Transcript The Nervous System

The Nervous System
Chapters 11-15
General Overview
• Master controlling and communications system of the
body
• Rapid & specific electrical impulses cause almost
immediate responses
Three overlapping functions:
1. Gather sensory input by monitoring internal and
external stimuli (changes) using millions of sensory
receptors
2. Integration - Processes and interprets sensory input
and makes decisions about what should be done
3. Effects a motor output (response) by activating
muscles or glands.
The nervous system works in
conjuction with the endocrine
system to maintain homeostasis.
The electrical impulses of the
N.S. cause a more rapid
response than the chemical
hormones of the endocrine
system.
Structural Classification
• Central Nervous System:
– Brain and spinal cord
– Integrating & command center:
• Interpret incoming sensory info
• Issue instructions based on past experience and current
conditions
• Peripheral Nervous System:
– Nerves that extend from the brain and spinal cord (spinal
& cranial nerves)
– Serve as communication lines
• Carries sensory input to the to CNS
• Carries motor output from CNS to appropriate effector
(gland/muscle)
Functional Classification
• Only applies to peripheral nervous system
• Two sub-divisions
– Sensory (afferent): Conveys impulses from sensory
receptors to CNS
• Somatic sensory fibers: from skin, skeletal muscles & joints
• Visceral sensory fibers: from visceral organs
– Motor (efferent):Carries impulses from CNS to effector
organs, muscles & glands & effect a motor response
• Somatic (voluntary): allows conscious or voluntary control of
skeletal muscles; reflexes are initiated involuntarily by same
fibers
• Autonomic (involuntary): regulates involuntary events of
smooth muscle, cardiac muscle & glands
– Sympathetic: Mobilizes body systems during emergency (speed up)
– Parasympathetic: Conserves energy & promotes non-emergent function
Nervous Tissue: Supporting Cells
• CNS supporting cells are categorized as
neurologia, (“nerve glue”) or simply glia
– 4 main categories
•
•
•
•
Astrocytes
Microglial cells
Ependymal cells
Oligodendrocytes
– support, insulate & protect the neurons:
• PNS supporting cells are either Schwann cells or
satellite cells
Neuroglia: Astrocytes
• Account for nearly half of
the neural tissue
• “star” shaped - numerous
projections with swollen
ends that cling to neurons
anchoring them to
capillaries; help mediate
exchange between the two
• Help control chemical
environment in the brain
– Pick up excess ions
– Recapture released
neurotransmitters
Neuroglia: Microglial Cells
• Spiderlike phagocytes
• Dispose of debris
– Dead brain cells
– Bacteria
Neuroglia: Ependymal Cells
• Lines the cavities of
the brain and spinal
cord
• Beating of their cilia
helps circulate
cerebrospinal fluid
within the cavities to
form a protective
cushion
Neuroglia: Oligodendrocytes
• Wrap flat extensions tightly around nerve fibers
• Produce fatty insulating covers around nerve cells
called myelin sheath
Schwann Cells
Form myelin sheaths of around nerve fibers of
the PNS
Satellite Cells
Act as protective, cushioning cells
Neuron Anatomy
• Cell body contains nucleus & metabolic
center
• Dendrites conduct impulses toward the
cell body
• Axons transmit impulses away from cell
body
• Axons branch into many axon
terminals at the end
• When impulses reach end of axon
terminals they stimulate the release
of neurotransmitters into the
extracellular space
• Synaptic clefts separate axon terminals
of one neuron from dendrites of the next
• Myelin protects & insulates nerve fibers
and increases transmission rate
• Gaps in myelin, called nodes of Ranvier,
exist at regular intervals b/c myelin is
formed from individual Schwann cells
CNS Myelin v. PNS Myelin
• PNS myelin is formed from Schwann cells and CNS myelin is
formed from oligodendrocytes
• Oligodendrocytes can coil around 60 fibers simultaneously
and the sheaths they form lack a neurilemma (outer
cytoplasmic layer of cells)
• The neurilemma remains mostly intact when neurons are
damaged and plays a large role in fiber regeneration, which is
essentially absent in the CNS.
Comparing Neurons: CNS v. PNS
• In the CNS:
– cell bodies are found in clusters
called nuclei within bony skull or
vertebral column
– Bundles of nerve fibers (neuron
processes) are tracts
– White matter refers to myelinated
regions (carry messages) and gray
matter refers to unmyelinated
regions (carry nutrients & convert
glucose to energy)
• In the PNS:
– small collections of cell bodies
called ganglia may be found
– Bundles of nerve fibers are called
tracts
Functional Classification of Neurons
• Neurons are grouped according to the direction the
impulse is traveling relative to the CNS
• Sensory (afferent) neurons:
– Always found in ganglia outside CNS
– Dendrite endings usually associated with specialized
receptors
• Interneurons:
– Connect motor & sensory neurons
– Cell bodies are always in CNS
• Motor (efferent) neurons:
– Carry impulses from CNS to viscera, muscles, glands
– Cell bodies always in CNS
Structural Classification of Neurons
• Based on number of processes extending from the cell
body
• Multipolar: several processes
– Most common structural type
– Includes all moter and association neurons
• Bipolar: two processes (axon and dendrite)
– Rare in adults
– Act as sensory receptors in special sense organs (eye & ear)
• Unipolar: single process
– Very short process; divides almost immediately into
proximal (central) and distal (peripheral) fibers
– Only small branches at the end of peripheral fibers are
dendrites, the rest function as axons and therefore carry
impulese both toward and away from cell body
Structure Differences in Neurons
Functional Properties of Neurons
• Irritability: Ability to respond to stimulus and convert
it to a nerve impulse
• Conductivity: Ability to transmit impulses to other
neurons, muscles or glands
The Nerve Impulse: Polarization
• Resting neurons have a polarized membrane
– Fewer positive ions inside the plasma membrane then in the
surrounding tissue fluid
– Major internal ion is K+, major external ion is Na+
– As long as internal environment is relatively negative,
neuron will stay inactive
The Nerve Impulse: Depolarization
• Neural response to stimuli is always the same
–
–
–
–
Permeability of plasma membrane changes briefly
Normally it is virtually impermeable to sodium
Adequate stimulus causes “sodium gates” to open
Sodium will rush into the neuron along its
concentration gradient
• Polarity of cell membrane is temporarily changed;
inside is now more positive and the cell is in a
depolarized state
• This activates the neuron to transmit an action
potential or nerve impulse
• Nerve impulses are an “all or none” response
The Nerve Impulse: Repolarization
• Membrane almost immediately becomes impermeable to
sodium again
• Potassium ions rapidly diffuse out of the neuron into the the
tissue space, restoring the electrical conditions of the resting
state
• The neuron is now repolarized and initial sodium and
potassium concentrations are restored by the Na/K pump
Propagation of Nerve Impulse
Animation
Nerve Impulses Along Myelinated Fibers
• Fibers that have myelin sheaths conduct impulses much faster
because the nerve impulse literally jumps from node to node
along the length of the fiber; “saltatory conduction”
• No current can flow along the axonal membrane where there
is no fatty insulation
• While an action potential is
occurring, the sodium channels
are open or recovering and
another action potential
definitely cannot occur. This is
often referred to as absolute
refractory period.
• During hyperpolarization, the
postassium channels are also
open. Action potentials are
more difficult to generate than
at resting potential thus making
this the relative refractory
period.
Rate of Nerve Impulses
• Myelin: myelin increases rate of impulse
– Continuous conduction: action potentials are
generated continuously along the axon
– Saltatory conduction: APs jump from node to node
• Axon diameter: larger diameter = faster
impulses
– Type A: Largest diameter & thick myelin (150m/s)
– Type B: lightly myelinated, intermediate diameter (15 m/s)
– Type C: smallest diameter and unmyelinated ((1m/s)
Synapses
Neural Communication at Synapses
• When the action potential reaches the axonal endings, the
axon terminals release chemicals called neurotransmitters
• These neurotransmitters diffuses across the synapse and
bind to receptors on the membrane of the next neuron
• If enough neurotransmitter is released a nerve impulse will
occur.
Post-Synaptic Potentials
• Neurotransmitter synapses are
categorized according to how they
affect the post-synaptic neuron,
excitatory or) inhibitory
• EPSPs (excitatory post-synaptic
potentials) open both Na+ and K
channels resulting in a net
depolarization. This doesn’t
generate an action potential but
helps trigger one at the axonal
hillok.
• IPSPs induce hyperpolarization by
making the membrane more
permeable to K+ or Cl-. This makes
the inside more negative and
threshold harder to reach.
Factors that Impair Nerve Impulses
• Alcohol, sedatives and anesthetics block nerve
impulses by reducing membrane permeability
to sodium ions
– If sodium cannot enter an action potential cannot
occur
• Cold or continuous pressure interrupt blood
circulation therefore reducing oxygen and
nutrient delivery to neurons
– Warming or removal of pressure results in a surge
of impulses and the uncomfortable ‘prickly’
feeling.