Neurons and Nervous Systems

Neurons and Nervous Systems

• Neural signaling involves the process by which an animal responds to a stimulus.
• Neurons perform neural signaling.
• Specialized sensory receptors carry out reception.
• Transmission is the sending of a message among neurons or to a muscle or gland.
• Integration involves sorting/interpreting neural messages to determine a response.
• The response to a stimulus is the output or action resulting from neural message integration.

Neural Signaling Classes

• Afferent (sensory) neurons transmit stimuli from sensory receptors to interneurons.
• Interneurons integrate information to formulate an appropriate response.
• Efferent neurons carry signals indicating a response away from the interneuron to the effectors (muscles and glands).
• Motor neurons are efferent neurons that carry signals to skeletal muscle.

Neuron Structure

• Dendrites receive signals and transmit them toward the cell body.
• The cell body contains the nucleus and most organelles.
• The axon conducts signals away from the cell body to another neuron or effector.
• The axon may be from ~1 mm to more than 1 m long.
• The axon hillock is the site of origin of the axon.
• The axon terminal connects a neuron functionally with an adjacent neuron or effector.

Glial Cells

• Glial cells are non-neuronal cells that provide nutrition and support to neurons.
• Astrocytes in the vertebrate CNS closely cover blood vessels and maintain concentrations of ions in the interstitial fluid.
• Oligodendrocytes in the CNS and Schwann cells in the PNS wrap axons to form insulating myelin sheaths.
• Nodes of Ranvier are gaps between Schwann cells that speed signal transmission.

Neural Circuits

• A typical neural circuit contains an afferent (sensory) neuron, one or more interneurons, and an efferent neuron.
• Circuits combine into networks to interconnect the parts of the nervous system.
• In vertebrates, afferent and efferent neurons form the peripheral nervous system (PNS).
• Interneurons form the central nervous system (CNS), consisting of the brain and spinal cord.

Ganglia Versus Nuclei

• Ganglia and nuclei are nerve cell clusters that typically perform related functions.
• Ganglia contain a number of cell bodies in the peripheral nervous system.
• Nuclei contain a number of cell bodies in the central nervous system.
• Ganglia form plexuses.
• Nuclei occur in the gray matter of the brain.
• Dorsal root, autonomic, and cranial nerve ganglia are examples.
• Caudate, putamen, dentate, emboliform, pallidum, substantia nigra, and subthalamic nuclei are examples.

Membrane Potential

• Potential difference exists across every cell's plasma membrane.
• Membrane potential indicates that a separation of positive and negative charges exists across the plasma membrane, this creates an electrical potential (voltage).

Resting Potential

• A resting potential exists when a neuron is not being stimulated.
• Resting potential ranges from -40 to -90 mV but it averages about -70 mV.

Electrochemical Potential

• Electrical potential produced by unequal distribution of charges & the concentration gradient produced by unequal concentrations of molecules.

Depolarization

• When a stimulus causes positive charges to flow inward, making the cytoplasmic side less negative, creating an action potential.
• Depolarization happens slowly until it reaches threshold potential, typically 10-20 mV more positive than resting potential.

Action Potential

• When a neuron transmits an electrical impulse, an abrupt change in membrane potential occurs during this process.

Voltage-Gated Na+ and K+ Channels

• Voltage-gated Na+ and K+ channels open and close when a neuron is stimulated, resulting in membrane potential changes.
• Axons and axon hillocks have voltage-gated channels mainly

Action Potential Refractory Period

• Threshold is reached, the membrane potential increases.
• The plasma membrane inside becomes positive because of positive ions influx.
• It reduces and drops below a resting value of -80 mV and rises to the resting potential.
• The membrane enters a refractory period which has 2 phases.
• The cell cannot be restimulated in the initial phase.
• The threshold required for generating an action potential is high during the 2nd phase.

All-or-Nothing Principle

• If the stimulus causes a depolarization and can reach the threshold it triggers changes independently of stimuli strength.

Action Potential Propagation

• Initiated at the dendrite end of the neuron, then travels away from the stimulation point as a wave of depolarization along the cell surface.
• In the axon, local current flow between the area of action potential and the inactive area depolarizes the downstream membrane to the threshold.
• The refractory period ensures one-way movement.

Saltatory Conduction

• In complex vertebrates, action potentials hop rapidly along myelin-coated axons.
• Uncoated nodes of Ranvier expose the axon membrane to extracellular fluids at regular intervals.
• Voltage-gated Na+ and K+ channels are at nodes which allow action potentials to develop.
• Na+ ions diffuse rapidly to the next node and cause depolarization so it activates an action potential.

Chemical Synapses

• Action potentials don't jump across the cleft in a chemical synapse.
• Action potential arrival causes neurotransmitter molecules to be released by the axon terminal plasma membrane.
• The neurotransmitter diffuses across the cleft and alters ion conduction by activating ligand-gated ion channels in the postsynaptic membrane.

Synapses

• A synapse is neurons connecting through direct electrical flow or through a chemical-based reaction using neurotransmitters.
• The dendrite or effector of cell receives the incoming signal at postsynaptic cells.
• The neuron transmits the signal on one side of the synapse.

Electrical Synapses

• Plasma membranes are in direct contact with each other and contain unregulated signal conduction.
• Electrical Synapses are found in cardiac/eye muscle, and teeth.

Chemical Synapses

• The presynaptic-postsynaptic cells create a narrow synaptic cleft.
• A neurotransmitter is released in the synaptic cleft.
• Neurotransmitter diffuses and binds to the postsynaptic cell membrane receptor.

Neurotransmitter Release

• Neurotransmitters are stored in synaptic vesicles in the cytoplasm of an axon terminal.
• Action potential releases neurotransmitters through exocytosis.
• Synaptic vesicles depend on voltage-gated calcium channels (Ca2+).
• Rise in Ca2+ concentration in the axon triggers a protein in the Synaptic vesicle membrane.
• Ca2+ is pumped outside.
• Cytoplasmic Ca2+ stops fusing w/presynaptic membrane and stops being released.

Neurotransmitters Reception

• Neurotransmitters open or close ion channels that conduct Na+, K+, or Cl- across the postsynaptic membrane.
• Altered ion flow in postsynaptic membranes stimulate or inhibit action potentials.
• Stimulatory & inhibitory neurotransmitters at chemical synapses can cause or prevent triggering potential.

Neurotransmitter Receptors

• Ionotropic receptors include a binding site & channel combined.
• Ionotropic receptors are independent of a secondary messenger.
• Ionotropic receptors use short latency action.
• Ionotropic receptors use rapid responses
• Ionotropic receptors: Post synaptic
• Metabotropic receptors use slow responses.
• Metabotropic receptors: Pre- and postsynaptic
• Nearly 100 substances are known neurotransmitters, with some axon terminals releasing one type and other releasing several types.
• The same neurotransmitter may stimulate or inhibit the generation of action potentials in the postsynaptic cell.

Integration of Incoming Signals

• Neurons receive many stimulatory and inhibitory signals via neurotransmitters.
• These signals are integrated by the postsynaptic neuron.
• Integration occurs based on varying patterns, number, types, & synapse activity.
• Inputs from other sources & signal molecules change integration.
• Excitatory postsynaptic potential (EPSP): A change that pushes the neuron closer to threshold.
• Inhibitory postsynaptic potential (IPSP): The change pushes a neuron off the threshold w/a channel that allows Cl- to flow in.

EPSP/ISPS Features

• EPSPs and IPSPs are graded potentials which change the membrane.
• Changes don't always result in triggering an action potential.