Organization of the Nervous System A.Central nervous system (CNS) 1.Components a) brain b) spinal cord 2.Function a) integrating and command center of.
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Organization of the Nervous System
A. Central nervous system (CNS)
1. Components
a) brain
b) spinal cord
2. Function
a) integrating and command center of the nervous system
dorsal body cavitydorsal body cavity
Three overlapping functionsThree overlapping functions
Marieb; Fig 11.1
Receptor
Central Nervous System
Peripheral Nervous System
Afferent neuron
Efferent neuron
Inter-neuron
Axon - peripheral processAxon - central process
Marieb; Fig 11.2
Marieb; Fig 11.2b
B. Peripheral nervous system (PNS)
1. Outside the central nervous system
2. a) functional subdivisions
(1) sensory, or afferent, division
(a) carries impulses toward the CNS from sensory receptors located throughout the body
Ascending Pathways
1. first-order neuron
2. second-order neuron
3. third-order neuron
pages 475pages 475 and 476and 476
Ascending Pathways
1. first-order neuron
2. second-order neuron
3. third-order neuron
sensory unipolar
sensory multipolar interneuron
sensory multipolar interneuron
Marieb; Fig 12.34
Marieb; Fig. 12.8Marieb; Fig 12.8
a) functional subdivisions
(2) motor, or efferent, division
(a) function
(i) carries impulses from the CNS to effector organs
(a) muscles
(b) glands
Marieb; Fig 12. 35
Somatic motor pathways
involve two neurons,
referred to as the upper and lower motor
neurons (anterior horn
neurons).
(page 478)
1. All neurons whose cell bodies are in the spinal cord gray matter are what structural type? ____________________
2. The posterior horns consist entirely of what functional type of neuron? ____________________
3. The anterior horns have some interneurons but mainly house the cell bodies of what functional type of neuron? ____________________
4. Within the spinal cord, where are the anterior horns the largest?
____________________
5. What are the names of the two motor divisions of the peripheral nervous system?
____________________ and ____________________
6. The cell bodies of the sensory neurons are found where? _________________
7. After entering the spinal cord, what route(s) does the axon of a sensory neuron take?
multipolar
interneurons
somatic motor neurons
limb-innervating cervical and lumbar regions of the cord
somatic autonomic
dorsal root (or cranial) ganglion
Some enter the posterior white matter of the cord directly and travel to synapse at higher cord or brain levels. Others synapse with interneurons in the posterior horns of the spinal cord gray matter at their entry level.
page 472page 472
(2) motor, or efferent, division
(b) two main parts
(i) somatic nervous system
(a) consists of somatic nerve fibers
(b) conduct impulses from the CNS to skeletal muscles, and allows conscious control of motor activities
(ii) autonomic nervous system
(a) involuntary system
(b) consists of visceral motor nerve fibers
(c) regulate the activity of smooth muscle, cardiac muscle, and glands
Histology of Nervous Tissue
A. Neuroglia, or glial cells, are closely associated with neurons, providing a protective and supportive network (pp. 389–391; Fig. 11.3)
1. Neuroglia of CNS
a) astrocytes – most abundant and versatile
(1) regulate the chemical environment around neurons
(2) exchange between neurons and capillaries
b) microglia
(1) monitor health and perform defense functions for neurons
Marieb; Fig 11.3
astrocytomaastrocytoma
1. Neuroglia of CNS
c) ependymal cells
(1) line the central cavities of the brain and spinal cord
(2) help circulate cerebrospinal fluid
d)oligodendrocytes
(1) wrap around neuron fibers
(a) form myelin sheaths Marieb; Fig 11.3
Marieb; Fig 11.3
2. Neuroglia of the PNS
a) satellite cells
(1) function is largely unknown
(2) found surrounding neuron cell bodies within ganglia
b) Schwann cells, or neurolemmocytes
(1) surround nerve fibers
(a) form the myelin sheath
Marieb; Fig 11.3
B. Neurons1. specialized cells that conduct messages in the form of electrical impulses throughout the body
Marieb; Fig 11.4
1. neurons function optimally for a lifetime, are mostly amitotic, and have an exceptionally high metabolic rate requiring oxygen and glucose.
a) “typical” neuron components
(1) cell body
(a) also called the perikaryon or soma
(b) major biosynthetic center containing the usual organelles except for centrioles
(2) dendrites
(a) cell processes that are the receptive regions of the cell
Clusters of cell bodiesCNS - nuclei
PNS - ganglia
Clusters of cell bodiesCNS - nuclei
PNS - ganglia
Dendrites transmit graded potentials
Dendrites transmit graded potentials
a) “typical” neuron components
(3) single axon
(a) generates and conducts nerve impulses away from the cell body to the axon terminals
(b) myelin sheath
(i) whitish, fatty, segmented covering
(ii) protects, insulates, and increases conduction velocity of axons
Axons do Axons do notnot have Nissl bodies have Nissl bodies or Golgi apparatior Golgi apparati
Axons do Axons do notnot have Nissl bodies have Nissl bodies or Golgi apparatior Golgi apparati
Axonal TransportAxonal Transport
SlowSlow - axoplasmic or cytoplasmic flow that moves from the cell body to the axon
FastFast - energy requiring process goes in both directions to deliver synaptic and secretory vesicles and mitochondria to the axon terminal (anterograde) or to return old membrane components to the cell body for recycling (retrograde)
Silverthorn; Fig. 8-4
No Nissl bodies or Golgi No Nissl bodies or Golgi apparati in cytoplasm of apparati in cytoplasm of axonaxon
Slow axonal transport – 2 mm/dayFast axonal transport – 400 mm/day
Slow axonal transport – 2 mm/dayFast axonal transport – 400 mm/day
Marieb; Fig 11.5
Adjacent Schwann cells do nottouch one another –
separated by nodes of Ranvier
Silverthorn; Fig. 8-6a
2. Structural classes of neurons
a) multipolar neurons
(1) three or more processes
2. Structural classes of neurons
b) bipolar neurons
(1) single axon and dendrite
2. Structural classes of neurons
c) unipolar neurons
(1)single process extending from the cell body that is associated with receptors at the distal end
3. Functional classes of neurons
a) sensory, or afferent, neurons
(1) conduct impulses toward the CNS from receptors
Cell bodies outside the CNS
3. Functional classes of neurons
b)motor, or efferent, neurons (multipolar)
(1) conduct impulses from the CNS to effectors
c) interneurons
(1) association neurons
(2) conduct impulses between sensory and motor neurons, or in CNS integration pathways
Cell bodies inside the CNS
Neurophysiology
A. Basic Principles of Electricity
1. voltage
a) measure of the amount of difference in electrical charge between two points, called the potential difference
2. current
a) flow of electrical charge from point to point
b) dependent on
(1) voltage
(2) resistance (hindrance to current flow)
c) in the body, electrical currents are due to the movement of ions across cellular membranes
Ohm’s Law
Current (I) = voltage (V)
resistance (R)
Ohm’s Law
Current (I) = voltage (V)
resistance (R)
1. current is directly proportional to voltage – the greater the potential difference, the greater the current (or flow of ions)
2. current is inversely proportional to resistance
1. current is directly proportional to voltage – the greater the potential difference, the greater the current (or flow of ions)
2. current is inversely proportional to resistance
B. Role of Membrane Ion Channels
1. ion channels of plasma membrane
a) some are always open
(1) called leakage channels
b) some have a protein “gate” that changes shape or opens in response to the proper signal
Marieb; Fig 3.7
(c) open (leak) channels: spend most time in open configuration
(d) gated channels: spend most time in closed state
(i) chemically gated: specific ligands open or close the channel
(ii) voltage gated: electrical state of cell opens or closes
(iii) mechanically gated: physical force opens or closes
regulate movement
of molecules through
them
Silverthorn; Fig. 5-8
Information from Chapter 3
Marieb; Fig 11.6
Ion movement follows an
electrochemical gradient
Electro – move toward an area
of opposite charge
Chemical – diffuse
passively from an area of high concentration to an area of
low concentration
Marieb; Fig 11.6
C. Resting Membrane Potential
1. Polarized neuron cell membrane
a) more negatively charged inside than outside
b) degree of this difference in electrical charge is the resting membrane potential (RMP)
(1) generation of RMP
(a) differences in ionic makeup of intracellular and extracellular fluids
(b) differential membrane permeability to solutes
Marieb; Fig 11.7
Marieb; Fig 11.7
Marieb; Fig 11.8K+ plays the most importantrole in generating the RMP
At rest, the membrane is 75X more permeable to K+ than Na+
D. Membrane Potentials That Act as Signals
1. neurons use changes in membrane potential as communication signals
a) changes in membrane permeability to any ion
b) alteration of ion concentrations on the two sides of the membrane
2. changes in membrane potential relative to resting membrane potential
a) depolarizations
(1) interior of the cell becomes less negative
b) hyperpolarizations
(1) interior of the cell becomes more negatively charged
Marieb; Fig 11.9
Increases probabilityof producing
nerve impulses
Increases probabilityof producing
nerve impulses Decreases probabilityof producing
nerve impulses
Decreases probabilityof producing
nerve impulses
3. Graded potentials
a) short-lived
b) local changes in membrane potentials
c) can either be
(1)depolarizations
(2)hyperpolarizations
d) critical to the generation of APs
Marieb; Fig 11.10
Graded – magnitude varieswith stimulus strength
Graded – magnitude varieswith stimulus strength
Names for Graded Potentials
(if located at)
sensory receptor
(if the result of a)
neurotransmitter stimulus
generator generator potentialpotential
postsynaptic postsynaptic potentialpotential
Marieb; Fig 11.11
Graded potentials are decremental –
they will die out quickly
Graded potentials are decremental –
they will die out quickly
4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate
Marieb; Fig 11.4
Site where graded potential transitions to AP
4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate
Marieb; Fig 11.12
Marieb; Fig 11.12
Marieb; Fig 11.12
Ion Movement During the
Action Potential
Ion Movement During the
Action Potential
Marieb; Fig 11.12
Ion Movement During the
Action Potential
Marieb; Fig 11.12
4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate
Marieb; Fig 11.12
4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate
a) generation of an action potential
(1) involves a transient increase in Na+ permeability
(2) followed by restoration of Na+ impermeability
(3) then a short-lived increase in K+ permeability
Depolar-ization
Repolarization and hyperpolarization
Marieb; Fig 11.12
http://bio.winona.msus.edu/berg/ANIMTNS/NaKpump2.htm
Repolarization restores Repolarization restores electricalelectrical conditions conditions
BUT BUT
sodium-potassium pump restores sodium-potassium pump restores ionicionic conditions conditions
Repolarization restores Repolarization restores electricalelectrical conditions conditions
BUT BUT
sodium-potassium pump restores sodium-potassium pump restores ionicionic conditions conditions
Marieb; Fig 11.13
b) propagation, or transmission, of an action potential
(1) local currents of an area undergo depolarization, causing depolarization of the forward adjacent area
4. Action potentials
c) repolarization
(1) restores RMP
(2) follows depolarization along the membrane
5. A critical minimum, or threshold, depolarization is defined by the amount of influx of Na+ that at least equals the amount of efflux of K+
Marieb; Fig 11.12
6. Action potentials
a) all-or-none phenomena
(1) either happen completely, in the case of a threshold stimulus
(2) or not at all, in the event of a subthreshold stimulus
7. Stimulus intensity
a) coded in the frequency of action potentials
Remember – all APs look alike!
Marieb; Fig 11.14
8. Refractory period of an axon
a) related to the period of time required so that a neuron can generate another action potentialMarieb; Fig 11.15
The absolute refractory period
ensures that a second action
potential will not occur before the first has finished.
The refractory period is a key characteristic
that distinguishesaction potentials from
graded potentials.
The refractory period is a key characteristic
that distinguishesaction potentials from
graded potentials.
E. Influence of Axon Diameter and the Myelin Sheath on Conduction Velocity
1. Axons with larger diameters
a) conduct impulses faster than axons with smaller diameters
Marieb; Fig 11.16
Largest axons have the lowest thresholdsGiant squid axon
Mammalian axon
E. Influence of Axon Diameter and the Myelin Sheath on Conduction Velocity
2. Unmyelinated axons
a) conduct impulses relatively slowly
3. Myelinated axons (saltatory conduction)
a) high conduction velocity
Marieb; Fig 11.16
PERIPHERAL NERVE AXON CLASSIFICATION SCHEME
Scheme 1
Scheme
2
Diameter (m)
CV (m/sec)
Type of nerve fiber
A
Ia
12 - 20
72 - 120
Muscle spindle primary afferent
Ib 12 - 20 72 - 120 Golgi tendon organ afferent
12 - 20 72 - 120 Skeletal muscle efferent
A
II
6 – 12
36 – 72
Touch-pressure receptor afferent
5 – 12
20 – 72
Muscle spindle secondary afferent
A
2 – 8
12 – 48
Muscle spindle efferent
A
III
1 – 5
6 – 30
Pain-temperature afferent
B
<3
2 – 18
Preganglionic autonomic efferent
C
IV
<1
<2
Pain-temperature afferent
<1
<2
Postganglionic autonomic efferent
Scheme 1: all peripheral fibers Scheme 2: sensory fibers only
PERIPHERAL NERVE AXON CLASSIFICATION SCHEME
Scheme 1
Scheme
2
Diameter (m)
CV (m/sec)
Type of nerve fiber
A
Ia
12 - 20
72 - 120
Muscle spindle primary afferent
Ib 12 - 20 72 - 120 Golgi tendon organ afferent
12 - 20 72 - 120 Skeletal muscle efferent
A
II
6 – 12
36 – 72
Touch-pressure receptor afferent
5 – 12
20 – 72
Muscle spindle secondary afferent
A
2 – 8
12 – 48
Muscle spindle efferent
A
III
1 – 5
6 – 30
Pain-temperature afferent
B
<3
2 – 18
Preganglionic autonomic efferent
C
IV
<1
<2
Pain-temperature afferent
<1
<2
Postganglionic autonomic efferent
Scheme 1: all peripheral fibers Scheme 2: sensory fibers only
Robinson & Snyder-Mackler; Table 3.1
F. Synapse
Marieb; Fig 11.17
F. Synapse
1. junction that mediates information transfer between neurons or between a neuron and an effector cell
2. presynaptic cell
a) neurons conducting impulses toward the synapse
3. postsynaptic cell
a) cell carrying impulses away from the synapse
F. Synapse
4. electrical synapses
a) neurons that are electrically coupled via protein channels
b) allow direct exchange of ions from cell to cell
ELECTRICAL SYNAPSES CHEMICAL SYNAPSES
Cytoplasmic continuity between pre- and post-synaptic cells
No cytoplasmic continuity between pre- and post-synaptic cells
Communicating agent is ionic current
Communicating agent is a chemical transmitter
Essentially no synaptic delay Significant synaptic delay
Typically bidirectional Unidirectional
F. Synapse
5. Chemical synapses Marieb; Fig 11.18
F. Synapse
5. chemical synapses
a) specialized for release and reception of chemical neurotransmitters
(1) termination of neurotransmitter effects
(a) degradation by enzymes from the postsynaptic cell or within the synaptic cleft
(b) reuptake by astrocytes or the presynaptic cell
(c) diffusion away from the synapse
6. synaptic delay
a) related to the period of time required for release and binding of neurotransmitters
G. Postsynaptic Potentials and Synaptic Integration
1. neurotransmitters mediate graded potentials
a) may be excitatory
b)may be inhibitory
EPSP
IPSP
Marieb; Fig 11.19
Silverthorn; Fig. 8-25
Likelihood of many graded
potentials reaching the trigger zone
simultaneously is quite high
2. summation by the postsynaptic neuron
a) temporal summation
(1) occurs in response to several successive releases of neurotransmitter
b) spatial summation
(1) occurs when the postsynaptic cell is stimulated at the same time by multiple terminals
Marieb; Fig 11.20
G. Postsynaptic Potentials and Synaptic Integration
3. synaptic potentiation
a) presynaptic cell is stimulated repeatedly or continuously
b) enhanced release of neurotransmitter
Appears to be important in learning and memory
G. Postsynaptic Potentials and Synaptic Integration
4. presynaptic inhibition
a) another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell (axoaxonic synapse)
5. neuromodulation
a) neurotransmitter acts via slow changes in target cell metabolism
b) chemicals other than neurotransmitter modify neuronal activity
H. Neurotransmitters and Their Receptors
1. neurotransmitters
a) one of the ways neurons communicate
b) several chemical classes
(1) Acetylcholine (ACh)
(2) Biogenic amines
(a)catecholamines
(i) dopamine
(ii) norepinephrine (NE)
(iii) epinephrine
(b) indolamines
(i) serotonin
(ii) histamine
Marieb; Fig 11.21
1. Neurotransmitters
(c) amino acids
(i) GABA
(d) peptides
(i) substance P
(ii) endorphins
(e) other messengers
(i) nitric oxide (NO)
2. Functional classifications
a) effects
(1) excitatory (cause depolarization)
(2) inhibitory (cause hyperpolarization)
b) mechanism of action
(1) direct – open ion channels
(2) indirect – work through second messengers so generally will have longer lasting effects
some do both
Marieb; Fig 3.16
3. Receptor types
a) channel-linked receptors
(1) mediate direct transmitter action
(2) brief, localized changes
Excitatory – usually Na+ entry
which contributes to membrane depolarization
Inhibitory – usually allow K+ or Cl- to move which
contributes to membranehyperpolarization
Marieb; Fig 11.22
3. Receptor types
b) G protein-linked receptors
(1) mediate indirect transmitter action
(2) slow, persistent, and often diffuse changes
G proteins often control the production of second messengers,
such as cAMP and Ca2+
G protein-linked receptors
Marieb; Fig 11.23
Receptor is activated by neurotransmitter on extracellular
surface
Binding of G protein causes GTP to ‘displace’ GDP – this ‘activating’ the G
protein
Inactive G protein binds to receptor on intracellular surface
Activated G protein binds to another membrane protein – such as adenylate cyclase which results in
its ‘activation’
Adenylate cyclase catalyzes conversion of ATP to cAMP
cAMP then acts as a second messenger usually by activating other cellular proteins
G protein-linked
receptors
Basic Concepts of Neural Integration
A. Organization of Neurons: Neuronal Pools
1. Neuronal pools
a) functional groups of neurons
b) integrate incoming information from receptors or other neuronal pools
c) relay the information to other areas
Marieb; Fig 11.23
B. Types of Circuits
1. Diverging, or amplifying, circuits
a) common in sensory and motor pathways
b) characterized by
(1) incoming fiber that triggers responses in ever-increasing numbers of fibers along the circuit
Marieb; Fig 11.24
2. Converging circuits
a) common in sensory and motor pathways
b) characterized by
(1) reception of input from many sources
(2) funneling to a given circuit
(3) strong stimulation or inhibition
Marieb; Fig 11.24
3. Reverberating, or oscillating, circuits
a) characterized by feedback by axon collaterals to previous points in the pathway
b) ongoing stimulation of the pathway
Marieb; Fig 11.24
rhythmic activities
Notice the positive feedback
system
4. Parallel after-discharge circuits
a) complex activities
b) characterized by stimulation of several neurons arranged in parallel arrays by the stimulating neuron
Marieb; Fig 11.24
C. Patterns of Neural Processing
1. Serial processing
a) exemplified by spinal reflexes
b) involves sequential stimulation of the neurons in a circuit
Marieb; Fig 11.25
C. Patterns of Neural Processing
2. Parallel processing
a) inputs stimulating many pathways simultaneously
b) vital to higher level mental functioning
Developmental Aspects of Neurons
A. Nervous system originates from a dorsal neural tube and neural crest
1. begins as a layer of neuroepithelial cells that ultimately become the CNS
B. Differentiation of neuroepithelial cells
1. occurs largely in the second month of development
So, some cueing must
exist between nerves and
muscles
Developmental Aspects of Neurons
C. Growth of an axon toward its target
1. Appears to be guided by
a) older “pathfinding” neurons and glial cells
b) nerve growth factor
c) cholesterol from astrocytes
d) tropic chemicals from target cells
Developmental Aspects of Neurons
D. Growth cone
1. growing tip of an axon
2. takes up chemicals from the environment
a) used by the cell to evaluate the pathway taken for further growth and synapse formation
Lieber; Fig 1.3
After initial NMJ formation, several other motoneuronsform synapses at this point.
Developmental Aspects of Neurons
E. Unsuccessful synapse formation
1. cell death
2. certain amount of apoptosis occurs before the final population of neurons is complete
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