Week 3 Neuro Physiology Part I (full slide set) pdf

Critical Thinking Questions

• Question 1: Solutions A and B are separated by a selectively permeable barrier. Over time, the level of fluid on side A increases. Which solution initially had the higher concentration of solute?
  • Answer: Solution A originally had more solute, causing water to move across the membrane to side A.
• Question 2: A molecule blocking ion channels in integral plasma membrane proteins would interfere with?
  • Answer: (c) Producing changes in the electrical charges across a plasma membrane.

Neurophysiology

• Part - Dr. Susan Prendergast PHD NP

Learning Outcomes

1. Explain how the resting membrane potential is established, maintained, and changed.
2. Describe action potential generation, propagation, and factors determining propagation speed.
3. Describe synapse structure and synaptic activity mechanisms.
4. Describe major neurotransmitter and neuromodulator types and their postsynaptic membrane effects.
5. Discuss neuronal pool significance and neuron interaction patterns within pools.
6. Describe neural reflex steps and classify reflex types.
7. Distinguish motor responses produced by reflexes and explain how reflexes interact to produce complex behaviors.
8. Explain how higher brain centers control and modify reflex responses.

Information Processing

• Integrates, processes, and coordinates sensory input and motor commands.
• Central Nervous System (CNS): Brain and spinal cord.
• Peripheral Nervous System (PNS): Nervous tissue outside the CNS and the ENS.
  • Sensory information within the afferent division.
  • Motor commands within the efferent division.
    • Somatic nervous system (SNS): Skeletal Muscle
    • Autonomic nervous system (ANS):
      • Parasympathetic division
      • Sympathetic division
  • Receptors:
    • Special sensory receptors monitor smell, taste, vision, balance, and hearing.
    • Visceral sensory receptors monitor internal organs.
    • Somatic sensory receptors monitor skeletal muscles, joints, and skin surface.
  • Effectors:
    • Skeletal muscle
    • Smooth muscle
    • Cardiac muscle
    • Glands
    • Adipose tissue

Neuron Structure

• Four general regions:
  • Dendrites
  • Cell body (Perikaryon)
  • Axon
  • Telodendria

Neuron Anatomy

• Nissl bodies: RER and free ribosomes.
• Axolemma: Axon membrane.
• Axon hillock: Where the axon originates from the cell body.
• Initial segment of axon
• Telodendria: Branches at the end of the axon.
• Axon Terminals
• Synaptic Cleft: Space between the presynaptic and postsynaptic cells

Neurons - Functional Classifications

1. Sensory neurons
2. Motor neurons
3. Interneurons

Functional Types of Neurons

• Sensory Neuron (Unipolar):
  • Receptor cell
  • Cell Body
  • Axon
• Interneuron (Multipolar):
  • Cell Body
  • Dendrite
  • Axon
• Motor Neuron (Multipolar):
  • Cell Body
  • Axon
  • Myelin Sheath
  • Neurofibril node (node of Ranvier)

Sensory Neurons

• Unipolar
• Cell bodies grouped in sensory ganglia
• Processes (afferent fibers) extend from sensory receptors to CNS
  • Somatic sensory neurons: Monitor external environment
  • Visceral sensory neurons: Monitor internal environment

Types of Sensory Receptors

• Interoceptors: Monitor internal systems (e.g., digestive, urinary), internal senses (stretch, deep pressure, pain).
• Exteroceptors: Monitor external environment (e.g., temperature), complex senses (e.g., sight, smell, hearing).
• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Motor Neurons

• Carry instructions from CNS to peripheral effectors via efferent fibers (axons).
  • Somatic motor neurons of SNS: Innervate skeletal muscles.
  • Visceral motor neurons of ANS: Innervate: smooth and cardiac muscle, glands, adipose tissue.
• Signals from CNS to visceral effectors cross autonomic ganglia that divide axons into:
  • Preganglionic fibers
  • Postganglionic fibers

Interneurons

• Located mostly in the brain and spinal cord, and some in autonomic ganglia.
• Positioned between sensory and motor neurons.
• Responsible for:
  • Distribution of sensory information
  • Coordination of motor activity
• Involved in higher functions:
  • Memory, planning, learning

Neuroglia

• Support and protect neurons.
• Make up half the volume of the nervous system.
• Many types in CNS and PNS:
  • Astrocytes
  • Ependymal cells
  • Oligodendrocytes
  • Microglia

Neuroglia in CNS

• Astrocytes:
  • Maintain blood-brain barrier
  • Provide structural support
  • Regulate ion, nutrient, and dissolved gas concentrations
  • Absorb and recycle neurotransmitters
  • Form scar tissue after injury
• Ependymal cells:
  • Line ventricles (brain) and central canal (spinal cord)
  • Assist in producing, circulating, and monitoring cerebrospinal fluid
• Oligodendrocytes:
  • Myelinate CNS axons
  • Provide structural framework
• Microglia:
  • Remove cell debris, wastes, and pathogens by phagocytosis

Oligodendrocytes & Astrocytes

• Oligodendrocytes:
  • Small cell bodies with few processes
  • Many cooperate to form a myelin sheath
    • Myelin insulates myelinated axons and increases the speed of action potentials; makes nerves appear white
    • Internodes—myelinated segments of axon
    • Nodes (nodes of Ranvier) lie between internodes where axons may branch
    • White matter: Regions of CNS with many myelinated axons
    • Gray matter of CNS: Contains unmyelinated axons, neuron cell bodies, and dendrites
• Astrocytes:
  • Large cell bodies with many processes
    • Maintain blood-brain barrier (BBB)
    • Create three-dimensional framework for CNS
    • Repair damaged nervous tissue
    • Guide neuron development
    • Control interstitial environment

Ependymal Cell & Microglia

• Ependymal cells:
  • Form epithelium that lines central canal of spinal cord and ventricles of brain
  • Produce and monitor cerebrospinal fluid (CSF)
  • Have cilia that help circulate CSF
• Microglia:
  • Smallest and least numerous neuroglia
  • Have many fine-branched processes
  • Migrate through nervous tissue
  • Clean up cellular debris, wastes, and pathogens

Neuroglia in PNS

• Insulate neuronal cell bodies and most axons.
• Two Types:
  • Satellite cells
  • Schwann cells
• Satellite cells: Surround ganglia (clusters of neuronal cell bodies) and regulate interstitial fluid around neurons.
• Schwann cells (neurolemmocytes): Form myelin sheath or indented folds of plasma membrane around axons.
  • Neurolemma—outer surface of Schwann cell
  • A myelinating Schwann cell sheaths only one axon
  • Many Schwann cells sheath entire axon

Neuroglia in Peripheral Nervous System

• Satellite cells: Surround neuron cell bodies in ganglia; regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia.
• Schwann cells: Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury.

Myelinated Axon

• Schwann cell nucleus
• Myelin covering internode
• Neurolemma
• Axon
• Axolemma

Unmyelinated Axons

• Enclosed by Schwann cells, but not myelinated
• Multiple axons can be enclosed by a single Schwann cell

Membrane Potential

• Living cells have a membrane potential that varies from moment to moment depending on activities.
• Resting membrane potential:
  • The membrane potential of a resting cell.
  • Difference between + and – ions on either side of the membrane.
  • Inside of cell is negative relative to outside of cell (ICF is -70mV).
• Graded potential:
  • Temporary, localized change in resting potential.
  • Caused by a stimulus.
• Action potential:
  • Is an electrical impulse.
  • Produced by graded potential.
  • Propagates along surface of axon to synapse.

Equilibrium Potential

• Membrane potential at which there is no net movement of a particular ion across the cell membrane.
  • K+ = –90 mV
  • Na+ = +66 mV
• Plasma membrane is highly permeable to K+, which explains the similarity of equilibrium potential for K+ and resting membrane potential (–70 mV).
• Resting membrane’s permeability to Na+ is very low; Na+ has a small effect on resting potential.

Resting Membrane Potential

• Three important concepts:

  1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition.
     • Extracellular fluid contains high concentrations of Na^+ and Cl^-.
     • Cytosol contains high concentrations of K^+ and negatively charged proteins.
  2. Cells have selectively permeable membranes.
  3. Membrane permeability varies by ion.

Resting Membrane Potential & Ion Channels

• Na^+ and K^+ are the primary determinants of membrane potential, using either passive or active channels.
  • Passive ion channels (leak channels): Always open; permeability changes with conditions.
  • Active ion channels (gated ion channels): Open and close in response to stimuli; at resting membrane potential, most are closed.

Processes That Produce the Resting Membrane Potential

• Passive Chemical Gradients
  • The intracellular concentration of potassium ions (K^+) is relatively high, so these ions tend to move out of the cell through potassium leak channels.
  • Similarly, the extracellular concentration of sodium ions (Na^+) is relatively high, so these ions move into the cell through sodium leak channels.
  • Each ion’s movement is driven by a concentration gradient, or chemical gradient.
• Active Na+/K+ Pumps
  • Sodium–potassium (Na^+/K^+) exchange pumps maintain the concentration gradients of sodium and potassium ions across the plasma membrane.
• KEY
  • Protein
  • Sodium ion (Na^+)
  • Potassium ion (K^+)
  • Chloride ion (Cl^−)
  • Na^+ leak channel
  • K^+ leak channel
  • Sodium– potassium exchange pump

Passive Electrical Gradients & Resting Membrane Potential

• Potassium ions leave the cytosol more rapidly than sodium ions enter because the plasma membrane is much more permeable to potassium than to sodium.
• As a result, there are more positive charges outside the plasma membrane.
• Negatively charged protein molecules within the cytosol cannot cross the plasma membrane, so there are more negative charges on the cytosol side of the plasma membrane. This results in an electrical gradient across the plasma membrane.
• Whenever positive and negative ions are held apart, a potential difference arises. We measure the size of that potential difference in millivolts (mV).
• The resting membrane potential for most neurons is about –70 mV. The minus sign shows that the inner surface of the plasma membrane is negatively charged with respect to the exterior.

Passive Processes

• Acting across cell membrane:
  • Chemical gradients: Concentration gradients of ions (Na^+, K^+).
  • Electrical gradients: Charges are separated by cell membrane; cytosol is negative relative to extracellular fluid.
  • Electrochemical gradient: Sum of chemical and electrical forces acting on an ion across the membrane and is a form of potential energy.

Electrochemical Gradients for Potassium and Sodium Ions

• Potassium Ion Gradients: At a neuron’s resting membrane potential, the chemical and electrical gradients are opposed for potassium ions (K^+).
  • The net electrochemical gradient tends to force potassium ions out of the cell.
  • If the plasma membrane were freely permeable to potassium ions, the outflow of K^+ would continue until the equilibrium potential (–90 mV) was reached. Note how similar it is to the resting membrane potential.
• Sodium Ion Gradients: At a neuron’s resting membrane potential, the chemical and electrical gradients for sodium ions (Na^+) are combined.
  • The net electrochemical gradient forces sodium ions into the cell.
  • If the plasma membrane were freely permeable to sodium ions, the inflow of Na^+ would continue until the equilibrium potential (+66 mV) was reached. Note how different it is from the resting membrane potential.

Active Processes – N^+ K^+ Pump

• Active processes across the membrane use the sodium–potassium exchange pump, powered by ATP.

• Ejects 3 Na^+ for every 2 K^+ brought in, balancing passive forces of diffusion and stabilizing resting membrane potential (–70 mV).

  • When ratio of Na^+ entry to K^+ loss through passive channels is 3:2

• Resting membrane potential = – 70 mV

Active Processes - Active Channels

1. Chemically gated ion channels
2. Voltage-gated ion channels
3. Mechanically gated ion channels

Chemically Gated Ion Channels

• Also called ligand-gated ion channels.
• Open when they bind specific chemicals (e.g., ACh).
• Found on cell body and dendrites of neurons.

Voltage Gated Ion Channels

• Respond to changes in membrane potential.
• Found in axons of neurons and sarcolemma of skeletal and cardiac muscle cells.
• Activation gate opens when stimulated.
• Inactivation gate closes to stop ion movement.
• Three possible states:
  • Closed but capable of opening
  • Open (activated)
  • Closed and incapable of opening (inactivated)

Mechanically Gated Ion Channels

• Respond to membrane distortion.
• Found in sensory receptors that respond to touch, pressure, or vibration.

Graded vs Action

• Graded potential:
  • Temporary, localized change in resting potential.
  • Caused by a stimulus that often triggers cell functions.
    • Exocytosis of glandular secretions
• Action potential:
  • Brief, rapid, large (100mv) change in membrane potential.
  • Needed for neurons to conduct impulse.
  • Produced by graded potential.

Graded Potentials

• Graded potentials (local potentials) are changes in membrane potential that cannot spread far from the site of stimulation.

• Produced by any stimulus that opens gated channels; for example, a resting membrane is exposed to a chemical:

  • Chemically gated Na^+ channels open
  • Sodium ions enter cell, causing the membrane potential to rise (depolarization).
  • Sodium ions move parallel to plasma membrane, producing local current, which depolarizes nearby regions of plasma membrane (graded potential).
  • Change in potential is proportional to stimulus.

Characteristics of Graded Potentials

• Membrane potential is most changed at the site of stimulation; the effect decreases with distance.

• The effect spreads passively due to local currents.

• Graded change in membrane potential may involve depolarization or hyperpolarization.

• Stronger stimuli produce greater changes in membrane potential and affect a larger area.

• Often trigger specific cell functions; for example, exocytosis of glandular secretions.

• ACh causes graded potential at the motor endplate at the neuromuscular junction.

Graded Potentials

• Repolarization: When the stimulus is removed, membrane potential returns to normal.

• Hyperpolarization: Results from opening potassium ion channels where positive ions move out, not into the cell, which is the opposite effect of opening sodium ion channels; increases the negativity of the resting potential.

Depolarization, Repolarization, and Hyperpolarization

• Chemical stimulus applied causes depolarization
• Chemical stimulus removed begins repolarization to resting membrane potential.
• Opening potassium channels causes hyperpolarization and the return to resting membrane potential.

Action Potentials

• Propagated changes in membrane potential.
• Affect an entire excitable membrane.
• Begin at the initial segment of the axon.
• Do not diminish as they move away from the source.
• Stimulated by a graded potential that depolarizes the axolemma to threshold.
• Threshold for an axon is –60 to –55 mV.

Action Potentials and Ion Channels

• Action potentials are propagated changes in the membrane potential that, once initiated, affect an entire excitable membrane. Action potentials depend on voltage-gated sodium and potassium ion channels.

All-or-None Principle

• Any stimulus that changes the membrane potential to threshold will cause an action potential.
• All action potentials are the same, no matter how large the stimulus.
• An action potential is either triggered or not triggered.

Generation of Action Potentials

• Step 1: Depolarization to threshold.
• Step 2: Activation of voltage-gated Na^+ channels.
  • Na^+ rushes into cytosol, and the inner membrane surface changes from negative to positive, resulting in rapid depolarization.
• Step 3: Inactivation of Na^+ channels and activation of K^+ channels.
  • At +30 mV, inactivation gates of voltage-gated Na^+ channels close.
  • Voltage-gated K^+ channels open, and K^+ moves out of cytosol, starting repolarization.
• Step 4: Return to resting membrane potential.
  • Voltage-gated K^+ channels begin to close as membrane reaches normal resting potential.
  • K^+ continues to leave cell, causing the membrane to briefly hyperpolarize to –90 mV.
  • After all voltage-gated K^+ channels finish closing, the resting membrane potential is restored, and the action potential is over.

Refractory Period

• From the beginning of action potential to return to resting state, during which the membrane will not respond normally to additional stimuli.
• Absolute refractory period: All voltage-gated Na^+ channels are already open or inactivated, so the membrane cannot respond to further stimulation.
• Relative refractory period: Begins when Na^+ channels regain resting condition and continues until membrane potential stabilizes; only a strong stimulus can initiate another action potential.

Action Potential: Ion Movement and Restoration

• Depolarization results from influx of Na^+.
• Repolarization involves the loss of K^+.
• The sodium–potassium exchange pump returns concentrations to pre-stimulation levels and maintains concentration gradients of Na^+ and K^+ over time.
• Uses one ATP for each exchange of two extracellular K^+ for three intracellular Na^+.

Propagation

• Moves an action potential along an axon in a series of steps.
• Types of propagation:
  • Continuous propagation
  • Saltatory propagation

Continuous Propagation of AP

• Occurs in unmyelinated axons.
• Affects one segment of an axon at a time.
• Step 1: Action potential develops at the initial segment, depolarizing membrane to +30 mV.
• Step 2: Local current develops, depolarizing the second segment to threshold.
• Step 3: Action potential occurs in the second segment as the initial segment begins repolarization.
• Step 4: Local current depolarizes the next segment, and the cycle repeats.
• Action potential travels in one direction (1 m/sec).

Saltatory Propagation of AP

• Occurs in myelinated axons.
• Faster than continuous propagation and requires less energy.
• Myelin prevents continuous propagation.
• Local current “jumps” from node to node, so depolarization occurs only at nodes.

Axon Diameter and Propagation Speed

• Axon diameter affects propagation speed of action potentials:
  • The larger the diameter, the lower the resistance and faster the speed.
• Types of axons based on diameter, myelination, and propagation speed:
  • Type A fibers: Myelinated, large diameter, transmit information rapidly
  • Type B fibers: Myelinated, medium diameter, transmit information at intermediate speeds
  • Type C fibers: Unmyelinated, small diameter, transmit information slowly
• Messages carried by nerves are routed according to priority; critical information is transmitted through Type A fibers (e.g., sensory information about survival threats, motor commands to prevent injury).

Fiber Types & Info Transmission

• Type A Fibers:
  • Myelinated
  • Large diameter
  • Transmit information to and from CNS rapidly (120 m/sec); examples:
    • Sensory information such as position and balance
    • Motor impulses to skeletal muscles
• Type B Fibers:
  • Myelinated
  • Medium diameter
  • Transmit information at intermediate speeds (18 m/sec)
• Type C Fibers:
  • Unmyelinated
  • Small diameter
  • Transmit information slowly (1 m/sec); example: most sensory information

Synapse

• Specialized site where a neuron communicates with another cell.
• Presynaptic neuron: Sends the message.
• Postsynaptic neuron: Receives the message.
• Types of synapses:
  • Electrical synapses: Direct physical contact between cells.
  • Chemical synapses: Signal transmitted across a gap by neurotransmitters.

Chemical Synapses - Types & Function

• Types of chemical synapses:
  • Neuromuscular junction: Synapse between neuron and skeletal muscle cell.
  • Neuroglandular junction: Synapse between neuron and gland cell.
• Function of chemical synapses:
  • The axon terminal releases neurotransmitters that bind to the postsynaptic plasma membrane, producing localized change in permeability and graded potentials.
  • The action potential may or may not be generated in the postsynaptic cell, depending on the amount of neurotransmitter released and the sensitivity of the postsynaptic cell.

Cholinergic Synapses

• Release acetylcholine (ACh) at:
  • All neuromuscular junctions involving skeletal muscle fibers
  • Many synapses in CNS
  • All neuron-to-neuron synapses in PNS
  • All neuromuscular and neuroglandular junctions in the parasympathetic division of ANS

Cholinergic Synapse - Events

• Action potential arrives at the axon terminal and depolarizes the membrane.
• Extracellular calcium ions enter the axon terminal and trigger exocytosis of ACh.
• ACh binds to receptors on postsynaptic membrane and depolarizes it.
• ACh is removed from the synaptic cleft by acetylcholinesterase (AChE).
• AChE breaks ACh into acetate and choline.

Steps in Impulse Transmission Across A Synapse

1. In axon terminal, AP opens Ca^{2+} channels in synaptic knob
2. Ca^{2+} causes release of neurotransmitter from synaptic vessels to synaptic cleft
3. Neurotransmitter diffuses across cleft and binds to receptor sites on sub-synaptic membrane
   • Excitatory synapse causes membrane to be more excitable & AP will occur
   • Inhibitory synapse causes membrane to be less excitable & AP will not occur

Neurotransmitters

• Chemical messengers are contained within synaptic vesicles in the axon terminal of the presynaptic cell.

• Released into the synaptic cleft.

• Affect receptors of postsynaptic membrane.

• Broken down by enzymes.

• Reabsorbed and reassembled by the axon terminal.

Neurotransmitters - Classes

• Excitatory neurotransmitters:
  • Cause depolarization of postsynaptic membranes
  • Promote action potentials
• Inhibitory neurotransmitters:
  • Cause hyperpolarization of postsynaptic membranes
  • Suppress action potentials
• After use, they may be broken down by enzymes, taken up by receptors on pre-synapse (re-uptake), or diffuse across.

Effect of Neurotransmitters

• The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the receptor, not on the nature of the neurotransmitter.
• Major classes of neurotransmitters include:
  • Biogenic amines
  • Amino acids
  • Neuropeptides
  • Dissolved gases

Biogenic Amines

• Norepinephrine (NE):
  • Released by adrenergic synapses
  • Excitatory and depolarizing effect
  • Widely distributed in the brain and portions of ANS
• Dopamine:
  • A CNS neurotransmitter
  • May be excitatory or inhibitory
  • Involved in Parkinson’s disease and cocaine use
• Serotonin:
  • A CNS neurotransmitter
  • Affects attention and emotional states
• Gamma-aminobutyric acid (GABA):
  • Inhibitory effect
  • Functions in CNS are not well understood

Neuromodulators

• Chemicals released by axon terminals that alter:
  • Rate of neurotransmitter release
  • Or response by postsynaptic cell
• Effects are long term and slow to appear; responses involve multiple steps and intermediary compounds.
• Affect presynaptic membrane, postsynaptic membrane, or both.
• Released alone or with a neurotransmitter.

Neuromodulators - Types

• Neuropeptides: Small peptide chains synthesized and released by axon terminal
• Opioids: Bind to the same receptors as opium and morphine
• Classes of opioids in CNS:
  • Enkephalins
  • Endorphins
  • Dynorphins
• Dissolved gases: Are important neurotransmitters
  • Nitric oxide (NO)
  • Carbon monoxide (CO)

Neurotransmitters & Modulators

• Have:
  • A direct effect on membrane potential by opening or closing chemically gated ion channels
    • ACh, glutamate, aspartate
  • An indirect effect through G proteins
    • E, NE, dopamine, serotonin, histamine, GABA
  • An indirect effect via intracellular enzymes
    • Lipid-soluble gases (NO, CO)

Indirect Effect

• G protein links first messenger (neurotransmitter) and second messengers (ions or molecules in cell).

• G proteins include an enzyme that is activated when an extracellular compound binds; for example, adenylate cyclase, which produces the second messenger cyclic-AMP (cAMP).

Information Processing

• Response of postsynaptic cell (integration of stimuli) at simplest level (individual neurons).
• Many dendrites receive neurotransmitter messages simultaneously; some excitatory, some inhibitory.
• The net effect on the axon hillock determines if an action potential is produced.

Postsynaptic Potentials

• Graded potentials developed in a postsynaptic cell in response to neurotransmitters.
• Types of postsynaptic potentials:
  • Excitatory postsynaptic potential (EPSP): Graded depolarization of postsynaptic membrane.
  • Inhibitory postsynaptic potential (IPSP): Graded hyperpolarization of postsynaptic membrane.

Information Processing - Action Potential Trigger

• A neuron that receives many IPSPs is inhibited from producing an action potential because the stimulation needed to reach threshold is increased.

• To trigger an action potential, one EPSP is not enough; EPSPs (and IPSPs) combine through summation (temporal and spatial).

Temporal Summation

• Occurs on a membrane that receives two depolarizing stimuli from the same source in rapid succession.

• The effects of the second stimulus are added to those of the first.

Spatial Summation

• Occurs when two stimuli arrive at the same time but at different locations.

• Local currents spread the depolarizing effects, and areas of overlap experience the combined effects.

Information Processing - Summation and its influence

• A neuron becomes facilitated as EPSPs accumulate and raise membrane potential closer to threshold until a small stimulus can trigger an action potential.

• Summation of EPSPs and IPSPs via neuromodulators and hormones can change membrane sensitivity to neurotransmitters, shifting the balance between EPSPs and IPSPs.

Information Processing - Information Conveyance

• Information may be conveyed simply by the frequency of action potentials received.
• Depends on degree of depolarization above threshold.
• Holding membrane potential above threshold has the same effect as a second, large stimulus.
• The maximum rate of action potentials is reached when the relative refractory period is eliminated.

Axoaxonic Synapses

• Synapses between axons of two neurons.
• Presynaptic inhibition: Decreases the rate of neurotransmitter release at presynaptic membrane.
• Presynaptic facilitation: Increases the rate of neurotransmitter release at presynaptic membrane.

Information Processing Summary

• Information is relayed in the form of action potentials.
• Neurotransmitters released at a synapse may have excitatory or inhibitory effects.
• Neuromodulators can alter the rate of neurotransmitter release or response of a postsynaptic neuron.
• Neurons may be facilitated or inhibited by chemicals other than neurotransmitters or neuromodulators.
• The response of the postsynaptic neuron can be altered by neuromodulators or other chemicals that cause facilitation or inhibition, activity underway at other synapses, and modification of the rate of neurotransmitter release through facilitation or inhibition.

Neuronal Pools

• Three Groups of Neurons:
  • Sensory Neurons: About 10 million; bring information to CNS (incoming)
  • Motor Neurons: Approx 0.5 million that send commands out from CNS (outgoing)
  • Interneurons: Approx 20 billion located in the CNS to interpret, plan, and coordinate signals coming in and going out.

Interneurons - Organization

• Organized into functional groups of interconnected neurons, each with limited input sources and output destinations.
• May stimulate or depress parts of the brain or spinal cord.
• Five patterns of neural circuits in neuronal pools:
  1. Divergence
  2. Convergence
  3. Serial processing
  4. Parallel processing
  5. Reverberation

Patterns of Neural Circuits in Neuronal Pools

• Divergence: Spreads information from one neuron or neuronal pool to many; especially common in sensory pathways.

• Convergence: Several neurons synapse on a single neuron; for example, subconscious and conscious control of the diaphragm in breathing where two neuronal pools synapse with the same motor neurons.

• Serial processing: Information moves along a single path sequentially from one neuron or neuronal pool to the next; for example, pain signals pass sequentially through two neuronal pools to reach the conscious brain.

• Parallel processing: Several neurons/neuronal pools process the same information at the same time; for example, stepping on a bee signals spread through several neuronal pools so you can shift your weight, lift your foot, and yell in pain at about the same time.

• Reverberation: Collateral branches of neurons extend back and continue stimulating presynaptic neurons, forming a positive feedback loop that continues until synaptic fatigue or inhibition occurs; examples may maintain consciousness, breathing, and muscle coordination.

Reflexes Classification

• Four ways to classify reflexes:
  1. Development
  2. Response
  3. Complexity of the neural circuit
  4. Site of information processing

Reflexes - Classification

• Development:
  • Innate reflexes: Genetically determined
  • Acquired reflexes: learned
• Response:
  • Somatic reflexes: Control skeletal muscle contractions; include superficial and stretch reflexes
  • Visceral reflexes: Control actions of smooth and cardiac muscles and glands
• Complexity of Circuit:
  • Monosynaptic: One synapse
  • Polysynaptic: Multiple synapses (two to several hundred)
• Processing Site:
  • Spinal reflexes: Processing in the spinal cord
  • Cranial reflexes: Processing in the brain

Development of Reflexes

• Innate reflexes are basic neural reflexes formed before birth; genetically programmed (inborn); examples: withdrawal, chewing, visual tracking.

• Acquired reflexes are rapid, automatic learned motor patterns; repetition enhances them; examples: braking a car in an emergency.

Response - Reflexes Types

• Somatic: Control skeletal muscle contractions
  • Superficial reflexes –stimuli in skin/mucous membranes
  • Stretch or deep tendon reflexes (such as patellar, or “knee-jerk,” reflex)
  • Immediate—important in emergencies (slipping, cutting finger)
• Visceral (autonomic): Control other effectors