SUSAN CAVAN, EMILY GEHRET AND HANNAH HOWINGTON

The Nervous System

INTRODUCTION Nervous system helps

coordinate body functions to maintain homeostasis

Enables body to respond to changing conditions

Nerve cells are called neurons- specialized to react to physical and chemical changes in surroundings

Nerve impulses are electrochemical changes that transmit info between neurons

TYPICAL NEURON MOTOR NEURON

NEURONS Neurons consist of:

-Cell body- rounded area with nucleus-Dendrites- extensions that receive electrochemical messages-Axons- extensions that send info in nerve impulses- axon hillock- conical region of axon where it joins the cell body- Schwann cell- a glial or supporting cell that forms myelin sheaths around the axons of neurons in the PNS - myelin sheath- fatty lipoprotein protection of axon that provides electrical insulation - neurons with myelin sheath= white matter- neurons without myelin= gray matter- nodes of Ranvier- gaps between adjacent Schwann cells

PNS AND CNS Organs can be split into two

groups:- Central nervous system (CNS)- brain and spinal cord-Peripheral nervous system (PNS)- nerves that connect the CNS to other body parts

Together they provide 3 general functions: sensory, integrative, motor

NEURON GROUPS Sensory neurons:

-Carry nerve impulses from PNS to CNS-Either receptor ends at tips of dendrites or dendrites closely associated with receptor cells in skin or sensory organs-Changes inside or outside body stimulate receptors; trigger sensory nerve impulses- impulses travel along axons, leading to one of CNS organs; other neurons process the impulses- internal and external environment ex) light, sound intensity, temperature, oxygen levels

For example, this man’s sensory neurons allow him to feel the heat of the stove

NEURON GROUPS Interneurons

- lie in the brain and spinal cord- link other neurons- transmit impulses from one part of brain/spinal cord to another; direct incoming sensory impulses

Motor Neurons- carry nerve impulses out of CNS to effectors- effectors- responsive structures that include muscles that contract and glands that secrete- motor impulses stimulate the effectors

This man touched the hot stove. His sensory neurons sent an impulse to the CNS, which sent a responding impulse to his motor neurons and effectors, in this case his muscles pulling his hand back.

CELL MEMBRANE POTENTIAL Resting potential

- it is the potential difference (or difference in electrical charge) between region inside membrane and region outside membrane in neurons not transmitting signals- depends on the ionic gradients; net movement of K+ and Na+ ions from higher to lower concentration- in a resting cell membrane K+ ions diffuse out of the cell more rapidly than Na+ ions can diffuse in ; outside gains “+” surplus and inside gains “-” surplus- meanwhile, Na+/K+ pumps actively transport ions in opposite directions to maintain this concentration gradient

CELL MEMBRANE POTENTIAL Potential changes:

- nerve cells are excitable; respond to changes in environment- changes affect resting potential in a particular region of nerve cell membrane- depolarization- membrane’s resting potential decreases (inside less negative than outside); opening of gated Na+ channels and K+ channels remain closed- Changes in resting potential are graded: change in potential is directly proportional to the intensity of the stimulations-threshold potential- summation of depolarizations to a certain membrane voltage, after which an action potential occurs…

CELL MEMBRANE POTENTIAL Action Potential:

- rapid sequence of depolarization and repolarization- at threshold potential, channels in membrane open and Na+ ions begin to diffuse freely inward and membrane loses its negative charge; becomes depolarized- then, the Na+ channels close and channels open that allow K+ ions to pass out, and inside becomes negatively charged again; it is repolarized and membrane returns to resting potential- this process forms the nerve impulse that is propagated along the axon

Action potential animation

NERVE IMPULSES Nerve impulse- movement of action potentials along

a nerve axon Local current: when action potential occurs in one

region of nerve cell membrane, causes bioelectric current to flow to adjacent portions;

stimulates to threshold level and trigger another action potential= chain reaction!

Wave of action potentials moves down the axon

IMPULSE CONDUCTION An unmyelinated axon conducts an impulse over its

entire surface Myelinated axon is different b/c myelin insulates and

prevents almost all ion flow through the membrane it encloses

Therefore it is not continuous; nodes interrupt the sheath and action potentials occur at these nodes; so impulse appears to jump from node to node

Termed saltatory and is much faster than unmyelinated Speed of nerve impulse conduction is proportional to

diameter of axon; greater diameter=faster impulse

NERVE IMPULSES

IMPULSE CONDUCTION Nerve impulse conduction is an “all-or-none-response” If neuron responds at all, it responds completely A greater intensity of stimulation does not produce a

stronger impulse, but rather more impulses per second Refractory period- short time following a nerve impulse

when a threshold stimulus will not trigger another impulse on an axon; limits the frequency of impulses

Calcium’s role: - too little Ca + = spasms - too much Ca+ = neuron is hard to fire, impulse hard to trigger

THE SYNAPSE How the impulse passes from neuron to neuron Synapse- junction between any two communicating neurons; not in

direct physical contact (axon to dendrite, or axon to cell body)- electrical synapse- gap junctions that allow current to flow directly from cell to cell; synchronize activity of neurons for certain rapid, stereotypical behaviors- chemical synapse- majority are this; release of chemical neurotransmitters

Synaptic cleft- gap between neurons; communication along a nerve pathway must cross these gaps

Presynaptic neuron- the sender; the neuron carrying the impulse into the synapse

Postsynaptic neuron- the receiver; receives the impulse Synaptic transmission- process of crossing the synaptic gap Neurotransmitters- biochemicals that carry out synaptic

transmission; it diffuses across the synaptic cleft and reacts with specific receptors on the postsynaptic neuron membrane

SYNAPSESynaptic knob- extension from axon

Synaptic vesicle- membranous sac that releases neurotransmitters

postsynaptic

THE SYNAPSE: THE BASIC PROCESS When an action potential reaches a synaptic knob

(terminal), it depolarizes the membrane, opening voltage-gated calcium channels.

Calcium ions (Ca 2+ ) then diffuse into the knob, and the increase in ion concentration causes some of the synaptic vesicles to fuse with the membrane

The vesicles then release their neurotransmitters, which diffuse across the synaptic cleft; subsequent affect on postsynaptic neuron may be:

Direct synaptic transmission- neurotransmitters bind to ligand-gated ion channels in membrane of postsynaptic cell; result is change in membrane potential

Indirect synaptic transmission- neurotransmitter binds to a receptor that is not part of an ion channel; activates a signal transduction pathway; slower but effects last longer

THE SYNAPSE Excitatory neurotransmitters- increase postsynaptic

membrane permeability to sodium ions which will bring it closer to the threshold and may trigger nerve impulses

Inhibitory neurotransmitters- decrease the potential that threshold will be reached; less chance that nerve impulse will occur

Synaptic knobs of 1000 or more neurons could potentially interact with one postsynaptic neuron; some knobs will release excitatory, some inhibitory; effect on the neuron depends on the ratio of each

synapse animation

NEUROTRANSMITTERS

Type/Name Functional Class Secretion SitesAcetylcholine Excitatory to vertebrate

skeletal muscles; either at other sites

CNS; PNS; vertebrate neuromuscular junction

Biogenic Amines: Dopamine Serotonin

Generally excitatoryReleased in brain and affects sleep, mood, attention, learning

Amino Acids GABA Glutamate

Inhibitoryexcitatory

At most inhibitory synapses in brainCNS

Neuropeptides Substance P

Met-enkephalin (an endorphin)

Excitatory

Generally inhibitory

CNS; PNS; mediates our perception of painDecreases pain perception

KNEE-JERK REFLEX