Download Structures and Functions Lecture 2

KayOnda Bayo
Organization and Division of the
Nervous System & Cranial Nerves:
Sensory, Motor, Mixed
Figure 11.1 The nervous system’s functions.
Sensory input
Integration
Motor output
Divisions of the Nervous System
• Central nervous system (CNS)
• Peripheral nervous system (PNS)
Peripheral Nervous System (PNS)
• Two functional divisions
• Sensory (afferent) division
• Motor (efferent) division
• Two divisions
• Somatic nervous system
• Autonomic nervous system
Motor Division of PNS: Somatic Nervous
System
• Somatic motor nerve fibers
• Conducts impulses from CNS to skeletal muscle
• Voluntary nervous system
Motor Division of PNS:
Autonomic Nervous System
• Visceral motor nerve fibers
• Regulates smooth muscle, cardiac muscle, and
glands
• Involuntary nervous system
• Two functional subdivisions
• Sympathetic
• Parasympathetic
Figure 11.2 Levels of organization in the nervous system.
Central nervous system (CNS)
Peripheral nervous system (PNS)
Brain and spinal cord
Cranial nerves and spinal nerves
Integrative and control centers
Communication lines between the CNS
and the rest of the body
Sensory (afferent) division
Motor (efferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory fiber
Skin
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Visceral sensory fiber
Stomach
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body systems
during activity
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Sympathetic motor fiber of ANS
Heart
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Bladder
Histology of Nervous Tissue
• Two principal cell types
• Neuroglia
• Neurons (nerve cells)
Neuron Cell Body (Perikaryon or
Soma)
• Biosynthetic center of neuron
• Synthesizes proteins, membranes, and other chemicals
• Rough ER (chromatophilic substance or Nissl bodies)
• Most active and best developed in body
• Spherical nucleus with nucleolus
• Some contain pigments
• In most, plasma membrane part of receptive region
• Most neuron cell bodies in CNS
• Nuclei – clusters of neuron cell bodies in CNS
• Ganglia – lie along nerves in PNS
Figure 11.4a Structure of a motor neuron.
Dendrites
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Nucleus
Nucleolus
Chromatophilic
substance (rough
endoplasmic
reticulum)
Axon hillock
Axon
(impulsegenerating
and -conducting
region)
Impulse
direction
Myelin sheath gap
(node of Ranvier)
Schwann cell
Terminal branches
Axon
terminals
(secretory
region)
Figure 11.4b Structure of a motor neuron.
Neuron cell body
Dendritic
spine
Myelin Sheath
• Composed of myelin
• Segmented sheath around most long or largediameter axons
• Function of myelin
• Nonmyelinated fibers conduct impulses more
slowly
Table 11.1 Comparison of Structural Classes of Neurons (1 of 3)
Table 11.1 Comparison of Structural Classes of Neurons (2 of 3)
Functional Classification of
Neurons
• Three types
• Sensory (afferent)
• Motor (efferent)
• Interneurons
Table 11.1 Comparison of Structural Classes of Neurons (3 of 3)
Role of Membrane Ion Channels
• Large proteins serve as selective membrane ion
channels
• Two main types of ion channels
• Leakage (nongated) channels
• Gated
Role of Membrane Ion Channels:
Gated Channels
• Three types
• Chemically gated (ligand-gated) channels
• Voltage-gated channels
• Mechanically gated channels
Figure 11.6 Operation of gated channels.
Chemically gated ion channels
Open in response to binding of the
appropriate neurotransmitter
Voltage-gated ion channels
Open in response to changes
in membrane potential
Neurotransmitter chemical
attached to receptor
Receptor
Membrane
voltage
changes
Chemical
binds
Closed
Open
Closed
Open
Resting Membrane Potential:
Differences in Ionic Composition
• ECF has higher concentration of Na+ than ICF
• ICF has higher concentration of K+ than ECF
• K+ plays most important role in membrane
potential
Action Potentials (AP)
• Principle way neurons send signals
• Principal means of long-distance neural
communication
• Occur only in muscle cells and axons of neurons
• Brief reversal of membrane potential with a change
in voltage of ~100 mV
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents.
The big picture
Resting state
Membrane potential (mV)
1
The key players
2
Voltage-gated Na+ channels
Voltage-gated K+ channels
Outside
cell
Outside
cell
Depolarization
+30
3
3 Repolarization
0
Action
potential
2
4 Hyperpolarization
Closed
Opened
Inactivated
Closed
Opened
The events
Threshold
–55
–70
1
0
4
1
2
3
Time (ms)
Sodium
channel
1
0
2
Action
potential
Na+
permeability
K+ permeability
–55
–70
1
0
1
4
1
2
3
Time (ms)
4
Relative membrane
permeability
+30
3
Potassium
channel
4
Activation
gates
Inactivation
gate
The AP is caused by permeability changes in the
plasma membrane:
Membrane potential (mV)
Inside
cell
Inactivation
gate
Inside Activation
cell
gate
1 Resting state
4 Hyperpolarization
2 Depolarization
3 Repolarization
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
1 Resting state. No
2 Depolarization
is caused by Na+
flowing into the cell.
Membrane potential (mV)
ions move through
voltage-gated
channels.
3 Repolarization is
caused by K+ flowing
out of the cell.
+30
3
4 Hyperpolarization is
0
Action
potential
2
Threshold
–55
–70
caused by K+ continuing to
leave the cell.
1
0
1
4
1
2
3
Time (ms)
4
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
1 Resting state. No
Membrane potential (mV)
ions move through
voltage-gated
channels.
+30
0
Action
potential
Threshold
–55
–70
1
0
1
1
2
3
Time (ms)
4
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
1 Resting state. No
2 Depolarization
is caused by Na+
flowing into the cell.
Membrane potential (mV)
ions move through
voltage-gated
channels.
+30
0
Action
potential
2
Threshold
–55
–70
1
0
1
1
2
3
Time (ms)
4
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
1 Resting state. No
2 Depolarization
is caused by Na+
flowing into the cell.
Membrane potential (mV)
ions move through
voltage-gated
channels.
3 Repolarization is
caused by K+ flowing
out of the cell.
+30
3
0
Action
potential
2
Threshold
–55
–70
1
0
1
1
2
3
Time (ms)
4
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
1 Resting state. No
2 Depolarization
is caused by Na+
flowing into the cell.
Membrane potential (mV)
ions move through
voltage-gated
channels.
3 Repolarization is
caused by K+ flowing
out of the cell.
+30
3
4 Hyperpolarization is
0
Action
potential
2
Threshold
–55
–70
caused by K+ continuing to
leave the cell.
1
0
1
4
1
2
3
Time (ms)
4
Importance of Myelin Sheaths:
Multiple Sclerosis (MS)
• Autoimmune disease affecting primarily young adults
• Myelin sheaths in CNS destroyed
• Treatment
• Drugs that modify immune system's activity improve lives
• Prevention?
• High blood levels of Vitamin D reduce risk of development
The Synapse
• Nervous system works because information flows
from neuron to neuron
• Neurons functionally connected by synapses
Important Terminology
• Presynaptic neuron
• Postsynaptic neuron
Information Transfer Across Chemical
Synapses
• AP arrives at axon terminal of presynaptic neuron
• Causes voltage-gated Ca2+ channels to open
• Ca2+ floods into cell
• Synaptotagmin protein binds Ca2+ and promotes
fusion of synaptic vesicles with axon membrane
• Exocytosis of neurotransmitter into synaptic cleft
occurs
• Higher impulse frequency  more released
Information Transfer Across Chemical
Synapses
• Neurotransmitter diffuses across synapse
• Binds to receptors on postsynaptic neuron
• Often chemically gated ion channels
• Ion channels are opened
• Causes an excitatory or inhibitory event (graded
potential)
• Neurotransmitter effects terminated
Termination of Neurotransmitter
Effects
• Within a few milliseconds neurotransmitter effect
terminated in one of three ways
• Reuptake
• By astrocytes or axon terminal
• Degradation
• By enzymes
• Diffusion
• Away from synaptic cleft
Neurotransmitters
• 50 or more neurotransmitters have been identified
• Most neurons make two or more neurotransmitters
• Usually released at different stimulation
frequencies