parts of the temporal lobes, the
hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory
49
Complex Functions of the Brain
• Alzheimer disease – Condition where memory and thought become
dysfunctional– Two causes have been proposed
1. Nerve cells are killed from the outside in– External protein: -amyloid
2. Nerve cells are killed from the inside out– Internal proteins: tau (
50
Spinal Cord
• Cable of neurons extending from the brain down through the backbone
• Enclosed and protected by the vertebral column and the meninges
51
Spinal Cord
• 2 zones– Inner zone is gray matter
• Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia
– Outer zone is white matter• Contains cables of sensory axons in the dorsal
columns and motor axons in the ventral columns
52
Spinal Cord
• It serves as the body’s “information highway”– Relays messages between the body and the
brain
• It also functions in reflexes– The knee-jerk reflex is monosynaptic– However, most reflexes in vertebrates involve
a single interneuron
53
54
Knee-jerk reflex is monosynaptic
55Most reflexes in vertebrates involve a single interneuron
The Peripheral Nervous System
• Consists of nerves and ganglia– Nerves are bundles of axons
bound by connective tissue– Ganglia are aggregates of
neuron cell bodies
• Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells
56
The Peripheral Nervous System
• Sensory neurons– Axons enter the dorsal surface of the spinal
cord and form dorsal root of spinal nerve– Cell bodies are grouped outside the spinal
cord in dorsal root ganglia
• Motor neurons– Axons leave from the ventral surface and form
ventral root of spinal nerve– Cell bodies are located in the spinal cord
57
The Autonomic Nervous System
• Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata
• In both, efferent motor pathway has 2 neurons– Preganglionic neuron – exits the CNS and
synapses at an autonomic ganglion– Postganglionic neuron – exits the ganglion and
regulates visceral effectors• Smooth or cardiac muscle or glands
58
59
The Autonomic Nervous System
• Sympathetic division– Preganglionic neurons originate in the
thoracic and lumbar regions of spinal cord – Most axons synapse in two parallel chains of
ganglia right outside the spinal cord
• Parasympathetic division– Preganglionic neurons originate in the brain
and sacral regions of spinal cord – Axons terminate in ganglia near or even
within internal organs
60
61