Exam 5

Nervous system is

master controlling and communicating system of body

Nervous System functions (3)

1. Sensory Input- Information gathered by sensory receptors about internal and external changes

2. Integration- Processing and interpretation of sensory input

3. Motor Output- activation of effectors produces a response

Nervous system is divided into two parts

1. Central Nervous System-Brain and spinal cord of dorsal body cavity

   1. Integration and control center

   2. Interprets sensory input and dictates motor output

2. Peripheral Nervous System-

   1. The portion of nervous system outside CNS

   2. Consists mainly of nerves that extend from brain and spinal cord–

      1. Spinal nerves to and from spinal cord

      2. Cranial nerves to and from brain

Walls of gastrointestinal tract also contain neurons called the

enteric nervous system

Peripheral nervous system (PNS) has two functional divisions

1. Sensory (afferent) division

   1. Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints toCNS

   2. Visceral sensory fibers: convey impulses from visceral organs toCNS

2. Motor (efferent) Division

   1. Transmits impulses fromCNS to effectors–Muscles and glands

   2. Two divisions–Somatic nervous system–Autonomic nervous system

Somatic nervous system, or voluntary nervous system function

• Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle

• Conscious control of skeletal muscles

Autonomic nervous system (ANS), or involuntary nervous system functions

• Consists of visceral motor nerve fibers

• Regulates smooth muscle, cardiac muscle, and glands

• Two functional subdivisions work in opposition of each other ▪

  • Sympathetic division- mobilize body systems during activity

  • Parasympathetic division- conserves energy, promotes house-keeping functions during rest

Neuroglia (glial cells)

small cells that surround and wrap delicate neurons

Neurons (nerve cells)

excitable cells that transmit electrical signals

Four main neuroglia support CNS neurons

1. Astrocytes

2. Microglial cells

3. Ependymal cells

4. Oligodendrocytes

Astrocytes

• Most abundant, versatile, and highly branched of glial cells

• Cling to neurons, synaptic endings, and capillaries

• Functions:

Support and brace neurons

• Play role in exchanges between capillaries and

• neurons

• Guide migration of young neurons

• Control chemical environment around neurons

  • Clean up leaked potassium ions

  • Recapture and recycle neurotransmitters

• Influence neuronal functioning

Microglial

• Small, ovoid cells with thorny processes that touch and monitor neurons

• Migrate toward injured neurons

• Can transform to phagocytize microorganisms and neuronal debris

Ependymal cells

• Range in shape from squamous to columnar

• May be ciliated

  • Cilia beat to circulate CSF

• Line the central cavities of the brain and spinal column

• Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells

Oligodendrocytes

• Branched cells with fewer processes than astrocytes

• Processes wrap CNS nerve fibers forming insulating myelin sheath

Two major neuroglia seen in PNS

• Satellite cells–

  • Surround neuron cell bodies in PNS

  • Function similar to astrocytes of CNS

• Schwanncells, or neurolemmocytes

  • Surround all peripheral nerve fibers and form myelin sheaths around the thicker nerve fibers

    • Similar function as oligodendrocytes–

  • Vital to regeneration of damaged peripheral nerve fibers

Neurons, or nerve cells, are

• structural units of nervous system

• Large, highly specialized cells that conduct impulses

• Special characteristics

  • Extreme longevity (lasts a person’s lifetime)

  • Amitotic (lose their ability to divide) with few exceptions

  • High metabolic rate and require continuous supply of oxygen and glucose

• All have a cell body and one or more slender processes

Cell body, soma, contains

spherical nucleus with nucleolus, surrounded by cytoplasm

Most neuron cell bodies are located in CNS

• Nuclei: clusters of neuron cell bodies in CNS

• Ganglia: clusters of neuron cell bodies in PNS

Neuron Processes

• Processes extend from cell body of all neurons

• CNS contains both neuron cell bodies and their processes

• PNS contains chiefly of neuron processes (most cell bodies in CNS)

• Two types of processes–Dendrites–Axon

Dendrites

• Motor neurons can contain hundreds of these short, tapering, diffusely branched processes

  • Contain same organelles as in cell body

• Receptive(input) regionsof neuron

• Convey incoming messages toward cell body as graded potentials(short-distance signals)

• In many brain areas, finer dendrites are highly specialized to collect information

  • Contain dendritic spines, appendages with bulbous or spiky ends

Axon Structure

• Each neuron has only one axon

• Axon length varies

• Some axons branch

Bundles of axons in CNS are called

tracts

Bundles of axons in PNS are called

nerves

Axon Functional Characteristics

• Axon is the conducting region of neuron

• Axon terminals are the secretory region of neuron

• Each neuron receives and sends signals with different neurons simultaneously

• Axons rely on cell bodies for membrane components and mRNA

• Axons quickly decay if cut or damaged

Axonal Transport

• Axons have efficient internal transport mechanisms

  • Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements

• Movement occurs in both directions

Myelin sheath function

Protect and electrically insulate axon

Increase speed of nerve impulse transmission

segmented sheath surrounds most long or large-diameter axons

Myelinated axons

do not contain sheath

Nonmyelinated axons

Conduct impulses more slowly

Myelination in the PNS

Formed by Schwann cells

Wrap around axon in jelly roll fashion

One cell forms one segment of myelin sheath

Myelin sheath gaps, or nodes of Ranvier

Gaps between adjacent Schwann cells

Sites where axon collaterals can emerge

Neuron Structural Classifications

Three types grouped by number of processes extending from cell body

1. Multipolar neurons: many processes (1 axon and many dendrites)–99% of neurons; major neuron type in CNS

2. Bipolar neurons: two processes (1 axon and 1 fused dendrite)

   1. Rare neuron type; found in retina, ear, and olfactory mucosa

3. Unipolar neurons: one process (T-shaped process exits cell body and divides into 2 branches of axon)

   1. Peripheral process: branch associated with sensory receptor

   2. Central process: branch enters CNS

   3. Also called pseudounipolar neurons

   4. Found mainly in PNS ganglia where function as sensory neurons

Neuron Functional Classification

Three types of neurons grouped by direction nerve impulse travels relative to CNS

1. Sensory (afferent) neurons

   1. Transmit impulses from sensory receptors toward CNS

      1. Almost all are unipolar neurons

      2. Cell bodies located in ganglia in PNS

2. Motor (efferent) neurons

   1. Carry impulses from CNS to effectors

   2. Multipolar neurons

   3. Most cell bodies located in CNS (except some autonomic neurons)

3. Interneurons, or association neurons

   1. Lie between motor and sensory neurons in neural pathways–

   2. Shuttle signals through CNS pathways, most are entirely within CNS–99% of body’s neurons are interneurons

a measure of potential energy generated by separated charge

Voltage

Measured between two points in volts (V)or millivolts (mV) ▪

Called potentialdifferenceor potential

Charge difference across plasma membrane results in potential

Greater charge difference between points = higher voltage

flow of electrical charge (ions) between two points

Current

Can be used to do work

Flow is dependent on voltage and resistance

Resistance

hindrance to charge flow

Insulator

substance with high electrical resistance

Conductor

substance with low electrical resistance

Ohms Law

gives relationship of voltage, current, resistance  Current(I) voltage(V)/resistance (R)

Role of membrane ion channels

Plasma membranes have large proteins that acts as ion channels

Two main types of ion channels

Leakage

Gated Channels

Leakage Channels

which are always open

Gated channels

• in which part of the protein changes shape to open/close the channel–

• Chemically gated channels or ligand gated channels

  • Open only with binding of a specific chemical (ex: neurotransmitter)

• Voltage-gated channels

  • Open and close in response to changes in membrane potential–

• Mechanically gated channels

  • Open and close in response to physical deformation of receptors, as in sensory receptors

Direction into or out of the cells is determined by the

Electrochemical gradient

ions move from area of higher concentration to area of lower concentration

Concentration Gradient

ions move toward area of opposite electrical change

Electrical gradient

Membrane Potential

Differences in ionic composition

• Cell cytosol has lower concentration of Na+ than ECF

  • Balanced chiefly by chloride ions Cl-

• Cell cytosol has higher concentration of K+ than ECF

  • Balanced by negatively charged proteins

• K+ plays most important role in membrane potential

Generating a resting membrane potential depends on

(1) differences in K and Na concentrations inside and outside cells,

(2) differences in permeability of the plasma membrane to these ions

stabilizes resting membrane potential

Sodium-potassium pump (Na KATPase)

Three Na are pumped out of cell while two K are pumped back in

Membrane potential changes when:

Concentrations of ions across membrane change

Membrane permeability to ions changes

Changes in membrane potential produce two types of signals

Graded potentials -Incoming signals operating over short distances

Action potentials -Long-distance signals of axons

decrease in membrane potential (moves toward zero and above)

Depolarization

Inside of membrane becomes less negative than resting membrane potential Probability of producing impulse increases

increase in membrane potential (away from zero)

Hyperpolarization

Inside of membrane becomes more negative than resting membrane potential ▪Probability of producing impulse decreases

short-lived, localized changes in membrane potential

Graded potentials

The stronger the stimulus, the more voltage changes and the farther current flows

Graded potentials are trigged by

stimulus that opens gated ion channels

graded potentials in receptors of sensory neurons

Receptor potential (generator potential):

neuron graded potential

Postsynaptic potential

special type of graded potential that occurs in muscle cell, instead of postsynaptic neuron, triggering action potential for muscle contraction

End-plate potential:

Once gated ion channel opens,

depolarization spreads from one area of membrane to next

a brief reversal of membrane potential with a change in voltage of ~100 mV

action potential

Action potentials only occur in

excitable membranes–Neurons and muscle cells

Steps in Generating an Action Potential - First Step

1. Resting state: All voltage-gated Na andK channels are closed

• Only leakage channels for Na andK   are open

  • Maintains the resting membrane potential

• Each Na channel has two voltage-sensitive gates

  • Activation gates: closed at rest; open with depolarization, allowing Na to enter cell

  • Inactivation gates: open at rest; block channel once it is open to prevent more Na from entering cell ▪

• Each K channel has one voltage-sensitive gate

  • Closed at rest

  • Opens slowly with depolarization

Steps in Generating an Action Potential - Second Step

Depolarization: voltage-gated  Na channels open ▪

• Depolarizing local currents open voltage-gated Na channels, and Na rushes into cell ▪

• Na activation and inactivation gates open ▪

• Na influx causes more depolarization, which opens more Na channels

  • As a result, ICF becomesless negative ▪

• At threshold ( 55 to 50 mV),   positive feedback causes opening of all Na channels–

  • Results in large action potential spike –

  • Membrane polarity jumps to +30 mV

Steps in Generating an Action Potential - Third Step

Repolarization:  Na channels are inactivating, and voltage-gated K channels open ▪

• Na channel inactivation gates close–

  • Membrane permeability to Na declines to resting state–

  • AP spike stops rising ▪

• Voltage-gated K channels open–

  • K exits cell down its electrochemical gradient ▪

• Repolarization: membrane returns to resting membrane potential

Steps in Generating an Action Potential - Fourth Step

Hyperpolarization: Some K channels remain open, and  Na channels reset ▪

• Some K channels remain open, allowing excessive K efflux–

  • Inside of membrane becomes more negative than in resting state ▪

• This causes hyperpolarization of the membrane (slight dip below resting voltage) ▪

• Na channels also begin to reset

Not all depolarization events produce APs

For an axon to “fire,” depolarization must reach threshold voltage to trigger AP

AP threshold

Membrane is depolarized by 15 to 20 mV

Na permeability increases–

Na influx exceeds K efflux

The positive feedback cycle begins

AP either happens completely, or does not happen at all

All-or-none phenomenon

allows AP to be transmitted from origin down entire axon length toward terminals

Propagation

time in which neuron cannot trigger another AP

Refractory period:

Absolute refractory period

Time from opening of Na channels until resetting of the channels

Ensures that each AP is an all-or-none event–

Enforces one-way transmission of APs

Relative refractory period

Follows absolute refractory period ▪

Most Na channels returned to their resting state, some K channels still open ▪

Repolarization is occurring–

Threshold for AP generation is elevated–Only exceptionally strong stimulus can stimulate an AP

Rate of AP propagation depends on two factors:

1. Axon diameter

▪Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction

2. Degree of myelination

▪Two types of conduction depending on presence or absence of myelin

APs can propagate in two ways:

Nonmyelinated axons: slower conduction, called continuous conduction, that occurs in nonmyelinated axons

Myelinated axons: faster conduction, called saltatory conduction, occurs only in myelinated axons and is about 30times faster

Axons (nerve fibers) are classified according to diameter, degree of myelination, and speed of conduction

A fiber, B fiber, C fiber

A Fiber

Largest diameter, heavily myelinated axons

Transmit at 150 m/s (over 300 mph)

B Fiber

Intermediate diameter, lightly myelinated axons

Transmit at  15 m/s ( 30 mph)

C Fiber

Smallest diameter, nonmyelinated axons ▪ Transmit at  1 m/s ( 2 mph) or less

junctions that mediate information transfer

From one neuron to another neuron

Or from one neuron to an effector cell

Synapses

neuron conducting impulses toward synapse (sends information)

Presynaptic neuron:

neuron transmitting electrical signal away from synapse (receives information)–In PNS may be a neuron, muscle cell, or gland cell

Postsynaptic neuron:

Types of Synaptic Connections

Axodendritic synapses: between axon terminals of one neuron and dendrites of others

Axosomatic synpases: between axon terminals of one neuron and soma (cell body) of others

Two main types of synapses

Electrical synapses

Chemical synapses

Electrical Synapse

Neurons are electrically coupled

Joined by gap junctions that connect cytoplasm of adjacent neurons–Communication is very rapid and may be unidirectional or bidirectional–

Found in some brain regions responsible for eye movements or hippocampus in areas involved in emotions and memory–

Most abundant in embryonic nervous tissue

Specialized for release and reception of chemical messengers

Neurotransmitters

Chemical Synapse composed of

Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter

Receptor region on postsynaptic neuron’s membrane: receives neurotransmitter

Usually on dendrite or cell body

Two parts separated by fluid-filled synaptic cleft

Transmission across synaptic cleft

Synaptic cleft prevents nerve impulses from directly passing from one neuron to next

Chemical event (as opposed to an electrical one)

Depends on release, diffusion, and receptor binding of neurotransmitters

Ensures unidirectional communication between neurons

Chemical Synapse Information Transfer Steps

1. AP arrives at axon terminal of presynaptic neuron

2. Voltage-gated 2 Ca channelsopen, and 2 Ca enters axon terminal

3. Ca entry causes synaptic vesicles to release neurotransmitter by exocytosis

4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane

5. Binding of neurotransmitter opens ion channels, creating graded potentials

6. Neurotransmitter effects are terminated

Acetylcholine(ACh)

Released at neuromuscular junctions

Synthesized from acetic acid and choline by enzyme choline acetyltransferase

Degraded by enzyme acetylcholinesterase (AChE)

Catecholamines - Biogenic Amines

Dopamine, norepinephrine (NE), and epinephrine: made from the amino acid tyrosine

Indolamines - Biogenic Amines

▪Serotonin: made from the amino acid tryptophan

Histamine: made from the amino acid histidine

Amino Acids

Amino acids make up all proteins: therefore, it is difficult to prove which are neurotransmitters

Amino acids that are proven neurotransmitters ▪

Glutamate ▪Aspartate ▪Glycine ▪Gamma-( )aminobutyric  acid (GABA

Peptides

Strings of amino acids that have diverse functions ▪

Substance P–Mediator of pain signals

▪Endorphins (beta endorphin, dynorphin, and enkephalins)–Act as natural opiates; reduce pain perception ▪

Gut-brain peptides–Somatostatin and cholecystokinin (CCK) play role in regulating digestion

Purines

Monomers of nucleic acids that have an effect in both CNS and PNS ▪

Adenosine triphosphate (ATP), the energy molecule, is now considered a neurotransmitter ▪

Adenosine, a part of ATP, is a potent inhibitor in brain

Caffeine blocks adenosine receptors ▪

Can induce 2 Ca influx in astrocytes

Gases and Lipids

Gasotransmitters ▪

Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide ( ) 2 HS gases

Central nervous system (CNS) consists of

brain and spinal cord

Adult brains have four regions:

1. Cerebral Hemispheres

2. Diencephalon

3. Brain stem

   1. midbrain

   2. pons

   3. medulla oblongata

4. Cerebellum

short, nonmyelinated neurons and cell bodies

gray matter

myelinated and nonmyelinated axons

white matter

Basic pattern found in CNS:

central cavity surrounded by gray matter, with white matter external to gray matter