Intro Animal Physiology Ch-2.1.ppt

Nervous System

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, briefHormonal 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 – star-

shaped 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.

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

Speed 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.


Vertebrates do not have giant axons.Instead, they achieve high speed

conduction by a cooperative relationship between axons and layers of myelin.


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.

Evolution of Nervous Systems

Metazoan phyla show a progressive increase in the complexity of their nervous systems.Reflects stages of evolution.


The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets.


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


Annelids 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).


Molluscs generally have three pairs of well-defined ganglia.

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


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

social behavior.


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

Nervering

Radialnerve

(b) Sea star (echinoderm)


In 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 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.

Cerebellum

The cerebellum controls balance posture, and muscle coordination.Birds have a highly developed cerebellum

because flying is complicated.

Brain Stem

The brain stem includes the midbrain, pons, and medulla oblongata. It connects the rest of

the brain to the spinal cord.

Controls breathing, swallowing, digestive processes, heartbeat, and diameter of blood vessels.

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.

Thalamus

The thalamus is the major site of sensory processing.Sensory information is received from the

sensory nerves processed in the thalamus and sent on to the cerebral cortex.

The thalamus also controls balance.

Hypothalamus

The hypothalamus integrates internal activities, regulating processes such as: Body temperature Blood pressure Respiration Heartbeat

The hypothalamus also controls the pituitary – a major hormone producing gland.

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.

Cerebrum

The right and left halves of the brain are connected by the corpus callosum.

The left side of the brain is associated with language, mathematical abilities, and leaning.

The right side of the brain is associated with spatial, intuitive, musical, and artistic abilities.

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 taste receptors of insects are located within sensory hairs called sensilla which are located on the feet and in mouthparts.

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)

Chemoreception

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

Few 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.

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.

Vision

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.


Two major types of image-forming eyes have evolved in invertebrates the compound eye and the single-lens eye.


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


Single-lens eyes are found in some jellies, polychaetes, spiders, and many molluscs.They work on a camera-like principle.

Vision in Vertebrates

The eyes of vertebrates are camera-like, but they evolved independently and differ from the single-lens eyes of invertebrates.


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.


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.

Compound Eyes

Present in Insects and Horseshoe CrabComposed of multiple units called

“ommatidia”Image-forming eyesVisual acuity less than for vertebrate eye

Mechanoreceptors

Respond to tactile stimuli, sound, vibrations, pressure

Produce graded response initiallyCan be “phasic” or “tonic” receptorsTypically exhibit “adaptation”Examples: Pacinian corpuscle;

statocysts; inner/middle ear of vertebrates

Vertebrate Ear: Phonoreception

Pinnacle of evolution as far as sound reception among animals is concerned

Detect waves that exhibit certain frequencies and amplitude (nearby and at a distance)

Involve external, middle and inner ear

Two principle organs involved:

CochleaOrgan of Corti

Chemoreceptors Provide information on “smell” and “taste”

Receptor organs: divided into 3 categories: Distance chemical receptors (olfaction) Contact chemical receptors (taste) General chemical receptors

Well-defined in higher vertebrates; harder to differentiate in lower orders (especially aquatic forms)

REFERENCES:

Hill, R. W., G. A. Wyse and M. Anderson. 2008. Animal Physiology, 2nd Ed. Sinauer Associates, Inc., Sunderland, MA.

Schmidt- Nielsen K., Animal Physiology 5th Edition. Cambridge U Press 2002

Thank you and God bless…

- Doc Irish

Intro Animal Physiology Ch-2.1.ppt

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