Download Neuroanatomy Handout #1: The Motor Neuron

Chapter 2
Nerve Cells and Nerve
Impulses
Animal Cells
• Membrane:
– separates
the inside of
the cell from
the outside
environment
– comprised
of two layers
of lipids with
proteins
embedded
Animal Cells
• Nucleus refers to the
structure that contains the
chromosomes
• Mitochondria perform
metabolic activities and
provide energy that the cell
requires.
• Ribosomes: Sites at which
the cell synthesizes new
protein molecules
• Endoplasmic reticulum:
Transports newly
synthesized proteins
The Human Nervous System—2 Kinds of Cells
•
•
Neurons
– Approx. 100 billion
in brain
– Receive and
transmit info
– Behavior depends
upon their
communication
Glia
– 10X the number of
neurons
– Support neural
communication
Neuroanatomy Handout #1: The Motor Neuron
• A motor neuron
– has its soma in the spinal cord
– receives excitation from other neurons
– conducts impulses along its axon to a muscle
or gland
– is the largest of the nerve cells
Neuroanatomy Handout #1: The Motor Neuron
• Neurons are similar to other cells of the body
• All neurons have a cell body (soma, A):
– responsible for the metabolic work of the
neuron
– surrounded by cell membrane (A1)
– Containing a nucleus (A2), mitochondria (A3),
ribosomes (A4), endoplasmic reticulum (A5)
Neuroanatomy Handout #1: The Motor Neuron
• Neurons are different from other cells of the
body because they have distinctive shape
and function
Neuroanatomy Handout #1: The Motor Neuron
• The 4 major components of a motor neuron:
– Soma/Cell body
– Dendrites
– Axon
– Presynaptic terminals
Neuroanatomy Handout #1: The Motor Neuron
• Dendrites (B)- branching fibers responsible for
receiving information from other neurons
• Dendritic spines (B1) further branch out and
increase the surface area of the dendrite
Neuroanatomy Handout #1: The Motor Neuron
• Axon (C) - thin fiber responsible for sending impulses to
other neurons, glands, or muscles
• Some neurons are covered with an insulating material called
the myelin sheath (D) with interruptions in the sheath
known as nodes of Ranvier (C2).
• Axon hillock (C1) – bulge in the cell body where axon begins
Neuroanatomy Handout #1: The Motor Neuron
• Presynaptic terminals (E) refer to the end
points of an axon responsible for releasing
chemicals (neurotransmitters) to
communicate with other neurons
Neuroanatomy Handout #1: The Motor Neuron
• Axons from other
neurons (F) converge on
receiving neuron
• Synapse: gap between
neurons
• Postsynaptic neuron (G)
and dendrite (G1)
Sensory and Motor Neurons
• A motor neuron receives
excitation from other neurons and
conducts impulses along its axon
to a muscle or gland
– It carries information from the
brain to the perimeter of the
body
Sensory and Motor Neurons
• A sensory neuron is specialized at
one end to be highly sensitive to a
particular type of stimulation (touch,
temperature, odor etc.)
– It carries information from the
perimeter of the body to the brain
Other Cells of the Nervous System
• Terms used to describe the
neuron include the following:
– Afferent axon - refers to
bringing information into a
structure.
– Efferent axon - refers to
carrying information away
from a structure.
– Interneurons or Intrinsic
neurons are those whose
dendrites and axons are
completely contained
within a structure.
Other Cells of the Nervous System
• Glia are the other major
component of the nervous
system and include the
following:
– Astrocytes help synchronize
the activity of the axon by
wrapping around the
presynaptic terminal and
taking up chemicals released
by the axon.
– Microglia - remove waste
material and other
microorganisms that could
prove harmful to the neuron.
The Cells of the Nervous System
– Oligodendrocytes & Schwann cells- build the myelin
sheath that surrounds the axon of some neurons.
– Radial glia- guide the migration of neurons and the
growth of their axons and dendrites during embryonic
development.
The Cells of the Nervous System
• Spaniard Santiago
Ramon y Cajal (18521934) was the first to
demonstrate that
neurons do not touch
one another.
• With this understanding
came new ideas about
how neurons
communicate.
The Nerve Impulse
• A nerve impulse is the electrical message that
is transmitted down the axon of a neuron.
• The impulse is regenerated at points along
the axon.
• The speed of nerve impulses ranges from
approximately 1 m/s to 100 m/s.
The Nerve Impulse
• The resting potential: state of the neuron prior to the
sending of a nerve impulse
• Electrical gradient: a difference in the electrical charge
inside and outside of the cell
– At rest, the membrane is slightly negative with
respect to the outside (approximately -70 millivolts)
Competing forces maintain a -70mV resting potential
1. Concentration gradient: The
difference in the distribution
of ions between the inside
and the outside of the
membrane
– Sodium (Na+) more
abundant outside cell
than inside (10:1)
– Potassium (K+) more
abundant inside cell than
outside (20:1)
2. Negatively charged proteins
inside cell
Cellular mechanisms of the resting potential
• Selective permeability of the
membrane allows some
molecules (e.g. water, oxygen)
to pass more freely than
others.
• Charged ions, like sodium
(Na+), potassium (K+), calcium
(Ca++) and chloride (Cl-) pass
through channels in the
membrane.
• When the membrane is at rest:
– Na+ channels are closed
– K+ channels are partially
closed allowing the slow
passage of sodium
Cellular mechanisms of the resting potential
• The sodium-potassium
pump puts three sodium
ions out of the cell while
drawing in two potassium
ions.
– helps to maintain the
electrical gradient
• The electrical gradient and
the concentration gradient
work to pull sodium ions
into the cell.
• The electrical gradient
tends to pull potassium
ions into the cells.
The resting potential allows a neuron
to respond quickly to a stimulus
The Action Potential
• The resting potential (-70mV) remains stable
until the neuron is stimulated.
• Electrical polarization: the difference in the
electrical charge between two places
Two things can happen to a resting neuron:
• Hyperpolarization: increasing the difference
(polarization) between the electrical charge of
two places (less likely to fire)
• Depolarization refers to decreasing the
polarization towards zero (more likely to fire)
• The threshold of excitement refers any
stimulation beyond a certain level that results
in a massive depolarization (action potential).
The Action Potential
• An action potential is a
rapid depolarization of
the neuron.
• Stimulation of the
neuron past the
threshold of excitation
triggers a nerve
impulse, action
potential, or “firing”
• -70mV can become
+50mV
The Nerve Impulse
• Voltage-activated channels
are membrane channels
whose permeability depends
upon the voltage difference
across the membrane.
– Sodium channels are
voltage activated channels.
• When sodium channels are
opened, positively charged
sodium ions rush in and a
subsequent nerve impulse
occurs.
The Nerve Impulse
• Scorpion venom attacks the
nervous system by keeping
sodium channels open and
closing potassium channels
• Local anesthetic drugs block sodium channels
and therefore prevent action potentials from
occurring.
– Example: Novocain
• General anesthetics open potassium channels
wider than usual
The Nerve Impulse
• The all-or-none law
states that the
amplitude and velocity
of an action potential
are independent of the
intensity of the stimulus
that initiated it.
– Action potentials are
equal in intensity and
speed within a given
neuron.
The Nerve Impulse
• A refractory period happens after an action
potential occurs, during which time the
neuron resists another action potential.
• The absolute refractory period: the first part,
when membrane cannot produce an action
potential
• The relative refractory period: the second
part, when it takes a stronger than usual
stimulus to trigger an action potential.
The Nerve Impulse
• In a motor neuron, the action potential begins
at the axon hillock (a swelling where the axon
exits the soma).
• Propagation of the action potential is the term
used to describe the transmission of the
action potential down the axon.
The Nerve Impulse
• The myelin sheath of axons are interrupted by short
unmyelinated sections called nodes of Ranvier.
• At each node of Ranvier, the action potential is
regenerated by a chain of positively charged ions
pushed along by the previous segment.
The Nerve Impulse
• Saltatory conduction: the “jumping” of the action potential
from node to node.
– Provides rapid conduction of impulses
– Conserves energy for the cell
• Multiple sclerosis: disease in which myelin sheath is
destroyed; associated with poor muscle coordination
The Nerve Impulse
• Not all neurons have lengthy axons.
• Local neurons have short axons, exchange
information with only close neighbors, and do
not produce action potentials.
• When stimulated, local neurons produce
graded potentials which are membrane
potentials that vary in magnitude and do not
follow the all-or-none law,.
• A local neuron depolarizes or hyperpolarizes
in proportion to the stimulation.