The Nervous SystemThe Nervous System

Nancy G. MorrisNancy G. Morris

Volunteer State Community CollegeVolunteer State Community College

Campbell Chapter 48Campbell Chapter 48

Nervous SystemNervous System Endocrine SystemEndocrine System

ComplexityComplexity More structurally complex; can integrate vast amounts of information & stimulate a wide range of responses

Less structurally complex; evolved from the nervous system

StructureStructure System of neurons that branch throughout the body

Endocrine glands secrete hormones into bloodstream where they are carried to the target organ

CommunicationCommunication Neurons conduct electrical signals directly to and from specific targets; allows fine pin-point control; transmission is hormonal: acetylcholine, epinepherin, norepinephrin, etc.

Hormones circulate as chemical messengers via the bloodstream; most cells are exposed but only target cells with receptors respond

Response TimeResponse Time Fast transmission of nerve impulses up to 100 m/sec

May take minutes, hours or days for hormones to be produced, diffuse to target organ, & for response to occur

EffectEffect Acts at the cellular level; Immediate and short-lived

Acts a the cellular level; Occur over time and are long-lived

Organization of nervous Organization of nervous systemssystems There is great diversity among animals. All phyla have a nervous “system”

except sponges. In the Hydra’s nerve net, impulses are

conducted in both directions causing movement of entire body.

Some cnidarians & echinoderms have modified nerve nets with rudimentary centralization.

Organization of nervous Organization of nervous systemssystems

Cephalization = evolutionary trend for concentration of sensory & feeding organs on the anterior end of a moving animal (Bilaterial symmetry).

Most bilateral animals have a PNS & a CNS (brain & one or more nerve cords).

Flatworms have a simple “brain” containing large interneurons.

Annelids & arthropods have a well-defined ventral nerve cord & prominent brain. Often coordinate ganglia in each segment to coordinate action.

Cephalopods have the most sophisticated invertebrate nervous system containing a large brain & giant axons.

Fig. 48.13 Diversity in Nervous Fig. 48.13 Diversity in Nervous SystemsSystems

Three overlapping functions Three overlapping functions of the Nervous System:of the Nervous System:

Sensory input is the conduction of signals from sensory receptors to integration centers of the nervous system.

Integration is a process by which information from sensory receptors is interpreted & associated with appropriate responses of the body.

Motor output is the conduction of signals from the processing center to effector cells (muscle & gland cells) that actually carry out the body’s response to stimuli.

Fig. 48.1Fig. 48.1 O Overview:verview: V Vertebrate ertebrate NNervous ervous

SSystemystem

Sensory neurons – convey information about the external & internal environments from sensory receptors to CNS; most synapse with interneurons.

Interneurons – integrate sensory input and motor input; located within the CNS; synapse only with other neurons

Motor neurons – convey impulses form the CNS to effector cells

Three majors classes of Three majors classes of neuronsneurons

Fig. 48.4 Fig. 48.4 SStructural tructural DDiversity of iversity of

NNeuronseurons

A single neuron on the surface of a microprocessor. A cm3 of the human brain will contain more than 50 million neurons.

Signals are conducted by nerves with many axons coming from many different neurons surrounded by connective tissue, the perineurium.

Found in both parts of the nervous system: 1) Central Nervous System (CNS) = comprised

of brain & spinal cord; responsible for integration of sensory input & associating stimuli with appropriate motor output

2) Peripheral Nervous System (PNS) = consists of a network of nerves extending into different parts of the body that carry sensory input to the CNS & motor output away form the CNS

Composition of Nervous Composition of Nervous SystemSystem

The Nervous System contains two types of cells:

1) Neurons – cells specialized for transmitting chemical & electrical signals form one location to another

2) Glia or supporting cells – structurally reinforce, protect, insulate, & generally assist neurons

NeuronsNeurons

Possess a large cell body located either in the CNS or a ganglion

Possess two fingerlike extensions (processes) that conduct messages:

1) Dendrites – convey signals to the neurons cell body Numerous, short, extensively branched to increase surface area

2) Axons – conduct impulses away from the cell body. Single, long process

Fig. 48.2 Fig. 48.2 SStructure of tructure of VVertebrate ertebrate

NNeuroneuron

Vertebrate axons in the PNS are wrapped

in concentric layers of Schwann cells, which form an insulating myelin sheath.

In the CNS, the myelin sheath is formed by ogliodendrites.

Extend from the neuron cell body to many branches (arborization of the axon) which are tipped with synaptic terminals that release neurotransmitters.

AXONSAXONS

Must cross the synapse, the gap between a synaptic terminal and a target cell (either another neuron or an effector cell).

Neurotransmitters are chemicals that cross the synapse to relay the impulse

Table 48.1: acetylcholine, norepinephrine, dopamine, serotonin, neuropeptides: endorphines

Transmission of the impulseTransmission of the impulse

Simple circuit: synapse between sensory neurons & motor neurons, resulting in a simple reflex.

Complex circuit: such as those associated with most behaviors, involve integration by interneurons in the CNS Convergent circuits Divergent circuits Reverberating circuits (memory

storage)

Neurons are arranged in Neurons are arranged in circuitscircuits

Figure 48.3Figure 48.3

The knee-The knee-jerk reflexjerk reflex

Coordination by clusterCoordination by cluster

Nerve cell bodies are often arranged into clusters; these clusters allow coordination of activities by only a art of the nervous system

A nucleus is a cluster of nerve cell bodies within the brain

A ganglion is a cluster of nerve cell bodies in the peripheral nervous system

Supporting cellsSupporting cells

Do not conduct impulses Outnumber neurons by 10- 50- fold Several types of glia cells: 1) astrocytes – encircle capillaries of the

brain 2) oligodendrocytes form the myelin

sheaths that insulate the CNS nerve processes 3) Schwann cells form the insulating

myelin sheath around axons in the PNS

Myelination of neuronsMyelination of neurons

Occurs when Schwann cells or oligodenrocytes grow around an axon so their plasma membranes form concentric layers

Provides electrical insulation

Increases speed of nerve impulse propagation

In MS, myelin sheaths deteriorate causing a disruption of nerve impulse transmission & consequent loss of coordination

Nature of Neural SignalsNature of Neural Signals

Signal transmission along a neuron depends on voltages created by ionic fluxes across neuron plasma membranes.

Membrane potentials arise from differences in ion concentrations between a cell’s contents and the extracellular fluid.

All cells have an electrical potential or voltage across their plasma membrane.

The charge outside is designated as zero, so the minus sign indicates that the cytoplasm inside is negatively charged compared to the extracellular fluid.

Nature of Neural SignalsNature of Neural Signals

Ion channel = integral transmembrane protein that allows a specific ion to cross the membrane.

May be passive all the time or it may be gated, requiring stimulus to change into an open conformation.

Is selective for a specific ion, such as Na+, K+, and Cl-

A shift in ionic gradients is prevented by sodium-potassium pumps which maintain the concentration gradient.

Action PotentialAction Potential

A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening & closing of voltage-gated ion channels.

There are four stages.

Four stages of an Action Four stages of an Action PotentialPotential

1) resting stage; no channels open. 2) depolarizing phase – membrane reverses

polarity (cell interior becomes + relative to exterior); the Na+ activation gates open, Na+ rushes in, potassium gates remain closed.

3) repolarizing phase - returns the membrane potential to resting level; inactivation gates close Na+ channels & K+ channels open.

4) undershoot phase – membrane potential is temporarily more negative than the resting stage (hyperpolarized); Na+ channels remain closed but K+ channels remain open since the inactivation gates have not had time to respond to repolarization of the membrane.

Figure Figure 48.948.9

Role of Role of gated gated ion ion channechannels in ls in the the action action potentipotentialal

Action Potential….Action Potential….

Refractory Period occurs during the undershoot phase.

During this period, the neuron is insensitive to depolarizing stimuli.

Limits the maximum rate at which action potentials can be stimulated in a neuron.

Action potentials are all-or-none events. The nervous system distinguishes between strong & weak stimuli based on the frequency of action potentials generated.

Action potential “travel” Action potential “travel”

Action potentials “travel” along the axon because they are self-propagating.

A neuron is stimulated at its dendrites or cell body and the action potential travels along the axon.

The signal travels in a perpendicular direction along the axon regenerating the action potential.

Figure 48.7Figure 48.7

Propagation Propagation of the of the action action potentialpotential

Action potential “travel” Action potential “travel”

Saltatory conduction – the action potential “jumps” from one node of Ranvier to the next, skipping myelinated regions of membranes

Figure 48.11

Figure 48.11 Saltatory Figure 48.11 Saltatory ConductionConduction

CommunicationCommunication Communication between cells happens across

the synapse Synapse – tiny gap between a synaptic

terminal of an axon & a signal- receiving portion of another neuron or effector cell

Presynaptic cell is the transmitting cell; postsynaptic cell is the receiving cell

There are two types of synapses: 1) electrical 2) chemical

Electrical SynapsesElectrical Synapses

Allow action potentials to spread directly from pre- to postsynaptic cells via gap junctions (intercellular channels)

Allow impulse travel without delay or loss of signal strength

Less common than chemical synapses

Common in crustaceans

Chemical SynapsesChemical Synapses

Synaptic vesicles containing thousands of neurotransmitter molecules are present in the cytoplasm of the synaptic terminal of the presynaptic neuron

Chemical synapses allow transmission in one direction only

Receptors for neurotransmitters are located only on postsynaptic membranes

Figure 48.12 A Chemical Figure 48.12 A Chemical SynapseSynapse

One neuron may receive information from thousands of synapses. Some synapses are excitatory, others are inhibitory (Fig. 48.11).

The same neurotransmitter can produce different effects on different types of cells

(Table 48.2)

Figure 48.13

Integration of multiple synaptic inputs

Numerous synaptic terminals communicating with a single postsynaptic cell (SEM).

Fig. 48.17 Fig. 48.17

Functional Functional hierarchy hierarchy of the of the peripheral peripheral nervous nervous systemsystem

Figure 48.18Figure 48.18ParasympathetParasympathetic ic – maintains – maintains normal activitynormal activity- slows down- slows down- acetylcholine- acetylcholineSympatheticSympathetic-”flight or -”flight or fight”fight”- speeds up- speeds up- - norepinephrinnorepinephrinee

Integration & ControlIntegration & Control

CNSCNS & PNSPNSAutonomic Somatic

NS NS (internal organs- (muscles, skin, viscera) joints)

Parasympathetic Sympathetic (normal activity) (“flight or fight”)

CNSCNS: Meninges: Meninges

MeningesMeninges series of 3 membrane layers which surround brain & spinal cord:

1) dura mater (outer)

2) arachnoid membrane 3) pia mater (inner)

CNS:CNS: Spinal Cord Spinal Cord

connecting mechanism between the body & the brain

part of the CNS enclosed in vertebral column 31 pairs of spinal nerves fluid-filled cavity – cerebrospinal

fluid

Fig. 48.16Fig. 48.16The nervous The nervous system of a system of a vertebrate:vertebrate:31 pairs31 pairs of of spinal nerves spinal nerves each each containing one containing one sensory sensory neuronneuron (afferent (afferent dorsal route) dorsal route) & one & one motor motor neuronneuron (efferent (efferent ventral route)ventral route)

Vertebrate BrainVertebrate Brain

100 billion neurons in human brain 10 billion in the cerebral cortex weights about 3 lbs composed of gray matter (cell

bodies) & white matter (axons & dendrites)

developed from the 3 bulges at the anterior end of the dorsal tubular nerve cord: hind – mid–fore brain)

Fig.48.19Fig.48.19 Embryonic Embryonic Development of the Vertebrate BrainDevelopment of the Vertebrate Brain

Fig.48.19 Fig.48.19

Embryonic Embryonic Development Development of the brainof the brain

Major parts of the human brain.Major parts of the human brain.

Vertebrate Brain Vertebrate Brain (Hindbrain)(Hindbrain)

Medulla –(upper spinal cord)

center for respiratory, cardiac function; vomiting, sweating, gastric secretion, heartbeat

Cerebellum – regulate & controls bodily muscular contractions; coordination, balance, equilibrium

Pons – bridge between two halves of the cerebellum; carries fibers that coordinate activity of muscles on two sides of the body

Vertebrate Brain (Vertebrate Brain (MidMid & & ForeFore))

Midbrain – relay center; visual & auditory reflexes

Thalamus – relay center for sensory impulses going to cerebrum control center for external manifestations of emotion (laughing, crying, etc.)

Hypothalamus – (floor of thalmus) regulates hunger, thirst, body temp, CHO & fat metabolism, blood pressure, sleep; regulates the pitiutary

Vertebrate Brain (Vertebrate Brain (ForebrainForebrain))

Cerebral hemispheres – Cerebrum – Controls learned behavior & memory; makes up about 80% of brain mass

1) frontal (speech, motor cortex)

2) parietal (taste, reading,)3) temporal (smell, hearing)4) occipital (vision)

Corpus Collosum – junction between two hemispheres

Figure 48.24 Figure 48.24 Structure and Functional areas of the Structure and Functional areas of the cerebrumcerebrum

Figure 48.26 Mapping Language Figure 48.26 Mapping Language areas of the cerebral cortex.areas of the cerebral cortex.

Figure 49.16 Figure 49.16

Neural Neural pathways for pathways for visionvision

Sensory transduction by a taste Sensory transduction by a taste

receptor.receptor.