Transcript neurons - Mount Carmel Academy

Chapter 7 – Part 2
The Nervous System
Axons and Nerve Impulses
 Axons end in axonal terminals
 Axonal terminals contain vesicles that contain
chemicals called neurotransmitters
 When the impulses reach the axonal
terminals, they stimulate the release of
neurotransmitters into the extracellular space.
 Axonal terminals are separated from the next
neuron by a gap
 Synaptic cleft – gap between adjacent
neurons
 Synapse – junction between nerves
Myelin
 Most long nerve fibers are
covered with a whitish, fatty
material called myelin
 Has a waxy appearance
 Protects and insulates the
fibers
 Increases the transmission
rate of nerve impulses
 Myelin sheaths – a tight coil
of wrapped membranes that
encloses the axon
Nerve Fiber Coverings
 Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion in axons
outside the CNS
 Nodes of Ranvier –
gaps in myelin
sheath along the
axon
Multiple Sclerosis (MS)
 The myelin sheaths around
the fibers are gradually
destroyed and converted to
hardened sheaths called
sclerosis.
 As this happens, the circuit is short-circuited.
 The affected person loses the ability to control
his or her muscles and becomes increasingly
disabled.
 Is an autoimmune disease in which a protein
component of the sheath is attacked.
Neuron Cell Body Location
 Most are found in the CNS
 Nuclei – clusters of cell bodies within the white
matter of the CNS
 Well-protected location within the bony skull or
vertebral column is essential
 Neurons do not routinely undergo cell division
after birth
 If it is damaged the cell dies and is not replaced
 Some are found outside the CNS
 Ganglia – collections of cell bodies outside the
CNS
Nerve Fibers
 Tracts – bundles of nerve fibers (neuron
processes) running through the CNS
 Nerves – bundles of nerve fibers (neuron
processes) running through the PNS
 White matter – consists of dense collections of
myelinated fibers (tracts)
 Gray matter – contains mostly unmyelinated
fibers and cell bodies
Functional Classification of Neurons
1. Sensory (afferent) neurons
 Carry impulses from the sensory receptors
to the CNS
 Keep us informed about what is happening
both inside and outside the body
 The dendrite endings of the sensory neuron
are usually associated with specialized
receptors.
1. Cutaneous sense organs – found in the skin
2. Proprioceptors – detect stretch or tension in
the muscles and tendons
Functional Classification of Neurons
2. Motor (efferent) neurons
 Carry impulses from the CNS to the
muscles and glands
3. Interneurons (association neurons)
 Found in neural pathways in the CNS
 Connect sensory and motor neurons
Neuron Classification
Structural Classification of Neurons
 Multipolar neurons – many extensions
from the cell body
 Most common type: all motor and
association neurons are multipolar
Structural Classification of Neurons
 Bipolar neurons – Neurons with two
processes - one axon and one dendrite
 Rare in adults
 Found only in some special sense organs
such as the ear and the eye, where they
act as sensory receptor cells
Structural Classification of Neurons
 Unipolar neurons – have a short single
process leaving the cell body
 The single process is short and divides
almost immediately into central and
peripheral fibers.
 In this case, the
axon conducts
nerve impulses
both toward and
away from the
cell body.
Functional Properties of Neurons
 Neurons have two major functional
properties:
1. Irritability – ability to respond to stimuli
2. Conductivity – ability to transmit an
impulse
Plasma Membrane of a Resting
Neuron
 The plasma membrane at rest is
polarized.
 Fewer positive ions are inside the cell than
outside the cell.
 The major positive ions inside the cell are
potassium (K+).
 The major positive ions outside the cell are
sodium (Na+).
 As long as the inside remains more negative
as compared to the outside, the neuron will
stay inactive.
Stimulus
 Many different stimuli excite neurons to
become active and generate an impulse.
 Light excites the eye receptors, sound excites
some of the ear receptors, and pressure
excites some cutaneous receptors of the skin.
 Most neurons in the body are excited by
neurotransmitters released by other neurons.
 Regardless of what the stimulus is, the result is
always the same – the “sodium gates” in the
membrane open allowing an inward rush of
sodium ions.
Starting a Nerve Impulse
•
A stimulus depolarizes the
neuron’s membrane.
•
•
A depolarized membrane
allows sodium (Na+) to flow
inside the membrane.
•
•
Depolarization – the loss of
a negative charge inside
the plasma membrane.
The inside becomes more
positive.
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.
Repolarization
 Almost immediately after the Na+ rush into the
neuron, the membrane permeability changes again:
It becomes impermeable to Na+, but permeable
to K+.
 K+ rush out of the neuron, which repolarizes the
membrane
 Repolarization - the outflow of positive ions, which
restores the electrical conditions at the membrane to
the resting state.
 Until repolarization occurs, a neuron cannot
conduct another impulse
Repolarization
 Until repolarization occurs, a neuron cannot
conduct another impulse
 Refractory Period – Period of repolarization of the
neuron during which it cannot respond to a second
stimulus
The Sodium Potassium Pump
 The sodium-potassium pump restores the
original configuration of sodium and potassium
ions inside and outside the neuron.
 This action requires ATP.
Nerve Impulse Propagation
 The impulse continues to
move toward the cell
body
 Impulses travel faster
when fibers have a
myelin sheath
 Nerve impulses literally
jumps or leaps from node
to node along the fiber.
 No current can flow
across the axonal
membrane where there is
fatty myelin insulation.
Blocking Nerve Impulse Conduction
 It is possible to block nerve impulses by reducing
membrane permeability to sodium ions.
 No sodium entry = no action potential
 Alcohol, sedatives, and anesthetics all do this.
 It is also possible to hinder impulse conduction by
interrupting blood circulation (interrupt the delivery of
oxygen and nutrients).
 Examples:
1. Cold – fingers get numb when you hold an ice cube.
2. Continuous pressure – when you sit on your foot, it
“goes to sleep”
 When you warm the fingers or remove the pressure from
the foot, the impulses begin to be transmitted again,
leading to an unpleasant prickly feeling.
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
How Neurons Communicate at
Synapses