Nervous Coordination
Chapter 33
Irritability
 The ability to respond to environmental stimuli is a
fundamental property of life.
 Single celled organisms respond in a simple way – e.g.
avoiding a noxious substance.
 The evolution of multicellularity required more complex
mechanisms for communication between cells.
 Neural mechanisms – rapid, brief
 Hormonal mechanisms – slower, long term
CNS & PNS
 Central Nervous
System (CNS) –
includes the brain and
spinal cord.
 Peripheral Nervous
System (PNS) –
includes motor and
sensory neurons.
Neurons
 A neuron (nerve cell) is the functional unit of the
nervous system.
 Sensory (afferent) neurons carry impulses from sensory
receptors to the CNS.
 Motor (efferent) neurons carry impulses away from the
CNS to effectors (muscles and glands).
 Interneurons connect neurons together.
Neurons
 Two types of cytoplasmic
processes extend from the cell
body.
 Dendrites bring signals in to
the cell body.
 Often highly branched.
 Axons carry signals away
from the cell body.
Nerves
 Nerve processes
(usually axons) are
often bundled together,
surrounded by
connective tissue,
forming a nerve.
 Cell bodies are located
in the CNS or in
ganglia (bundles of cell
bodies outside the
CNS).
Glial Cells
 Non-neural cells that
work with neurons are
called glial cells.
 Astrocytes – starshaped cells that serve
as nutrient and ion
reservoirs for neurons.
Glial Cells
 The axon is
covered with an
insulating layer of
lipid-containing
myelin, which
speeds up signal
propagation.
 Concentric rings of
myelin are formed
by Schwann cells
in the PNS and
oligodendrocytes
in the CNS.
Action Potential
 A nerve signal or action potential is an
electrochemical message of neurons.
 An all-or-none phenomenon – either the fiber is
conducting an action potential or it is not.
 The signal is varied by changing the frequency of signal
conduction.
The Nerve Impulse
 Across its plasma membrane, every cell has a voltage
called a membrane potential.
 The inside of a cell is negative relative to the outside.
The Nerve Impulse
 Neuron at rest – active transport channels in the
neuron’s plasma membrane pump:
 Sodium ions (Na+) out of the cell.
 Potassium ions (K+) into the cell.
 More sodium is moved out; less potassium is moved in.
 Result is a negative charge inside the cell.
 Cell membrane is now polarized.
Sodium-Potassium Exchange Pump
 Na+ flows into the
cell during an
action potential, it
must be pumped
out using sodium
pumps so that
the action
potential will
continue.
http://youtu.be/SdUUP2pMmQ4
potassium
The Nerve Impulse
 Resting potential – the charge that exists
across a neuron’s membrane while at rest.
 -70 mV.
 This is the starting point for an action potential.
The Nerve Impulse
 A nerve impulse starts when pressure or other
sensory inputs disturb a neuron’s plasma
membrane, causing sodium channels on a
dendrite to open.
 Sodium ions flood into the neuron and the
membrane is depolarized – more positive inside
than outside.
The Nerve Impulse
 The nerve impulse travels along the axon or dendrites
as an electrical current gathered by ions moving in and
out of the neuron through voltage-gated channels.
 Voltage-gated channels – protein channels in the
membrane that open & close in response to an electrical
charge.
The Nerve Impulse
 This moving local reversal of voltage is called an
action potential.
 A very rapid and brief depolarization of the cell membrane.
 Membrane potential changes from -70 mV to +35 mV.
 After the action potential has passed, the voltage
gated channels snap closed and the resting potential
is restored.
 The membrane potential quickly returns to -70 mV during the
repolarization phase.
 An action potential is a brief all-or-none depolarization
of a neuron’s plasma membrane.
 Carries information along axons.
 An action potential is self-propagating – once started it
continues to the end.
High Speed Conduction
 Insulating layers of the
myelin sheath are
interrupted by nodes of
Ranvier where the surface
of the axon is exposed to
interstitial fluid.
 Action potentials depolarize
the membrane only at the
nodes.
 This is saltatory
conduction, where the
action potential jumps from
node to node.
Synapses: Junctions Between
Nerves
 Eventually, the impulse
reaches the end of the
axon.
 Neurons do not make
direct contact with each
other – there is a small
gap between the axon of
one neuron and the
dendrite of the next.
 This junction between a
neuron & another cell is
called a synapse.
Synapses: Junctions Between
Nerves
 Thousands of
synaptic knobs may
rest on a single nerve
cell body and its
dendrites.
 Two types of
synapses:
 Electrical synapses
 Chemical Synapses
Electrical Synapse
 Electrical synapses are points where ionic currents
flow directly across a narrow gap junction from one
neuron to another.
 No time lag – important in escape reactions.
Chemical Synapse
 Presynaptic neurons bring action potentials toward
the synapse.
 Postsynaptic neurons carry action potentials away
from the synapse.
 A synaptic cleft is the small gap between the two
neurons.
Neurotransmitters
 Chemical
messengers called
neurotransmitters
carry the message
of the nerve
impulse across the
synapse.
Neurotransmitters
 Neurotransmitters are released into the
synapse and bind with receptors on the
postsynaptic cell membrane, which cause ion
channels to open in the new cell.
Acetylcholine – Example
Neurotransmitter
Kinds of Synapses
 There are many types of neurotransmitters, each
recognized by certain receptor proteins.
 Excitatory synapse – the receptor protein is a
chemically gated sodium channel (it is opened by a
neurotransmitter).
 When opened, sodium rushes in and an action potential
begins in the new neuron.
Kinds of Synapses
 Inhibitory synapse – the receptor protein is a
chemically gated potassium channel.
 When opened, potassium ions leave the cell which
increases the negative charge and inhibits the start of an
action potential.
Kinds of Synapses
 An individual nerve cell can have both types of
receptors.
 Sometimes both excitatory and inhibitory
neurotransmitters arrive at the synapse.
 Integration is the process where the various
neurotransmitters cancel out or reinforce each other.
Vertebrate Nervous System
 Vertebrates have a hollow, dorsal nerve cord
terminating anteriorly in a large ganglionic mass – the
brain.
 Invertebrate nerve cords are solid and ventral.
 Encephalization – the elaboration of size, configuration,
and functional capacity of the brain.
Spinal Cord
 The spinal cord begins as
an ectodermal neural
groove, which becomes a
hollow neural tube.
 The spinal cord is protected
by the vertebrae (derived
from the notochord).
 White, myelinated sheath of
axons & dendrites surround
the gray matter containing
cell bodies.
Reflex Arc
 A simple reflex produces a very fast motor response to
a stimulus because the sensory neuron bringing
information about the stimulus passes the information
directly to the motor neuron.
Reflex Arc
 Usually, there are interneurons between sensory and
motor neurons.
 An interneuron may connect two neurons on the same
side of the spinal cord, or on opposite sides.
Brain
 The vertebrate brain has
changed dramatically from the
primitive linear brain of fishes
and amphibians.
 It has expanded to form the
deeply fissured, intricate brain of
mammals.
The Vertebrate Brain
 The vertebrate brain has three parts:
 Hindbrain – extension spinal cord responsible for
hearing, balance, and coordinating motor reflexes.
 Midbrain – contains optic lobes and processes
visual information.
 Forebrain – process olfactory information.
The Hindbrain
 The hindbrain consists of the medulla oblongata, the
pons, and the cerebellum.
 The medulla oblongata, is really a continuation of the
spinal cord.
 The pons carries impulses from one side of the
cerebellum to the other and connects the medulla and
cerebellum to other brain regions.
 The cerebellum controls balance posture, and muscle
coordination.
 Birds have a highly developed cerebellum because flying is
complicated.
Midbrain
 The midbrain consists of the tectum, including optic
lobes, which contain nuclei that serve as centers for
visual and auditory reflexes.
Forebrain
 Vertebrates other than fishes have a complex forebrain:
 Diencephalon contains:
 Thalamus – relay center between cerebrum & sensory
nerves.
 Hypothalamus – participates in basic drives & emotions.
Also controls pituitary gland.
 Telencephalon (cerebrum in mammals) is devoted to
associative activity.
Cerebrum
 The cerebrum is the control center of the
brain.
 Right and left halves called cerebral hemispheres.
 Functions in language, conscious thought, memory,
personality development, vision.
Cerebrum
 The gray outer layer of the cerebrum is the cerebral
cortex and is the most active area.
 Gray color comes from the many cell bodies.
 The inner white area contains myelinated nerve fibers
that shuttle information between the cortex and the rest
of the brain.
Peripheral Nervous System
 The peripheral nervous system includes all nervous
tissue outside the CNS.
 Sensory nerves bring sensory info to the CNS.
 Motor nerves carry motor commands to muscles and
glands.
 Somatic nervous system innervates skeletal muscle.
 Autonomic nervous system innervates smooth muscle,
cardiac muscle, and glands.
Autonomic Nervous System
 The autonomic nervous system is involuntary.
 Works all the time carrying messages to muscles and
glands that work without you even noticing.
 Works to maintain homeostasis.
Autonomic Nervous System
 The sympathetic nervous system (fight or flight)
dominates in times of stress.
 Increases blood pressure, heart rate, breathing rate &
blood flow to muscles.
 The parasympathetic nervous system (rest & digest)
conserves energy by slowing the heartbeat and
breathing rate and promoting digestion.
Sense Organs
 Sense organs are specialized receptors for detecting
environmental cues.
 A stimulus is some form of energy – electrical,
mechanical, chemical, or radiant.
 A sense organ transforms energy from the stimulus into
an action potential.
 Perception of a sensation is determined by which part of
the brain receives the action potential.
Classification of Receptors
 Exteroceptors receive information about the
external environment.
 Based on the energy they transduce, sensory
receptors fall into five categories





Mechanoreceptors
Chemoreceptors
Electromagnetic receptors
Thermoreceptors
Pain receptors
 Interoceptors receive information about
internal organs.
Chemoreception
 Chemoreceptors include general receptors that
transmit information about the total solute concentration
of a solution.
 Unicellular organisms use contact chemical
receptors to locate food or avoid toxins.
 Chemotaxis is orientation toward or away from a
chemical.
 Metazoans use distance chemical receptors
(olfaction).
Chemoreception
 The perceptions of gustation (taste) and olfaction
(smell) are both dependent on chemoreceptors that
detect specific chemicals in the environment.
Chemoreception
 The receptor cells for taste in
humans are modified epithelial cells
organized into taste buds.
 Olfactory receptor cells are neurons
that line the upper portion of the
nasal cavity.
 When odorant molecules bind to
specific receptors a signal
transduction pathway is triggered,
sending action potentials to the
brain.
Chemoreception
 Many animals produce species-specific
compounds called pheromones.
 Pheremones released into the environment carry
information about territory, social hierarchy, sex
and reproductive state.
Mechanoreceptors
 Mechanoreceptors
sense physical
deformation caused by
stimuli such as
pressure, stretch,
motion, and sound.
 The mammalian sense
of touch relies on
mechanoreceptors that
are the dendrites of
sensory neurons.
Mechanoreceptors
 Thermoreceptors, which respond to heat or cold help
regulate body temperature by signaling both surface
and body core temperature.
Mechanoreceptors
 In humans, pain receptors are a class of naked
dendrites in the epidermis that respond to excess heat,
pressure, or specific classes of chemicals released
from damaged or inflamed tissues.
Mechanoreceptors
 Most fishes also have a
lateral line system along
both sides of their body.
 The lateral line system
contains mechanoreceptors
with hair cells that respond to
water movement.
 Allows the fish to detect any
changes in current associated
with nearby prey or predators.
Hearing
 Vertebrate
ears
originated as
a balance
organ, or
labyrinth.
 A part of the
labyrinth
elaborated
into the
cochlea.
Hearing
 Vibrating objects create percussion waves in the air
that cause the tympanic membrane to vibrate.
 The three bones of the middle ear transmit the
vibrations to the oval window on the inner ear, or
cochlea.
Hearing
 These vibrations
create pressure
waves in the fluid
in the cochlea that
travel through the
vestibular canal
and ultimately
strike the round
window.
Hearing
 The pressure waves in the vestibular canal cause the
basilar membrane to vibrate up and down causing its
hair cells to bend.
 The bending of the hair cells depolarizes their
membranes sending action potentials that travel via
the auditory nerve to the brain.
Hearing
 The cochlea can
distinguish pitch
because the basilar
membrane is not
uniform along its
length.
 Each region of the
basilar membrane
vibrates most
vigorously at a
particular frequency
and leads to excitation
of a specific auditory
area of the cerebral
cortex.
Equilibrium
 Most invertebrates
have sensory organs
called statocysts
that contain
mechanoreceptors
and function in their
sense of equilibrium.
 When an animal
changes position,
statoliths shift,
disturbing cilia.
Equilibrium
 In most
terrestrial
vertebrates the
sensory organs
for hearing and
equilibrium are
closely
associated in
the ear.
Equilibrium
 Several of the organs of the inner ear detect
body position and balance.
Electromagnetic Receptors
 Electromagnetic receptors detect various forms of
electromagnetic energy such as visible light, electricity,
and magnetism.
Electromagnetic Receptors
 Some snakes have very
sensitive infrared
receptors that detect
body heat of prey
against a colder
background.
 Many mammals appear
to use the Earth’s
magnetic field lines to
orient themselves as
they migrate.
Vision
 Many types of light detectors have evolved in the
animal kingdom and may be homologous.
 Light sensitive receptors are called photoreceptors.
 Even some unicellular organisms have
photoreceptors.
 Dinoflagellate
Vision in Invertebrates
 Most invertebrates
have some sort of
light-detecting organ.
 One of the simplest
is the eye cup of
planarians which
provides information
about light intensity
and direction but
does not form
images.
Vision in Invertebrates
 Two major types of image-forming eyes have evolved
in invertebrates the compound eye and the single-lens
eye.
Vision in Invertebrates
 Compound eyes are
found in insects and
crustaceans and
consist of up to several
thousand light
detectors called
ommatidia.
Vision in Vertebrates
 The main parts of the
vertebrate eye are:
 The sclera, white,
includes the
transparent cornea.
 The iris, colored,
regulates the pupil.
 The retina, which
contains
photoreceptors.
 The lens, which
focuses light on the
retina.
Vision in Vertebrates
 The human retina contains
two types of
photoreceptors:
 Rods are sensitive to light
but do not distinguish
colors.
 Cones distinguish colors
but are not as sensitive.
Color Vision
 Cones contain three
types of visual pigments:
red, green, and blue.
 Colors are perceived by
comparing levels of
excitation of the three
different kinds of cones.
 Color vision is found in
some fishes, reptiles,
birds, and mammals.