NERVOUS SYSTEM

Maintenance of homeostasisMajor regulatory and control center

FUNCTIONS

Sensing changes (INPUT) Processing information (INTEGRATING) Responding (OUTPUT) SEAT OF ALL MENTAL ACTIVITY

• CONSCIOUSNESS• MEMORY• THINKING

Divisions of the Nervous System

Central Nervous SystemPeripheral Nervous System

Central Nervous System

Involves the brain and spinal cord Part of the dorsal body cavity Contains gray and white matter, and

has fluid filled spaces

Peripheral Nervous System

Cranial and Spinal nerves Afferent Nervous System - input Efferent Nervous System - output

•somatic (voluntary) NS• autonomic (involuntary) NS

•sympathetic and parasympathetic divisions

Cells of the Nervous System

Neuroglial CellsNeurons

Neuroglial Cells Found in CNS

•Astrocytes•Star-shaped; most

abundant; anchor to blood vessels

•Microglia – •Protect

Neuroglial Cells Found in CNS

•Oligodendrocytes•Produce insulating

myelin sheath in the CNS

•Ependymal •Line cavities•Circulate

cerebrospinal fluid

Neuroglial Cells

Found in PNS•Schwann cell

•Form myelin sheath in PNS cells

•Satellite cell•Surround cell•Function relatively unknown

Neuron Structure

Cell body (Soma) Processes

•Dendrites •Receiving branches

•Axon•Sending branch

Classifications of Neurons

StructureFunction

Area served

Neurons by structure

Pseudounipolar (unipolar) – one process

Bipolar – one dendrite & one axon Multipolar

•3 or more processes; most common

•1 axon and 2 or more dendrites

Neurons by function

Afferent neurons - sensory neurons Efferent neurons - motor or secretory

neurons Association neurons - interneurons or

intermediate neurons

Neurons by the area served

Visceral neurons •serving the internal organs•visceral afferent, visceral efferent

Somatic neurons •serving the body wall•somatic afferent, somatic efferent

Neuron Characteristics

Excitable Conductive Transmits

information Cannot reproduce

Regeneration • cell body intact• presence of

neurolemma sheath• regeneration tunnel

aligned• very, little scar

tissue

Nerves

Bundles of nerve cell processes in the PNS

Link the PNS to the CNS Sensory, motor, or mixed Connective tissue

• epineurium, perineurium, endoneurium Tract - bundles of nerve fibers in the

CNS

White / Gray Matter

White matter contains myelinated processes• PNS - myelinated nerves• CNS - myelinated tracts

Gray matter- consists of cell bodies and unmyelinated processes• Nuclei- collection of cell bodies (CNS)• Ganglia - cell bodies outside CNS

Neurophysiology

Basic electrical principles• Voltage – measures potential difference

between to charges• Resistance – hindrance to charge flow• Insulators – resist charge flow• Conductors – allow charge flow• Ohm's law: Current = Voltage/Resistance• Ions – charged atoms flow across membranes

Ion channels

Passive channels – always open Active (gated) channels.

• Chemically-gates channels – open when combined with appropriate neurotransmitter

• Voltage-gates channels – open in response to changes in membrane potential

Establishing the resting membrane potential.

The neuron membrane is positively charged on the outside and negativelycharged on the inside.

Inequality of charged particles occurs only at membrane. The total numberof positive and negative ions and molecules inside and outside the cell are equal.

Distribution of charged particles

Sodium ions (Na+) Potassium ions (K+) Chloride ions (Cl-) Negatively charged

proteins (A-)

Factors that help establish the RMP.

Differences in the resting permeability of the plasma membrane to Na+ and K+ ions –

ATPase, Na+ ion/K+ ion pump

Impermeability of the plasma membrane to proteins –

Membrane potentials that act as signals

Communication in neurons and muscle cells involves changing the membrane potential.

Factors that change membrane potential.

Changing the permeability of the plasma membrane to any ion.

Changing the concentration of ions across the plasma membrane.

Depolarization - membrane potential decreases

Hyperpolarization – membrane potential increases

Graded membrane potential.

Short-lived depolarization or hyperpolarization of the plasma membrane.

Caused by opening of gated ion channels in the plasma membrane.

Magnitude of the change in potential is directly related to intensity of the stimulus.

Action potential

Rapid reversal of membrane potential.

Occurs only in neurons and muscle cells.

Also called a nerve impulse in a neuron.

Only axons can generate an action potential.

Steps in generating an action potential.

Resting membrane potential. • Voltage-gated Na+ channels closed. • Voltage-gated K+ channels closed.

Depolarization. • Opening voltage-gated Na+ channel in the

axon. • Influx of Na+ ions results in depolarization of

the axonal membrane. • Closing of Na+ channels stops the influx of

Na+ ions.

Repolarization.

• Open voltage-gated K+ channel in the axon.

• Efflux of K+ ions results in repolarization of the axonal membrane.

• Membrane potential moves back to RMP.

• Na+/K+ ATP pump helps re-establish the Na+ and K+ ion concentrations inside and outside the neuron.

Propagation of the action potential.

An action potential travels away from its point of origin.

An action potential is self-propagating. (domino effect)• Changes in membrane potential in one

section stimulates depolarization in next section of the membrane

Threshold and the all-or-none phenomenon -

All action potentials are the same regardless of the strength of stimulus.

Strong stimuli lead to more action potentials during a time frame.

Weak stimuli lead to fewer action potentials during a time frame.

Refractory period.

Absolute refractory period - • Time period in which no impulse can

be generated Relative refractory period –

• Time period in which only a strong stimulus will generate an impulse

Conduction velocities of axons -

Speeds up to 100 m/s or more Influenced by

• Axon diameter: larger = faster• Myelinated versus unmyelinated axons

•Saltatory Conduction = impulse leaps from one node of Ranvier to the next

Synapse = junctions between neurons

Axodendritic – axon to dendrite Axosomatic - axon to cell body Axoaxonic - axon to axon Dendrodendritic – dendrite to dendrite Presynaptic neurons – conducts to the

synapse Postsynaptic neuron – conducts away

from the synapse

Synapses

Electrical synapse - direct connections allow current to flow

from one cell to the next

Chemical synapse – use chemical neurotransmitters to

conduct impulses across the synapse

Parts of a chemical synapse

Presynaptic neuron's axon terminal contains synaptic vesicles containing a neurotransmitter.

Synaptic clefts - Fluid-filled space between the pre- and postsynaptic neurons.

Postsynaptic neuron has receptors for neurotransmitters released from the synaptic vesicle.

Information flow across a chemical synapse.

Action potential opens calcium channels in the presynaptic membrane.

Synaptic vesicles fuse with the membrane of the axon terminal and neurotransmitter is released into the synaptic cleft.

Neurotransmitter binds with receptors on the postsynaptic membrane.

Ions channels open leading to the depolarization or hyperpolarization of the postsynaptic membrane.

Termination of the neurotransmitter effect.

Neurotransmitter degraded by an enzyme.

Neurotransmitter taken up by the presynaptic terminal.

Diffusion of the neurotransmitter from the synaptic cleft.

Postsynaptic potentials. Excitatory postsynaptic potential (EPSP).

• Causes depolarization of postsynaptic

membrane.• Opens channels allowing Na+ and K+ ions to

cross the membrane. Inhibitory postsynaptic potential (IPSP).

• Causes hyperpolarization of postsynaptic membrane.

• Opens K+ or chloride channels allowing one of both of these ions to cross the membrane.

Modification of synaptic events.

Summation = effects add up • Temporal summation – rapid

stimulation• Spatial summation – stimulation from

mutiple presynaptic terminals Synaptic potentials – continued use

increases ability to excite the postsynaptic membrane

Neurotransmitters - Acetylcholine - Biogenic amines.

•Catecholamines.•Dopamine -•Norepinephrine -•Epinephrine -•Indolamines.•Serotonin -•Histamine -

Neurotransmitters Amino acids.

• Gamma amino butyric acid -

• Glutamate -• Glycine -• Aspartate -

Peptides.• Substance P -• Endorphins -• Enkephalins -

Novel messengers • ATP• NO• CO

Neurotransmitters by function.

Excitatory - cause depolarization

Inhibitory – cause hyperpolarization

SENSORY SYSTEMS

Levels of Sensation

Sensation - is the arrival of a sensory impulse to the brain

Perception - is the interpretation of the sensation

Sensory Receptors

Simple receptors - General senses Complex receptors - Special senses Selectivity Types by location

• Exteroceptors - outside• Interoceptors – visceral (more general)• Proprioceptors – musculoskeletal (more

specific

Receptors classified by stimulus

Mechanoreceptor – touch, pressure, vibration, etc.

Photoreceptor - light Thermoceptor - temperature Chemoreceptor - chemicals Nociceptor – damage / pain

Cutaneous Sensation

Tactile sensations • touch • pressure• vibration• cold, heat• pain

Crude / Discriminative Touch

Types of Tactile Receptors

Meissner’s corpuscles – light touch Hair root plexuses – light touch Merkel discs – light touch Pacinian corpuscles – deep pressure Itch/tickle Thermoreceptors - heat Nociceptors - pain