Download Nervous Systems

Topic 6.5 - Nerves
Overview of the Nervous System
• Three major functions:
• Sensory input – sensory receptors receive
signal – peripheral nervous system (nerves,
eyes, ears, etc.)
• Integration – signal is interpreted and
response started – central nervous system
(brain and spinal cord)
• Motor output – response to stimulus –
peripheral nervous system (nerves, muscle
or gland cells)
6.5.1
Overview of the Nervous System
Overview of the Nervous System
Neurons
• Function - conduct messages to help
communication between parts of nervous
system.
• Neurons are helped by numerous
supporting cells, which provide structural
support, protection, and insulation of
neurons.
6.5.2
Neuron Structure
• Cell body – large central part of neuron
– Contains nucleus and other organelles
• – Dendrites – receive and move signal
from tips to cell body (into neuron)
– Axons – carry signals away from cell body to
tips (out of neuron)
6.5.2
Neuron Structure
• Schwann cells – supporting cells that form
insulating myelin sheath layer.
– Increases speed of signal
• Nodes of Ranvier – spaces in between the
Schwann cells
• Synaptic terminal – end of axon where
neurotransmitters are released into
synapse
6.5.2
Neuron Structure
6.5.2
Types of Neurons
• Sensory neurons – communicate
information about external and internal
environments to central nervous system
(input)
• Interneurons – link sensory response to
motor output.
• Motor neurons – communicate response
from central nervous system to effector
cells (motor output)
• All combined, these neurons create a reflex
arc, which integrates a stimulus and
response.
6.5.3
A Reflex Arc
Membrane Potential
• Membrane potential – the difference in
electrical charge across the plasma
membrane.
• The inside of the cell is negative with
respect to the outside.
• Neurons have a resting membrane
potential of -70mV
Membrane Potential
• Inside the cell:
– Cations: potassium (K+) and few sodium (Na+)
– Anions: proteins, sulfate, phosphate
(collectively A-) and few chloride (Cl-)
• Outside the cell:
– Cations: Sodium (Na+) and few potassium (K+)
– Anions: chloride (Cl-)
Membrane Potential
Membrane Potential – How it’s Created
• The plasma membrane is more permeable (more
membrane channels) to K+ than to Na+.
– Therefore, large amounts of K+ are transferred out of
the cell (down the concentration gradient)
– Small amounts of Na+ are transferred into the cell
(down the concentration gradient)
• The movement of K+ and Na+ across the
membrane generate a net negative membrane
potential (-70mV)
• A sodium-potassium pump is used to move K+
back into the cell and Na+ back out of the cell to
maintain the constant concentration gradients.
Membrane Potential
Changes in Membrane Potential
• Neurons are excitable cells – a stimulus
can change the neuron’s membrane
potential
• Resting potential – membrane potential of
unexcited neuron (-70mV)
• Neurons become “excited,” when a
stimulus opens a gated ion channel and
increases the movement of K+ or Na+
across the plasma membrane
6.5.4
Changes in Membrane Potential
• Hyperpolarization:
• A stimulus opens a K+ ion channel and
efflux of K+ out of the cell increases
• Membrane potential becomes more
negative
Hyperpolarization
Changes in Membrane Potential
• Depolarization:
• A stimulus opens a Na+ ion channel and
influx of Na+ into the cell increases
• Membrane potential becomes more positive
6.5.4
Depolarization
6.5.4
Action Potential
• When depolarization reaches a certain
point, the threshold potential is achieved.
• When threshold potential is reached, an
action potential is triggered.
– Action potential is a nerve impulse.
• Action potentials consist of a rapid
depolarization, a rapid repolarization, and
undershoot (hyperpolarization)
6.5.4
Action Potential
6.5.2
Action Potential
• Caused by voltage-gated channels
– Open and close in response to changes in
membrane potential
– K+ channels – one gate; closed at resting
potential; opens slowly during depolarization
– Na+ channels – two gates:
• Activation gate – closed at resting potential;
opens rapidly during depolarization
• Inactivation gate – open at resting potential;
closes slowly during depolarization
6.5.5
Steps in Action Potential
• Depolarization: Na+ activation gates open
and Na+ enters cell.
• Repolarization: Na+ inactivation gate
closes (prevents Na+ influx) and K+ gate
opens and K+ exits cell.
• Undershoot: K+ gates remain open and
K+ continues to leave cell
• Resting state: All gates closed, Na+/K+
pump (active transport) moves Na+ out and
K+ in to restore resting potential.
6.5.5
Steps in the Action Potential
Steps in Action Potential
Propagation of the Action Potential
• Action potentials are all-or-none events
– There is no BIG action potential or small action
potential
• The nervous system determines the
strength of a stimulus by the frequency of
action potentials
• Action potentials do not travel along the
axons of neurons, but are continually
regenerated.
Synapses
• Synapse – junction between
two neurons
– Transmitting cell –
presynaptic cell
– Receiving cell – postsynaptic
cell
• Neurons are separated by a
gap called the synaptic cleft.
• Messages are transmitted
across the synaptic cleft by
chemical neurotransmitters.
6.5.6
Steps in Synaptic Transmission
1. A nerve impulse reaches end of
presynaptic neuron.
2. Presynaptic membrane depolarizes,
opening voltage-gated Ca2+ channels.
– Ca2+ ions diffuse into presynaptic neuron
3. Influx of Ca2+ causes neurotransmitter
vesicles to fuse to presynaptic membrane
and release neurotransmitters into the
synaptic cleft (exocytosis)
6.5.6
Steps in Synaptic Transmission
4. Neurotransmitter diffuses across synaptic
cleft and bind to receptors on postsynaptic
membrane.
5. Receptors open gated ion channels in
postsynaptic membrane.
– Specific receptors open specific ion channels
– May open Na+, K+, or Cl- channels
– Different ions have different responses
(excitatory or inhibitory)
6.5.6
Steps in Synaptic Transmission
6. Enzymes quickly degrade
neurotransmitter, ending its activity.
– E.g. acetylcholine is degraded by
cholinesterase.
7. Ca2+ is pumped out of presynaptic cell
back into synaptic membrane.
6.5.6
Chemical Synapse