Transcript The Nervous System

The Nervous System
Ch. 48-49
The Nervous System…
• Performs three basic continuous
functions:
– Sensory input
– Integration
– Motor output
It Is Brain surgery…
Sensory Input…
Sensory
Receptors
collect
information
from the
outside
environment.
Integration…
• Input is interpreted and linked to
appropriate responses
• Accomplished by the CNS (central
nervous system)
– Brain
– Spinal cord
Motor Output…
• Signal conduction from CNS to effector
cells in PNS (peripheral nervous system)
• CNS is connected to effector cells via
neurons, or nerve cells
The Nervous System
PNS
CNS
Afferent
Brain
Efferent
Spinal
Cord
Autonomic
Sympathetic
Somatic
Parasympathetic
Neurons and the Connections
They Love…
• The NEURON, or nerve cell is the
functional unit of the nervous system.
• Composed of cell body, dendrites, axon,
myelin sheath, synaptic terminals (bulbs)
• Dendrites: receive afferent signals, incoming
from other neurons or receptors
• Axon: only one per neuron; efferent pathway
to other neuron or an effector cell (muscle,
gland)
• Myelin sheath: lipid layer that insulates axon;
produced by Schwann cells.
• Synaptic Terminals (bulbs): transmit signals
from axon by release of neurotransmitters
(Ach)
• Synaptic cleft (synapse): site of contact
between two neurons or neuron and effector
• Postsynaptic cell: the receiver.
Label This Drawing…
A
H
B
G
F
E
D
C
How do
Scientists Study
Nerves?
From One Neuron to its
Neighbor…
The Simplest Nerve Circuit…
The Reflex
Arc:
Often involves
only two nerve
cells, the sensory
neuron and the
motor neuron.
This action is not
integrated by the
CNS.
Supporting Cells…
• GLIA =“glue”
• Used to believe they were wholly supportive,
new research says not!
• Provide nutrition and protection.
• Lead neurons from neural tube along
pathway in embryonic development.
Three Types of Glial Cells…
• Astrocytes: form connections between capillaries
and neurons for feeding and waste disposal; in
brain, they form tight junctions which form the
blood-brain barrier.
• Microglial cells: immune system cells which engulf
microbes in the brain; alcohol kills microglial cells in
fetuses.
• Oligodendrocytes (CNS) and Schwann Cells (PNS):
form around axons like burritos; insulate electrical
impulses and speed up nerve signal transduction.
How a Nerve Cell Passes a
Signal…
• All cells have a membrane potential, a difference in
charge between inside and outside. Developed 2
weeks post conception, maintained through life.
• The resting potential of an unstimulated nerve cell is
about -70mV; negative inside the cell.
• The resting membrane potential is maintained by the
Sodium-Potassium Pump.
• Neurons have a 50X greater permeability to K+ than
Na+.
Resting MP
Extracellular:
1° cation:
1° anion:
Intracellular:
1° cation:
1° anion:
Resting Potential Video
Excitable cells…
• Cells in the body like muscle and nerve
cells can create large changes in their
membrane potentials.
• Environmental stimuli can cause these
cells to alter their membrane potential,
possibly causing an action potential.
The Action Potential in Words…
• Stimulus causes Na+ gates to open.
• Na+ influx changes membrane potential.
• If Na+ influx is great enough to achieve threshold
potential (-50mV), then all Na+ gates open.
• “All or none” phenomenon…at threshold, all gates
will be opened (below threshold, no extra gates will
open) and stimulus is transmitted.
• Additional Na+ influx causes depolarization of
membrane (action potential).
• K+ channels remain closed. Cell becomes positive.
• Repolarization begins when K+ gates open and Na+
gates are closed. (~ +50mV)
• K+ ions leave the cell, causing the interior to become
more negative.
• BUT…The ions are in reversed concentrations!
• When K+ gates finally close, there is slightly more K+
outside than inside… hyperpolarization.
• Refractory period returns ions to resting state.
• Sodium-Potassium pump restores resting
potential…no stimuli can be transmitted during this
phase…the neuron is BUSY!
Action Potential Video
How Fast Are Impulses
Conducted?
• Campbell p. 1020
How Do Muscle Contractions
Fit in?
• All motor neurons are associated with
muscle fibers at their peripheral
end…the neuromuscular junction.
• There is a space between a neuron and
a muscle fiber…the synaptic cleft.
• The depolarization wave cannot pass
across the cleft!
Three Types of Muscle in the
Body…
• Cardiac…found only in the
heart; striated; involuntary
• Smooth…lines internal
organs…digestive tract, blood
vessels, rep. tract, bladder;
not striated; involuntary.
• Skeletal…attaches to
skeleton; striated; voluntary;
multinucleate
General Anatomy of a
Muscle Fiber Cell w/ myofibrils
Muscle…
A Little Bit Closer…
ONE MUSCLE FIBER…(cell)
Myofibril Anatomy…
• Skeletal
muscle is
striated
• Individual
units are
called
sarcomeres.
• Thin filaments
– actin
• Thick
filaments –
myosin
The Neuromuscular Junction…
The Neuromuscular
Junction
Signal Transduction…
• The depolarization wave in the neuron
cannot traverse the synaptic cleft.
• When action potential reaches the
synaptic end bulb, Ca++ in the cleft
flows into the bulbs.
• This calcium causes vesicles filled with
neurotransmitters (acetylcholine) to
migrate to the neural membrane.
• The vesicles fuse with the cell
membrane and exocytose their
contents into the synaptic cleft.
• Receptors on sarcolemma bind
acetylcholine causing gated Na+
channels to open. As Na+ comes into
the sarcoplasm, what happens?!
• DEPOLARIZATION !
• The depolarization wave (DW) passes across
the sarcolemma which extends into the
muscle fiber via the T-tubules.
• T-tubules have close associations with the
sarcoplasmic reticulum (SR), the Ca++
warehouse.
• The DW opens gated Ca++ channels allowing
Ca++ to flow into the sarcoplasm…this is the
important part!!!!
• Ca++ binds the troponin complex.
• Once troponin is removed, the tropomyosin
shifts away from the myosin binding sites.
• Ca++ serves as an enzyme cofactor with
myosin and they become ATPase!
• The ATP is broken down into ADP and Pi.
This allows myosin to bind to the actin
filaments and contract the filament to the
center of the sarcomere.
Muscle Contraction
So, How Do You Stop It?
• The binding of the acetylcholine receptors on
the sarcolemma signals the release of
acetylcholinesterase from the sarcolemma.
• This enzyme breaks down acetylcholine in the
synaptic cleft.
One molecule of acetylcholinesterase breaks down 25,000 molecules
of acetylcholine each second. This speed makes possible the rapid
"resetting" of the synapse for transmission of another nerve impulse.
• How do you get the contraction to continue?
Belgian Blue Bull
The myostatin gene is effectively blocked by
being mistranscribed (it’s truncated)…leads
to “double-muscled” animal.
• Negative Feedback Control (depends on four detector
types so that the CNS knows what the muscles are
doing and can make adjustments accordingly)
• • Muscle spindles (so-called stretch receptors) –
actually length detectors
• • Golgi tendon organs – detectors of tension in
tendons
• • Joint angle receptors – indicate the angle of a
joint
• • Skin stretch and compression receptors – give
information about how the skin is deformed around a
joint
Storytime…
The Function of the Sympathetic Nervous
System and Wartime…