Download Unit 8 Nervous System

Chapter Nine
Nervous
System
Part 1
Functions of the Nervous System
Sensory input- information gathered by
sensory receptors about internal and
external changes
Integration- interpretation of sensory input
Motor output- activation of effector
(muscles and glands) produces a response
Divisions of the Nervous System
Central Nervous System (CNS)
Brain and spinal cord
Integration and command
center
Peripheral nervous system (PNS)
Paired spinal and cranial nerves
carry messages to and from the
CNS
Peripheral Nervous System Functions
Sensory (afferent) division
1 Somatic afferent fibers- convey
impulses from skin, skeletal muscles,
and joints
2 Visceral afferent fibers- convey
impulses from visceral organs
Motor (efferent) division
Transmits impulses from the CNS to effector organs
Somatic (voluntary) nervous system
Conscious control of skeletal muscles
Autonomic (involuntary) nervous system (ANS)
Visceral motor nerve fibers
Regulated smooth muscle, cardiac muscle, and
glands
Functional divisions
Sympathetic
good pic
Parasympathetic
next slide
Up next! The neuron! (AKA Nerve Cell)
Histology of nervous system
Neurons- excitable
cells that transmit
electrical signals
Characteristics
Long lived, amitotic,
high metabolic rate,
electrical signaling and
cell-to-cell interactions
during development
Parts of a neuron
Cell body- biosynthetic center of a
neuron, network of neurofibrils
(neurofilaments)
Axon hillock- cone shaped area where the
axon arises
Clusters of cell bodies in the CNS are called
nuclei, and in the PNS are called ganglia
Parts of a neuron
Processes- bundles are classed
tracts (CNS) and nerves (PNS)
Dendrites
Short, tapering and diffusely
branched
Receptive (input) region of the
neuron
Convey electrical signals
toward the cell body
Parts of a neuron
Axons
One long axon per cell body
Long axons are called nerve fibers
Knoblike axon terminals
Secretory region of the neuron
Release neurotransmitters to excite or inhibit other cells
Conducting region of the neuron, generates and
transmits nerve impulses away from the cell body
Nerve bundles
Unmyelinated
axons are thin
nerve fibers and
one Schwann cell
can incompletely
enclose 15 or more
unmyelinated
axons
End of Day 1
Structural classifications of neurons
Multipolar- 1 axon and several dendrites (most
abundant)
Bipolar- 1 axon and 1 dendrite (rare)
Unipolar (pseudounipolar)- single, short process
that has two branches
Peripheral process- most distal branch
Central process- branch entering the CNS
Functional classification of neurons
Sensory (afferent)- transmit impulses from sensory
receptors toward the CNS
Motor (efferent)- carries impulses from the CNS
to effectors
Interneurons (association neurons)- Shuttle
signals through CNS pathways
Neuroglia (glial cells)- supporting cells
Astrocytes (CNS)
Most abundant, versatile, and highly branched glial
cells
Cling to neurons, synaptic endings, and capillaries
Support and brace neurons
Determine capillary permeability
Guide migration of young neurons
Control chemical environment
Information processing in the brain
Neuroglia (glial cells)- supporting cells
Neuroglia (glial cells)- supporting cells
Microglia (CNS)
Migrate toward injured neurons
Phagocytize microorganisms and neuronal debris
Ependymal cells (CNS)
May be ciliated
Line the central cavities of the brain and spinal
column
Separate the CNS interstitial fluid from the
cerebrospinal fluid in the cavities
Neuroglia (glial cells)- supporting cells
 Oligodendrocytes (CNS)
 Branched cells
 Processes wrap CNS nerve fibers, forming insulating myelin
sheaths
 Myelin sheath- concentric layers of Schwann cell membrane
 Neurilemmal- peripheral bulge of Schwann cell cytoplasm
 Nodes of Ranvier-myelin sheath gaps between Schwann cells, sites
where axon collaterals can emerge
 CNS
 Formed by processes of oligodendrocytes, Nodes of Ranvier not
present, no neurolimma, thinnest fibers are unmyelinated
 White matter
 Dense collections of myelinated fibers
 Grey matter
 Mostly neuron cell bodies and unmyelinated fibers
Neuroglia (glial cells)- supporting cells
Satellite cells (PNS)
Surround neuron cell bodies in the PNS
Schwann cells (PNS)
Surround peripheral nerve fibers and form myelin
sheaths
Vital to regeneration of damaged peripheral nerve
fibers
Membrane potentials
Role of membrane ion channels
Proteins serve as membrane ion channels
Two main types of channels
Leakage (nongated) channels- always open
Membrane potentials
Gated channels
Three types
Chemical gated (ligand-gated) channels- open
with binding of a specific neurotransmitter
Voltage-gated channels- open and close in
response to changes in membrane potentials
Mechanically gated channel- open and close in
response to physical deformation of receptors
When gated channels are open
Ions diffuse quickly across the membrane
along their electrochemical gradients
Chemical gradients go from high to low
Electrical gradients go from low to high
Ion flow creates an electrical current and
voltage changes across the membrane
Resting membrane potential
Potential difference across the membrane of a
resting cell
Approximately -70mV in neurons
Generated by
Differences in ICF (intracellular fluid) and ECF
(extracellular fluid)
Differential permeability of the plasma membrane
Resting membrane potential
Differences in ionic makeup
ICF has lower concentration of Na+ and Cl- than ECF
ICF has higher concentration of K+ and negatively
charged proteins (A-) than ECF
Sodium-potassium pump stabilizes the resting
membrane potential by maintaining the
concentration gradients for Na+ and K+
Membrane potentials act as signals
Changes when concentrations of ions across
the membrane change and permeability of
membrane to ions changes
Signals used to receive, integrate, and send
information
Graded potentials- incoming short-distance signals
Depolarization
 Reduction in membrane potential
 Inside of the membrane becomes less negative than the resting
potential
 Increases the probability of producing a nerve impulse
Hyperpolarization
 An increase in membrane potential
 Inside of the membrane becomes more negative than the
resting potential
 Reduces the probability of producing a nerve impulse
Occur when a stimulus causes gated ion channels to open
Decrease in magnitude with distance as ions flow and diffuse
through leakage channels
Action potentials- long-distance signals of
axons
Brief reversal of membrane potential with an
amplitude of ~100mV
Occur in muscle cells and axons of neurons
Does not decrease in magnitude over
distance
Nerve fiber classification
Group A fibers
Large diameter, myelinated somatic sensory and
motor fibers
Group B fibers
Intermediate diameter, lightly myelinated ANS fibers
Group C fibers
Smallest diameter, unmyelinated ANS fibers
The synapse
A junction that mediated information transfer
form one neuron to another neuron or an
effector cell
Presynaptic neuron- conducts impulses toward
the synapse
Postsynaptic neuron- transmits impulses away
from the synapse
synapses
Types of synapses
Axodendritic- between the axon of one neuron and
the dendrite of another
Axosomatic- between the axon of one neuron and
the soma of another
Less common
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrite to soma)
Synapses
Electrical synapses
Less common than chemical synapses
Neurons are electrically coupled (joined by gap
junctions)
Communication is very rapid and may be
unidirectional or bidirectional
Important in embryonic nervous tissue and some brain
regions
Synapses
Chemical synapses
Specialized in the release of neurotransmitters
Composed of two parts
Axon terminal of the presynaptic neuron
Receptor region on the postsynaptic neuron
Synaptic cleft
 Fluid-filled space separating the presynaptic and
postsynaptic neurons
 Prevents nerve impulses from directly passing from one
neuron to the next
 Transmission across the synaptic cleft
 Is a chemical event that involves the release, diffusion, and
binding of neurotransmitters that ensures unidirectional
communication between neurons
Neurotransmitters and their receptors
Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
50 or more neurotransmitters have been
identified
Classified by chemical structure and by
function
Acetylcholine (Ach)
Released at neuromuscular junctions and
some ANS neurons
Synthesized by enzyme choline
acetyltransferase
Degraded by the enzyme
acetylcholinesterase (AChE)
Classified by chemical structure and by
function
Biogenic amines include
Catecholamines
Dopamine, norepinephrine (NE), and epinephrine
Indolamines
Serotonin and hisamine
Broadly distributed in the brain
Play roles in emotional behaviors and the biological
clock
Classified by chemical structure and by
function
Amino acids
GABA (Gamma-aminobutyric acid)
Glycine
Aspartate
Glutamate
Classified by chemical structure and by
function
Peptides (neuropeptides)
Substance P
Mediator of pain signals
Endorphins
Act as natural opiates; reduce pain perception
Gut-brain peptides
Somatostatin and cholecystokinin
Classified by chemical structure and by
function
 Purines such as ATP
 Act in both the CNS and PNS
 Provoke pain sensation
 Gases and lipids
 Nitric oxide (NO)
 Involved in learning and memory
 Carbon monoxide (CO)
 Regulator of cGMP in the brain
 Endocannabinoids
 Lipid soluble and involved in learning and memory
Neurotransmitters
 Direct action
 Neurotransmitter binds to channel-linked receptor and opens ion
channels
 Promotes rapid responses
 Ach and amino acids
 Indirect action
 Neurotransmitter binds to a G-protein-linked receptor and acts through
an intracellular second messenger
 Promotes long –lasting effects
 Biogenic amines, neuropeptides, and dissolved gases
Neurotransmitters
Types of neurotransmitter receptors
Channel-linked receptors
G protein-linked receptors
Basic concepts of neural integration
 Neuronal pools integrate incoming information and
forward the processes information to other destinations
 Simple neuronal pool
 Single presynaptic fiber branches and synapses with several
neurons in the pool
 Discharge zone- neurons most closely associated with the
incoming fiber
 Facilitated zone- neurons farther away from incoming fiber
Types of circuits in neuronal pools
Diverging circuit
One incoming fiber stimulated an ever-increasing
number of fibers, often amplifying circuits
May affect a single pathway or several
Common in both sensory and motor systems
Types of circuits in neuronal pools
Converging circuit
Opposite of diverging circuits; strong
stimulation or inhibition
Also common in sensory and motor systems
Types of circuits in neuronal pools
Reverberating (oscillating) circuit
Chain of neurons containing collateral synapses with
previous neurons in the chain
Parallel after-discharge circuit
Incoming fiber stimulates several neurons in parallel
arrays to stimulate a common output cell
Neural processing
 Serial processing
 Input travels along one pathway to a specific destination
 Works in all-or-none manner to produce a specific response
 Ex. reflexes
 Parallel processing
 Input travels along several pathways
 Important in higher-level mental functioning
 Ex. Smell reminds you of an odor and associated experience
Developmental aspects of neurons
 Originates from the neural tube and neural crest formed
from ectoderm
 Neural tube becomes the CNS
 Cell death
 About 2/3 of neurons die before birth
 Death results in cells that fail to make functional synaptic
contacts
 Many cells also die due to apoptosis (programmed cell death)
during development
Multiple Sclerosis (MS)
 An autoimmune disease that mainly affects young
adults
 Myelin sheaths in the CNS become nonfunctional
scleroses
 Shunting and short-circuiting of nerve impulses occurs,
impulse condition slows and eventually ceases
 Symptoms: visual disturbances, weakness, loss of
muscular control, speech disturbances, and urinary
incontinence.