Download Nervous System Notes

The Nervous
System
Functions of the Nervous System
 Sensory input – gathering information
 To monitor changes occurring inside and
outside the body
 Changes = stimuli
 Integration
 To process and interpret sensory input
and decide if action is needed
 Motor output
 A response to integrated stimuli
 The response activates muscles or glands
Structural Classification
 Central nervous system
(CNS)
 Brain
 Spinal cord
 Peripheral nervous
system (PNS)
 Nerves outside the brain
and spinal cord
Functional Classification: PNS
 Sensory (afferent) division
 Nerve fibers that carry information to the
central nervous system
 Motor (efferent) division
 Nerve fibers that carry impulses away from the
central nervous system
Subdivisions of Motor division
 Somatic nervous system = voluntary
 Somatic motor division carries information to the
skeletal muscles
 Autonomic nervous system = involuntary
 Efferent division of ANS carries information to
the autonomic or visceral effectors
 Sympathetic division: prepares the body to deal with
immediate threats to the internal environment; produces
fight-or-flight response
 Parasympathetic division: coordinates the body’s normal
resting activities; sometimes called the rest-and-repair
division
 Visceral sensory division carries feedback
information to autonomic integrating centers in
the CNS
Organization
of the
Nervous
System
Nervous Tissue: Support Cells
(Neuroglia)
 Astrocytes
 Abundant, star-shaped cells
 Brace neurons
 Form barrier
between capillaries
and neurons
 Control the chemical
environment of
the brain
Nervous Tissue: Support Cells
 Microglia
 Spider-like phagocytes
 Dispose of debris
 Ependymal cells
 Line cavities of the
brain and spinal cord
 Circulate
cerebrospinal
fluid
 Some produce
cerebrospinal fluid
Nervous Tissue: Support Cells
 Oligodendrocytes
 Produce myelin
sheath around
nerve fibers in the
central nervous
system
Nervous Tissue: Support Cells
 Satellite cells
 Protect neuron cell bodies
 Schwann cells
 Form myelin sheath in the peripheral
nervous system
Nervous Tissue: Neurons
 Neurons = nerve cells
 Cells specialized to transmit messages
 Major regions of neurons
 Cell body – nucleus and metabolic
center of the cell
 Processes – fibers that extend from
the cell body
Neuron Anatomy
 Cell body
 Nissl bodies –
specialized
rough
endoplasmic
reticulum
 Produce
proteins
 Neurofibrils –
intermediate
cytoskeleton
that maintains
cell shape
Neuron Anatomy
 Cell body
 Nucleus
 Large
nucleolus
Neuron Anatomy
 Extensions
outside the
cell body
 Dendrites –
conduct
impulses
toward the
cell body
 Axons –
conduct
impulses away
from the cell
body
Nerve Fiber Coverings
 Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion
 Nodes of Ranvier –
gaps in myelin
sheath along the
axon
Connective Tissue Layers
 Endoneurium- delicate
layer of fibrous
connective tissue
surrounding each nerve
 Perineurium- connective
tissue holding together
bundles of fibers
(fascicles)
 Epineurium- fibrous coat
surrounding numerous
fascicles and blood vessels
to form a complete nerve
Neuron Cell Body Location
Most are found in the central nervous
system
 Gray matter – cell bodies and unmyelinated
fibers
 Nuclei – clusters of cell bodies within the white
matter of the central nervous system
 Ganglia – collections of cell bodies outside the
central nervous system
Neuron Cell Body Location
 White matter
 PNS- myelinated nerves
 CNS- myelinated tracts
 In the CNS nerve fibers are
referred to as tracts
CYCLE OF LIFE:
NERVOUS SYSTEM CELLS
• Nerve tissue development
• Begins in ectoderm
• Occurs most rapidly in womb and in first 2 years
• Nervous cells organize into body network
• Synapses
• Form and re-form until nervous system is intact
• Formation of new synapses and strengthening or
elimination of old synapses stimulate learning and
memory
• Aging causes degeneration of the nervous
system, which may lead to senility
Functional Classification of
Neurons
 Sensory (afferent) neurons
 Carry impulses from the sensory receptors
 Cutaneous sense organs
 Proprioceptors – detect stretch or
tension
 Motor (efferent) neurons
 Carry impulses from the central nervous
system
Functional Classification of
Neurons
 Interneurons (relay neurons)
 Found in neural pathways in the
central nervous system
 Connect sensory and motor neurons
 Mixed Nerves
 Contain both sensory and motor
neurons
Neuron Classification
Structural Classification of
Neurons
 Multipolar neurons – many extensions
from the cell body
 One axon and many dendrites
 Most of the neurons in the brain and
spinal cord
Structural Classification of
Neurons
 Bipolar neurons – one axon and one
dendrite
 Least numerous
 Found in retina, inner ear and olfactory
pathways
Structural Classification of
Neurons
 Unipolar neurons – have a short single
process leaving the cell body
 Always sensory neurons, conduct towards
the CNS
Repair of Nerve Fibers
 Mature neurons can not undergo
mitosis, damage is permanent
 If damage is not extensive, it can be
repaired…only if
 Cell body and neurilemma intact and
there is no scarring
 In the CNS, repair is unlikely
Repair of Nerve Fibers
 Following injury, distal
portion of axon and
myelin sheath
degenerates
 Macrophages remove
debris
 The neurilemma and
endoneurium form a
tunnel from the point of
injury to the effector
Repair of Nerve Fibers
 New Schwann cells grow in the tunnel
to maintain the path for the regrowth
of the axon
 Cell body reorganizes its Nissl bodies
to provide proteins
 Axon “sprouts” and begins to fill tunnel
 Skeletal muscle atrophies until nervous
connection reestablished
Axons and Nerve Impulses
 Axons end in axonal terminals
 Axonal terminals contain vesicles with
neurotransmitters
 Acetylcholine, amines, amino acids, nitric
oxide, neuropeptides, etc.
 Axonal terminals are separated from the
next neuron by a gap
 Synaptic cleft – gap between adjacent neurons
 Synapse – junction between nerves
The Reflex Arc
 Reflex – rapid, predictable, and
involuntary responses to stimuli
 Reflex arc – direct route from a
sensory neuron, to an interneuron, to
an effector
Simple Reflex Arc
Types of Reflexes and Regulation
 Autonomic reflexes
 Smooth muscle regulation
 Heart and blood pressure regulation
 Regulation of glands
 Digestive system regulation
 Somatic reflexes
 Activation of skeletal muscles
Functional Properties of
Neurons
 Irritability – ability to respond to stimuli
 Conductivity – ability to transmit an
impulse
 Action potential refers to the membrane
potential of a neuron that is conducting an
impulse
 The plasma membrane at rest is polarized
 Fewer positive ions are inside the cell than
outside the cell
Starting a Nerve Impulse
 Depolarization – a
stimulus depolarizes
the neuron’s
membrane
 A depolarized
membrane allows
sodium (Na+) to flow
inside the membrane
 The exchange of ions
initiates an action
potential in the
neuron
The Action Potential
 If the action potential (nerve impulse)
starts, it is propagated over the entire
axon
 Potassium ions rush out of the neuron
after sodium ions rush in, which
repolarizes the membrane
 The sodium-potassium pump restores
the original configuration
 This action requires ATP
The Action Potential
 The membrane
potential of a
nonconducting
neuron’s plasma
membrane is -70 mV
 Depolarization allows
the membrane potential
to move towards zero
Nerve Impulse Propagation
 Impulses travel
faster when fibers
have a myelin
sheath
Continuation of the Nerve Impulse
between Neurons
 Impulses are able to cross the synapse
to another nerve
 Neurotransmitter is released from a
nerve’s axon terminal
 The dendrite of the next neuron has
receptors that are stimulated by the
neurotransmitter
 An action potential is started in the
dendrite of the next neuron
How Neurons Communicate at Synapses
Synaptic Transmission
 Two types of synapses (junctions)
 Electrical synapse occurs where cells
joined by gap junctions allow an action
potential to simply continue along the
postsynaptic membrane
 Chemical synapse occurs where
presynaptic cells release
neurotransmitters across a tiny gap to
the postsynaptic cell, inducing an
action potential
Synaptic Transmission
 Action potential reaches a synaptic
knob, causing release of calcium ions
to diffuse into the knob
 Increased calcium concentrations
trigger the release of
neurotransmitters via exocytosis
 Neurotransmitters diffuse across the
synaptic cleft and bind to receptor
molecules causing ion channels to
open
 This causes postsynaptic potential
Synaptic Transmission
 Neurotransmitter’s action is quickly
terminated by returning
neurotransmitters back into synaptic
knob, or metabolized into inactive
compounds
NEUROTRANSMITTERS
• Neurotransmitters: means by which neurons
communicate with one another; more than 30 compounds
are known to be neurotransmitters, and dozens of others
are suspected
• Common classification of neurotransmitters:
• Function
• Chemical structure
NEUROTRANSMITTERS
• Acetylcholine
• Present at various locations, sometimes
in an excitatory role and other times
inhibitory
• Amines
• Synthesized from amino acid molecules
• Two categories: monoamines and
catecholamines
• Found in various regions of the brain,
affecting learning, emotions, motor
control
• Amino acids
• Believed to be among the most common
neurotransmitters of the CNS
• In the PNS, amino acids are stored in
synaptic vesicles and used as
neurotransmitters
• Other small-molecule transmitters