Neural Physiology and Receptors

Receptor Types

Neural receptors are specialized proteins that respond to specific stimuli, initiating changes in membrane potential.

• Chemically-gated receptors: Open in response to ligands such as acetylcholine (ACh).

• Voltage-gated receptors: Respond to changes in membrane potential.

• Mechanically-gated receptors: Activated by physical stimuli like pressure or vibration.

Exam Tip: Be careful not to confuse the types; questions may switch their definitions.

Glial Cells: CNS vs PNS

Types and Functions

Glial cells support and protect neurons. Their functions differ between the central and peripheral nervous systems.

• Astrocytes (CNS): Maintain the blood-brain barrier and regulate nutrient flow.

• Oligodendrocytes (CNS): Produce myelin sheaths in the CNS.

• Schwann cells (PNS): Produce myelin sheaths in the PNS.

• Microglia (CNS): Act as phagocytes, removing debris and pathogens.

• Ependymal cells (CNS): Line ventricles and produce cerebrospinal fluid (CSF).

Common Misconceptions:

• Schwann cells are not found in the CNS.

• Astrocytes do not produce myelin.

Neuron Structure and Function

Basic Anatomy

Neurons are the functional units of the nervous system, specialized for communication.

• Dendrites: Receive incoming signals.

• Axon: Sends outgoing signals to other cells.

Exam Trap: Dendrites do not send signals; axons do not receive signals.

Membrane Potential and Action Potentials

Phases of Membrane Potential

Neurons communicate via changes in membrane potential, leading to action potentials.

• Polarized: Resting state; inside is negative relative to outside.

• Depolarization: Sodium ions (Na+) enter, making the inside less negative.

• Repolarization: Potassium ions (K+) exit, restoring negativity.

• Hyperpolarization: Membrane becomes more negative than resting.

Trap: Hyperpolarization means more negative, not less.

Propagation of Action Potentials

Action potentials travel along axons by two main mechanisms:

• Continuous propagation: Occurs in unmyelinated axons; slower.

• Saltatory propagation: Occurs in myelinated axons; faster, as the impulse 'jumps' between nodes.

Trap: Saltatory propagation is not for unmyelinated axons.

Muscle Structure and Function

Muscle Cell Anatomy

Muscle fibers have specialized structures for contraction.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Muscle cell cytoplasm.

• Sarcomere: Functional contractile unit.

• Z line: Boundary of each sarcomere.

Myofibril Anatomy

Myofibrils are composed of repeating sarcomeres, which contain thick and thin filaments.

• A band: Contains thick filaments (myosin).

• I band: Contains thin filaments (actin).

• H zone: Region with only thick filaments.

During contraction: Sarcomeres shorten, but filaments do not shrink.

Filaments and Muscle Proteins

Muscle contraction depends on the interaction of several proteins:

• Actin: Thin filament.

• Myosin: Thick filament.

• Troponin: Binds calcium ions (Ca2+).

• Tropomyosin: Blocks myosin binding sites on actin.

• Titin: Prevents overstretching of the sarcomere.

Common Misconceptions:

• Calcium binds troponin, not tropomyosin.

• Myosin is a thick filament, not thin.

Steps of Muscle Contraction

Muscle contraction is a multi-step process:

1. ACh (acetylcholine) is released at the neuromuscular junction.

2. Depolarization of the sarcolemma occurs.

3. Signal travels down the sarcolemma and T-tubules.

4. Calcium ions are released from the sarcoplasmic reticulum (SR).

5. Calcium binds to troponin.

6. Tropomyosin moves, exposing binding sites.

7. Myosin binds to actin.

8. Power stroke occurs.

9. Sarcomere shortens.

Motor Units and Muscle Fiber Types

Motor Unit

A motor unit consists of one neuron and all the muscle fibers it innervates. All fibers contract together when the neuron fires.

• Force increases by recruitment: More motor units activated = greater force.

Muscle Fiber Types

Muscle fibers are classified by their arrangement, affecting force and function.

• Parallel: Long fibers, less force.

• Convergent: Fan-shaped arrangement.

• Bipennate: High force; example: rectus femoris.

• Circular: Sphincter muscles.

Exam Tip: Rectus femoris is bipennate.

Muscle Movement Terms

Roles in Movement

Muscles work together to produce and control movement.

• Agonist: Main mover.

• Antagonist: Opposes the agonist.

• Synergist: Assists the agonist.

• Fixator: Stabilizes the origin.

Smooth Muscle Characteristics

Structure and Function

Smooth muscle is found in internal organs and blood vessels.

• Non-striated

• Involuntary

• Slow contractions

• Location: Organs and vessels

ATP Production in Muscle

Energy Pathways

Muscle cells produce ATP through three main pathways:

• Creatine Phosphate: Provides immediate energy for about 15 seconds.

• Glycolysis (Anaerobic): Occurs in the cytoplasm, does not require oxygen, produces 2 ATP and lactic acid, uses glycogen.

• Aerobic Metabolism: Occurs in mitochondria, requires oxygen, produces about 34 ATP, generates CO2 and H2O.

Trap: Aerobic metabolism occurs in mitochondria, not cytoplasm.

Summary Table: Muscle Energy Pathways

Common Exam Traps and Always-True Facts

Frequently Tested Misconceptions

• Aerobic metabolism does not occur in the cytoplasm.

• Dendrites do not send signals.

• Myosin is not a thin filament.

• Calcium binds troponin, not tropomyosin.

• Hyperpolarization is more negative, not less.

• Saltatory propagation is not for unmyelinated axons.

Always-True Exam Facts

• Sarcomeres shorten during contraction.

• Calcium is released from the sarcoplasmic reticulum.

• ACh is released by exocytosis.

• Grey matter consists of cell bodies.

• White matter consists of axons.

• All fibers in a motor unit contract together.

Key Equations and Concepts

ATP Yield from Glucose

• Glycolysis (Anaerobic): per glucose

• Aerobic Metabolism: per glucose

Membrane Potential Changes

• Depolarization:

• Repolarization:

• Hyperpolarization:

Additional info: Expanded explanations and context were added to clarify brief points and exam traps, ensuring completeness and academic quality.