Neural Circuits and Reflex Arcs

Functional Classification of Neurons

• Motor or efferent neurons carry impulses away from the CNS to effector organs
• Most efferent neurons are multipolar
• The cell bodies of motor neurons are within the CNS
• Motor neurons form junctions with effector cells
• Interneurons (or association neurons) are multipolar
• Interneurons lie between afferent and efferent neurons
• Interneurons are confined to the CNS

Reflex Arc Components

• Receptors (in dermis)
• Afferent or sensory neuron
• Integrating center (interneuron- spinal cord)
• Efferent or motor neuron
• Effector organ (muscle or gland)

Types of Neurons (Based on Function)

• Somatic sensory neuron
• Somatic motor neuron
• Parasympathetic motor neuron
• Sympathetic trunk ganglion

Cutaneous Receptors (Exterioreceptors)

• Cutaneous receptors are nerve endings (dendrites/sensory axons)
• They convert chemical, mechanical, and light stimuli into electrical signals
• Cutaneous receptors are located in the dermis, the wall of organs including skeletal muscles, the wall of blood vessels, and joint surfaces (proprioceptors)

Interoreceptors (Visceroreceptors)

• Cells in taste buds have receptors for salt (Na+), sour (H+), sweet (CHO), and bitter (amino acids and alkaloids)
• The sense of taste is also known as gustation
• Taste cells or gustatory cells give rise to nerve fibers that carry taste sensations to the brain
• The retina of the eye has photoreceptors, which are stimulated by light and generate electrical signals
• These photoreceptor signals are transmitted by the optic nerve to the brain

Interoreceptors (Internal Ear)

• One group of hair cells in the internal ear is stimulated in response to the movement of the head and generates electrical signals
• The other group of cells is stimulated by sound vibrations entering the external acoustic meatus and generates electrical signals
• These cells are known as mechanoreceptors generating electrical signals in response to mechanical stimuli

Interoreceptors (Stretch Receptors)

• Stretch receptors are located in the wall of the gastrointestinal and urinary systems (e.g., stomach and urinary bladder)
• These receptors are stimulated when organs are distended, sending signals of fullness to the CNS

Proprioceptors (Mechanoreceptors)

• Proprioceptors are sensors providing information about joint angle, muscle length, and muscle tension
• This information is integrated to give information about the position of the body in space

Receptors and Neural Pathways

• Receptors include interoceptors, exteroceptors and proprioceptors
• The peripheral nervous system (PNS) has afferent fibers, efferent fibers and sensory neurons
• The central nervous system (CNS) has somatic motor neurons and interneurons

Resting Membrane Potential

• The resting membrane potential is defined as the electrical potential difference across the plasma membrane in a non-excited state
• A neuron at rest is negatively charged, with the inside of the cell approximately 70 millivolts more negative than the outside (-70 mV)
• The resting membrane potential is caused by differences in the concentrations of ions inside and outside the cell
• If the membrane were equally permeable to all ions, each type of ion would flow across the membrane until equilibrium is reached
• The resting membrane potential results from different concentrations inside and outside the cell
• The number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential
• Extracellular Fluid (ECF): Na+ = 142 mEq/L, K+ = 4 mEq/L, Cl- = 103 mEq/L, Ca++ = 2.4 mEq/L, PO4 = 4, Proteins = 5
• Intracellular Fluid (ICF): Na+ = 10 mEq/L, K+ = 140 mEq/L, Cl = 4 mEq/L, Ca++ = 0.0001 mEq/L, PO4 = 75, Proteins = 40, SO4 = 2
• The resting membrane potential arises from:
  • Unequal distribution of ions in the ECF and cytosol (ICF) due to the selective permeability of the cell membrane
  • Inability of most anions to leave the cell
  • Electrogenic nature of the Na+/K+ ATP ases
• The Resting Membrane Potential is usually negative on the interior
• Skeletal muscle is -90 mV
• Neurons are closer to -70 mV
• The negative sign denotes that the cytoplasm is electrically negative to the ECF

Action Potential

• An action potential is a momentary change in electrical potential on the surface of a cell (nerve or muscle)
• It occurs on application of a threshold stimulus, resulting in an electrical impulse along a nerve fiber or excitable cell transmission
• Action potentials are initiated in the nerve cell body and action potential is initiated at the initial segment of the axon (axon hillock) and then propagated along the nerve

Depolarization and Repolarization

• Depolarization: The movement of a cell's membrane potential to a more positive value
• Repolarization: The change in membrane potential from a positive to a negative value

Types of Synapses

• A synapse is the junctional region between two neurons, neurons and muscle, or neurons and glands where an electrical impulse (action potential) is transmitted from one excitable cell (neuron) to another (neuron/muscle/gland)
  • Anatomical classification
    • Axo-somatic
    • Axo-dendritic
    • Axo-axonic
  • Type of Transmission
    • Chemical synapse
    • Electrical synapse
• In electrical synapses, cytoplasmic connections exist between adjacent cells through gap junctions
• Action potential (flow of ions) spreads directly from one cell to another, eliminating the need for chemicals for synaptic transmission
• Synaptic delay is minimal

Neuromuscular Junction

• The neuromuscular junction is the junction between the axonal terminal and the muscle fiber membrane; it is a chemical synapse
  • Anatomy
    • The axonal terminal that stimulates the muscle fiber is called the presynaptic terminal
    • The muscle fiber membrane is called the postsynaptic terminal
    • A gap between the presynaptic and postsynaptic terminals is known as the synaptic cleft
  • Presynaptic Terminal
    • Consists of voltage-gated Ca2+ channels and vesicles containing neurotransmitters
  • Postsynaptic Terminal (Motor End Plate)
    • The highly-excitable region of muscle fiber's plasma membrane
    • Immediately adjacent to the presynaptic axon terminal
    • Terminals have junctional folds with high density of cholinergic receptors where neurotransmitters (acetylcholine) from the presynaptic terminal bind
    • Cholinergic receptors act as ligand-gated Na+ channels
    • The membrane also has voltage-gated Na+ channels

Neuromuscular Transmission

• The process of conversion of a nerve action potential arriving at the presynaptic terminal into a muscle action potential at the postsynaptic terminal

Applied Physiology

• Diseases of the Neuromuscular Junction: Myasthenia gravis (post-synaptic disorder)
• Neuromuscular Junction Blockers:
  • Non-depolarizing blockers (Tubocurarine)
  • Depolarizing blockers (succinylcholine)