NERVOUS SYSTEM AND SENSE ORGANS

The Nervous System

a rapid communication system that interacts continuously with the endocrine system to control coordination of body function.

The basic unit of nervous integration in all animals is the neuron, a highly specialized cell designed to conduct self-propagating electrical events, called action potentials, to other cells.

Action potentials are transmitted from one neuron to another across synapses which may either be electrical or chemical.

The thin gap between neurons at chemical synapses is bridged by a chemical neurotransmitter molecule, which is released from the synaptic knob, and can be either stimulatory or inhibitory.

Neurons: Functional units

of Nervous System

Neuron

Axon terminals

Muscle fiber

Axon

Axon hillock

DendritesSoma

Schwann cell

Direction of signal

Nodes of Ranvier

A neuron or nerve cell may assume many shapes, depending on its function and location. Involves two types of cytoplasmic processes: one or more dendrites and a single axon. These processes are profusely branched. They are the nerve cell’s receptive apparatus that receives information from several different sources at once.

NucleusMyelin sheath

The axon is often covered with an insulating sheath of myelin, which speeds up signal propagation.

Neurons are commonly classified as afferent(sensory), efferent(motor) and interneurons, which are neither sensory nor motor but connect neurons with otherneurons.

Afferent and efferent neurons lie mostly in the peripheral nervous system, while interneurons lie entirely within the central nervous system.

Afferent neurons are connected to receptors. Receptors function to convert external and internal environmental stimuli into nerve signals, which are carried by afferent neurons into the central nervous system. Nerve signals also move to efferent neurons, which carry them via the peripheral nervous system to effectors, such as muscles or glands.

Cell bodies of the nerve processes are located either in the:*Central nervous system*Ganglia-discrete bundles of nerve cell bodies located outside the CNS

Neuroglial cells (glial cells) Extremely numerous in the vertebrate brain and may form almost half the volume of

the brain.

Vertebrate nerves are often enclosed by concentric rings of myelin, produced by special glial cells called Schwann cells in the PNS and oligodendrocytes in the CNS.

Astrocytes are radiating and star like glial cells that serve as nutrients and ion reservoirs for neurons, as well as scaffold during brain development.

Astrocytes, and microglial cells, are essential for the regenerative process that follows brain injury.

Astrocytes also participate in several diseases of the nervous system, including Parkinson’s disease, multiple sclerosis and brain tumor development.

Nature of a Nerve Action Potential A nerve signal or action potential is an electrochemical message of neurons, the

common functional denominator of all nervous system activity. An action potential is an “all-or-none” phenomenon; either the fiber is conducting an

action potential, or it is not. Action potentials are alike, the only way a nerve fiber can vary its signal is by

changing the frequency of signal conduction. Frequency change is the language of a nerve fiber. The higher the frequency (or rate) of conduction, the greater is the level of

excitation.

Resting Membrane Potential The membrane of a neuron is selectively permeable to K+, which can transverse the

membrane through special potassium channels. The permeability to Na+ is nearly zero because Na+ channels are closed in a resting membrane. Potassium ions tend to diffuse outward through the membrane, following the gradient of potassium concentration. Very quickly the positive charge outside reaches a level that prevents anymore K+ from diffusing out of the axon, and because large ions cannot pass through the membrane, positively charged potassium ions are drawn back into the cell. Now the resting membrane is at equilibrium, with an electrical gradient that exactly balances the concentration gradient.

This resting membrane potential is usually -70mV, with the inside of the membrane negative with respect to the outside.

Sodium PumpA complex of protein subunits embedded in the plasma membrane of the axon.Uses energy from the breakdown of ATP to transport sodium form the inside to the outside of the membrane.the sodium pump in nerve axons, as in many other cell membranes, also moves K+ into the axon while it is moving Na+ out.It is a Sodium-potassium exchange pump that helps to restore the ion gradients of both Na+ and K+.

Action Potential A very rapid and brief depolarization of the membrane of the nerve fiber. This means

that the membrane potential changes from rest in a positive direction and overshoots 0 mV to about +35 mV.

The membrane potential reverses for an instant so that the outside becomes negative compared with the inside.

As the action potential moves ahead, the membranes returns to its normal resting membrane potential, ready to conduct another signal.

The entire event occupies approximately a millisecond.

What causes the reversal of polarity in the cell membrane during passage of an action

potential?When an action potential arrives at a given point in a neuron membrane, the change in membrane potential causes voltage-gated Na+ channels to suddenly open,

permitting a flood of Na+ to diffuse into the axon from the outside, moving down the concentration gradient for Na+. Only a very minute amount of Na+ moves

across the membrane but this sudden rush of positive ions cancels the local resting membrane potential and the membrane is depolarized. Then, as the Na+ channels close, the membrane quickly regains its resting properties as K+ ions quickly diffuse

out of voltage-gated K+ channels that open briefly in response to the membrane depolarization. The membrane once again become practically impermeable to Na+

the outward movement of K+ is checked as the voltage-gated K+ channels close, and the membrane again becomes leaky to movement of K+ as the resting membrane

potential is reestablished.

Increased potassium permeability causes the action potential to drop rapidly toward the resting membrane level, during the repolarization phase. The membrane is now ready to transmit another action potential.

Synapses: Junctions between

nerves

Synapse is a small gap that separates another neuron or effector organ from an axon terminal when an action potential passes down.

Two distinct types of synapses:*electrical synapses-points at which ionic currents flow directly across a narrow gap junction from one neuron to another.*chemical synapses-much more complex than electrical synapses which contain packets or vesicles or specialized chemicals called neurotransmitters.

+presynaptic neurons-neurons bringing action potentials toward chemical synapses.+postsynaptic neurons-neurons carrying action potentials away from chemical synapses.

Synaptic cleft-a narrow gap, having a with of approximately 20nm, that separates membranes at a synapse.

Synaptic vesicles-found inside the synaptic knobs, each containing several thousand molecules of acetylcholine.

Acetylcholine-most common neurotransmitter of the PNS, which illustrates typical synaptic transmission.

Whether the postsynaptic excitatory potential is large enough to trigger an action potential depends on how many acetylcholine molecules are released and how many channels are opened.

Acetylcholinesterase-enzyme that rapidly destroys acetylcholine, which converts acetylcholine into acetate and choline.

The final step in the sequence is reabsorption of choline into the presynaptic terminal, resynthesis of acetylcholine and its storage in synaptic vesicles, ready to respond to another action potential.

Excitatory synapses-releases chemical neurotransmitters that depolarize postsynaptic membranes .

Inhibitory synapses-releases chemical neurotransmitters that move the resting membrane potential in a more negative direction (hyperpolarization).

Neurotransmitters that are :both excitatory and inhibitory-acetylcholine, norepinephrine, dopamine, serotoninalways excitatory-glutamatealways inhibitory-glycine, GABA(gamma amino butyric acid)

The synapse is a crucial part of the decision making equipment of the central nervoussystem, modulating flow of information from one neuron to the next.

Evolution of Nervous Systems

Invertebrates: Development of Centralized

Nervous Systems The simplest pattern of invertebrate nervous system is the nerve net of radiate

animals, such as sea anemones, jellyfishes, hydras, and comb jellies. There are no differentiated sensory, motor or connector components in the strict

meaning of those terms. However, there is evidence of organization into reflex arcs with branches of a nerve net connecting to sensory receptors in the epidermis and to epithelial cells that have contractile properties.

This type of nervous system is found among vertebrates in nerve plexuses located, for example, in the intestinal wall.

Bilateral nervous system represent a distinct increase in complexity over the nerve net of radiate animals.

The flatworm’s nervous system is the simplest nervous system showing differentiation into a PNS and a CNS which coordinates everything.

More complex invertebrates exhibit a more centralized nervous system (brain), with two longitudinal fused nerve cords and many ganglia.

Vertebrates: Fruition of Encephalon The basic plan of the vertebrate nervous system is a hollow, dorsal nerve cord

terminating anteriorly in a large ganglionic mass, or brain. The most important trend in evolution of vertebrate nervous system is the great

elaboration of size, configuration, and functional capacity of the brain, a process called encephalization.

Spinal CordThe brain and spinal cord compose the CNS. They begin as an ectodermal neural groove, which by folding and enlarging becomes a long, hollow neural tube.Segmental nerves of the spinal cords of the vertebrates are separated into dorsal sensory roots and ventral motor roots.Sensory nerve cell bodies are gathered together into dorsal root (spinal) ganglia. Both dorsal (sensory) and ventral (motor) roots meet beyond the spinal cord to form a mixed spinal nerve.

Spinal cord

Ventral root

Dorsal root

Dorsal ganglion

Spinal nerve

Meninges

Sympathetic ganglion

Vertebra

A reflex act is a response to a stimulus acting over a reflex arc. It is involuntary. Some reflex acts are innate; others are acquired through learning.

BrainA primitive linear brain, as seen in fishes and amphibians, expanded to form a deeply

fissured and enormously Intricate brain in the lineage leading to mammals. The spinal cord encloses a central spinal canal and is additionally wrapped in three

layers of membranes called meninges. An inner zone of gray matter contains the cell bodies of motor neurons and

interconnecting interneurons. An outer zone of white matter contains bundles of axon and dendrites linking different levels of the spinal cord with each other and with the brain.

Reflex Arc Reflex arcs appear to be the fundamental unit of neural operation.

Parts of a typical reflex arc Receptor-a sense organ in skin, muscle, or another organ. Afferent-sensory neuron, which carries impulses toward the CNS. CNS-where synaptic connections are made between sensory and interneurons.

Efferent-motor neuron, which makes the synaptic connection with the interneuron and carries impulses from the CNS.

Effector-by which an animal responds to environmental changes.

A reflex arc in vertebrates in its simplest form contains only two neurons: sensory(afferent) neuron and a motor(efferent) neuron. Interneurons are interposed between sensory and motor neurons.

Brains of early vertebrates had three principal divisions:*prosencephalon (forebrain)*mesencephalon (midbrain)*rhombencephalon (hindbrain)

HindbrainThe medulla oblongata, the most posterior division of the brain, is really a conical

continuation of the spinal cord. The medulla, together with the more anterior midbrain, constitutes the “brainstem”, an area that controls numerous vital and largely subconscious activities such as heartbeat, respiration, etc. The pons contains a thick bundle of fibers that carry impulses from one side of the cerebellum to the other.

The cerebellum, lying dorsally to the medulla, controls equilibrium, posture and movement. It does not initiate movement but operates as a precision error-control center, or servomechanism, that programs a movement initiated somewhere else, such as motor cortex of the cerebrum.

MidbrainThe midbrain consists mainly of the tectum, which contains nuclei that serve as centers

for visual and auditory reflexes. It mediates the most complex behavior of fishes and amphibians, integrating visual, tactile, and auditory information. In mammals, the midbrain is mainly a relay center for information on its way to higher brain centers.

ForebrainJust anterior to the midbrain lie the thalamus and hypothalamus, the most posterior

element of the forebrain. The thalamus is a major relay station the analyzes and passes sensory information to higher brain centers. In the hypothalamus are several “house keeping” centers that regulate all functions concerned with maintenance of internal consistency (homeostasis).

The anterior portion of the forebrain, or cerebrum, can be divide into two anatomical distinct areas, the paleocortex and neocortex. In mammals and especially in primates the paleocortex is a deep-lying area called rhinencephalon, because many of its functions depend on olfaction. Better known as the limbic system, it mediates several species-specific behaviors that relate to fulfilling needs such as feeding and sex.

The neocortex (cerecral cortex)completely overshadows the paleocortex and has become so expanded that it envelops much of the forebrain and all of the midbrain.

The cortex contains discrete motor and sensory ares. The motor ares control voluntary muscle movements, while the sensory cortex is the center of conscious perception of touch, pain, pressure, temperature, and taste.

The right and left hemispheres of the cerebral cortex are bridged through the corpus callosum, a neural connection through which the two hemispheres are able to transfer information and coordinate mental activities. In humans, the left hemisphere is for language development, mathematical and learning capabilities, and sequential thought processes; the right hemisphere is for spatial, musical, artistic, intuitive, and perceptual activities. Each hemisphere also controls the opposite side of the body.

Peripheral Nervous System

The peripheral nervous system includes all nervous tissue outside the CNS.Two functional divisions:

*sensory or afferent division, which brings sensory information to the CNS*motor or efferent division, which conveys motor commands to muscles and glandsEfferent divisions:

+somatic nervous system-innervates skeletal muscles+autonomic nervous system-innervates smooth muscle, cardiac muscle,

and glandsAutonomic NS subdivisions:

->parasympathetic NS-associated with non stressful activities->sympathetic NS-active under conditions of physical or

emotional stress

Sense Organs

Sense organs are specialized receptors designed for detecting environmental status and change. Sense organs are its first level of environmental perception; they are channels for bringing information to the CNS.

A stimulus is some form of energy-electrical, chemical, mechanical, or radiant. A sense organs transforms energy from a stimulus into nerve action potentials. Sense organs are biological inducers.

Sense organs are specific for one kind of stimulus*eyes respond to light, ears to sound, pressure receptors to pressure, and chemoreceptors to chemicals

Classification of ReceptorsBy location: Exteroceptors-near the external surface that keep an animal

informed about its external environment.Interoceptor-internal parts of the body which receive stimuli

from internal organs.Proprioceptors-in muscles, tendons, and joints which are

sensitive to changes in tension of muscles and provide an organism with a sense of body position.

By the form of energy to which it responds:Chemical, Mechanical, Light, or Thermal

ChemoreceptionChemoreception is the oldest and most universal sense in the animal kingdom.

*Contact chemical receptors-to locate food and adequately oxygenated water and to avoid harmful substances. Chemotaxis, orientation behavior toward or away from the chemical source.*Distance chemical receptors-often developed to a remarkable degree of sensitivity. Distance chemoreception is usually called smell or olfaction that guides feeding behavior, location and selection of sexual mates, etc.

In vertebrates, taste receptors are found in the mouth cavity and especially on the tongue, where they provide a means for judging foods before they are swallowed. A taste bud consists of a cluster of receptor cells surrounded by supporting cells; it is provided with a small external pore through which slender tips of sensory cells project.

Taste sensations are categorized as sweet, salty, acid, bitter, and possibly umami (Jap. For “meaty” or “savory”)

Taste discrimination depends on assessment by the brain of the relative activity of many different taste receptors.

Taste buds have short life (5-10 days in mammals) and are continually being replaced.

Olfactory sense is a primal sense for many animals, used for identification of food, sexual mates, and predators.

Olfactory endings are located in a special epithelium covered by a thin film of mucus, positioned deep in the nasal cavity.

Social insects and many other animals produce species-specific compounds called pheromones that constitute a highly developed chemical language.

Pheromones are a diverse group of organic compounds that an animal releases to affect the physiology or behavior of another individual of the same species.

MechanoreceptionMechanoreceptors are sensitive to quantitative forces such as touch, pressure, or in

short, in motion.

Touch: Pacinian corpuscles, relatively large mechanoreceptors that register deep touch and pressure in mammalian skin, illustrate the general properties of mechanoreceptors.

Pain: Pain receptors are relatively unspecialized nerve fiber endings that respond to a variety of stimuli signaling possible or real damage tissues. *Slow pain-Pain fibers respond to small peptides which are released by the injured cell.*Fast pain-more direct response of the nerve endings to mechanical or thermal stimuli.

Lateral-line System of Fish and Amphibians: a lateral line is a distant touch receptor system for detecting wave vibrations and currents in water.

Receptors called neuromasts are located on the body surface in aquatic amphibians and some fishes. Each neuromast is a collection of hair cells with sensory endings or cilia, embedded in a gelatinous, wedge-shape mass, the cupula.

Hearing: An ear is a specialized receptor for detecting sound waves in the surrounding environment.

Equilibrium: The vertebrate organ of equilibrium is the labyrinth, or vestibular organ. Specialized sense organs for monitoring gravity and low- frequency vibrations often appear as statocysts, a simple sac lined with hair cells and containing a heavy calcareous structure, the statolith.

Photoreception: VisionLight –sensitive receptors are called photoreceptors.

these receptors range from simple light-sensitive cells scattered randomly on the body surface of many invertebrates to the exquisitely developed camera-type eye of vertebrates and cephalopods.

A dinoflagellate bears a lens, a light-gathering chamber, and a photoreceptive pigment cup-all developed within a single-celled oragnism.

Vertebrates have a camera eye with focusing optics. Photoreceptor cells of the retina are two of kinds:

*Rods-designed for high sensitivity with dim light

*Cones-designed for color vision in daylight.

Cones predominate in fovea centralis of human eyes, the area of keenest vision. Rods are more abundant in peripheral areas of the retina.