Nervous CoordinationChapter 33

IrritabilityThe 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, briefHormonal mechanisms – slower, long term

CNS & PNSCentral Nervous

System (CNS) – includes the brain and spinal cord.

Peripheral Nervous System (PNS) – includes motor and sensory neurons.

NeuronsA 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.

NeuronsTwo 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.

NervesNerve 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 CellsNon-neural cells that

work with neurons are called glial cells.Astrocytes – star-

shaped cells that serve as nutrient and ion reservoirs for neurons.

Glial CellsThe 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 PotentialA 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 ImpulseAcross 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 ImpulseNeuron 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 PumpNa+ flows into the

cell during an action potential, it must be pumped out using sodium pumps so that the action potential will continue.

potassium

http://youtu.be/SdUUP2pMmQ4

The Nerve ImpulseResting 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 ImpulseA 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 ImpulseThe 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 ImpulseThis 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 ConductionSpeed is related to the diameter of the axon.

Larger axons conduct faster.A squid’s giant axon can carry impulses 10x faster than

their normal axons.Used for powerful swimming.

High Speed ConductionVertebrates do not have giant axons.

Instead, they achieve high speed conduction by a cooperative relationship between axons and layers of myelin.

High Speed ConductionInsulating 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 synapsesChemical Synapses

Electrical SynapseElectrical 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 SynapsePresynaptic 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.

NeurotransmittersNeurotransmitters 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 SynapsesThere 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 SynapsesInhibitory 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 SynapsesAn 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.

Evolution of Nervous SystemsMetazoan phyla show a progressive increase in the

complexity of their nervous systems.Reflects stages of evolution.

Evolution of Nervous SystemsThe simplest animals with

nervous systems, the cnidarians, have neurons arranged in nerve nets.

Evolution of Nervous SystemsIn relatively simple

cephalized animals, such as flatworms, a central nervous system (CNS) is evident.

Evolution of Nervous SystemsAnnelids have a bilobed brain,

a double nerve cord with segmental ganglia (clusters of neurons) and distinctive sensory and motor neurons.

These ganglia connect to the CNS and make up a peripheral nervous system (PNS).

Evolution of Nervous SystemsMolluscs generally

have three pairs of well-defined ganglia.

In cephalopods, these ganglia have developed into complex nervous centers with highly developed sense organs.

Evolution of Nervous SystemsThe arthropod plan

resembles that of annelids, but ganglia are larger and sense organs are better developed.Often elaborate social

behavior.

Evolution of Nervous SystemsSea stars have a

nerve net in each arm connected by radial nerves to a central nerve ring.

Nervering

Radialnerve

(b) Sea star (echinoderm)

Evolution of Nervous SystemsIn vertebrates, the

central nervous system consists of a brain and dorsal spinal cord.The PNS connects

to the CNS.

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)

Vertebrate Nervous SystemVertebrates 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 CordThe 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 ArcA 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 ArcUsually, 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.

BrainThe 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 BrainThe 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 HindbrainThe 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.

MidbrainThe midbrain consists of the tectum, including optic

lobes, which contain nuclei that serve as centers for visual and auditory reflexes.

ForebrainVertebrates 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.

CerebrumThe cerebrum is the control center of the

brain.Right and left halves called cerebral hemispheres.Functions in language, conscious thought, memory,

personality development, vision.

CerebrumThe 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 SystemThe 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 SystemThe 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 SystemThe 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 OrgansSense 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 ReceptorsExteroceptors receive information about the

external environment.Based on the energy they transduce, sensory

receptors fall into five categoriesMechanoreceptorsChemoreceptorsElectromagnetic receptorsThermoreceptorsPain receptors

Interoceptors receive information about internal organs.

ChemoreceptionChemoreceptors 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).

ChemoreceptionThe 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.

Five taste perceptions : Sweet Sour, Salty Bitter Umami (meaty or savory)

ChemoreceptionOlfactory 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.

ChemoreceptionMany animals produce species-specific

compounds called pheromones.Pheremones released into the environment

carry information about territory, social hierarchy, sex and reproductive state.

MechanoreceptorsMechanoreceptors

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.

MechanoreceptorsThermoreceptors, which respond to heat or cold help

regulate body temperature by signaling both surface and body core temperature.

MechanoreceptorsIn 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.

MechanoreceptorsMost 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.

HearingFew invertebrates can hear.Exceptions include insects that have simply designed

ears that allow the insects to hear calls of potential mates, rival males, or predators.Moths can detect the ultrasonic sounds of bats.

HearingVertebrate

ears originated as a balance organ, or labyrinth.

A part of the labyrinth elaborated into the cochlea.

HearingVibrating 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.

HearingThese 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.

HearingThe 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.

EquilibriumMost 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.

EquilibriumIn most

terrestrial vertebrates the sensory organs for hearing and equilibrium are closely associated in the ear.

EquilibriumSeveral of the organs of the inner ear detect

body position and balance.

Electromagnetic ReceptorsElectromagnetic receptors detect various forms of

electromagnetic energy such as visible light, electricity, and magnetism.

Electromagnetic ReceptorsSome 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.

VisionMany 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 InvertebratesMost 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 InvertebratesTwo major types of image-forming eyes have evolved

in invertebrates the compound eye and the single-lens eye.

Vision in InvertebratesCompound eyes are

found in insects and crustaceans and consist of up to several thousand light detectors called ommatidia.

Vision in VertebratesThe 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 VertebratesThe 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 VisionCones 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.