Neurophysiology

Basic Structure and Function of Neurons

Neurons are specialized cells of the nervous system responsible for transmitting electrical and chemical signals. Understanding their structure and function is essential for studying neurophysiology.

• Neuron Parts: Dendrites (receive signals), cell body (soma), axon (transmits signals), axon terminals (release neurotransmitters).

• Signal Transmission: Signals travel from dendrites to axon terminals, passing through the cell body.

• Example: Sensory neurons transmit information from sensory organs to the central nervous system.

Ion Channels and Action Potentials

Ion channels are proteins in the neuronal membrane that allow ions to pass, generating electrical signals known as action potentials.

• Stimuli and Ion Channels: Various stimuli (chemical, electrical, mechanical) can open ion channels, allowing ions such as Na+ and K+ to flow.

• Depolarization: When Na+ channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: K+ channels open, K+ exits the cell, restoring the negative membrane potential.

• Threshold, Subthreshold, and Suprathreshold:

  • Threshold: The minimum stimulus needed to trigger an action potential.

  • Subthreshold: Stimulus too weak to trigger an action potential.

  • Suprathreshold: Stimulus stronger than threshold, ensuring action potential generation.

• Graded vs. Action Potentials:

  • Graded potentials: Vary in size, occur in dendrites and cell body.

  • Action potentials: All-or-none, occur in axons.

• Refractory Period: Time after an action potential when a neuron cannot fire another action potential.

• Directionality: Action potentials travel in one direction due to refractory periods.

• Equation: Nernst equation for equilibrium potential:

Synaptic Transmission

Neurons communicate at synapses, where neurotransmitters are released to transmit signals to other neurons or effector cells.

• Events at the Synapse:

  1. Action potential arrives at axon terminal.

  2. Calcium channels open, Ca2+ enters.

  3. Neurotransmitter vesicles fuse with membrane, releasing neurotransmitter.

  4. Neurotransmitter binds to receptors on post-synaptic cell.

  5. Signal is terminated by reuptake, degradation, or diffusion.

• EPSP vs. IPSP:

  • EPSP (Excitatory Post-Synaptic Potential): Depolarizes the post-synaptic membrane, increasing likelihood of action potential.

  • IPSP (Inhibitory Post-Synaptic Potential): Hyperpolarizes the membrane, decreasing likelihood of action potential.

• Ion Channel Effects:

  • Opening Na+ channels: EPSP

  • Opening Cl- channels: IPSP

• Temporal and Spatial Summation:

  • Temporal: Multiple signals from one neuron over time.

  • Spatial: Signals from multiple neurons at the same time.

Neurotransmitters and Receptors

Neurotransmitters are chemical messengers released at synapses. Their effects depend on the type of receptor they bind to.

• Major Neurotransmitters: Acetylcholine, dopamine, serotonin, norepinephrine, GABA, glutamate.

• Agonists vs. Antagonists:

  • Agonists: Activate receptors.

  • Antagonists: Block receptors.

• Example: Acetylcholine is the main neurotransmitter at neuromuscular junctions.

Organization of the Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

• CNS: Brain and spinal cord.

• PNS: All neural tissue outside the CNS, including sensory and motor neurons.

• Example: Sensory neurons in the PNS transmit signals to the CNS for processing.

Muscle Physiology

Types of Muscle Tissue

Muscle tissue is classified into three types: skeletal, cardiac, and smooth muscle.

• Skeletal Muscle: Voluntary, striated, attached to bones.

• Cardiac Muscle: Involuntary, striated, found in the heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of organs.

• Example: Skeletal muscles enable movement; cardiac muscle pumps blood.

Muscle Structure and Contraction

Muscle contraction is driven by the sliding filament theory, involving actin and myosin filaments.

• Hierarchy of Structure:

  • Myofibril → Muscle fiber → Fascicle → Muscle

• Sarcomere: The functional unit of muscle contraction, composed of thick (myosin) and thin (actin) filaments.

• Steps in Contraction:

  1. Action potential triggers Ca2+ release.

  2. Ca2+ binds to troponin, moving tropomyosin and exposing binding sites.

  3. Myosin binds to actin, forming cross-bridges.

  4. ATP hydrolysis powers the sliding of filaments.

• Equation: Muscle force: (where m is mass, a is acceleration)

Types of Skeletal Muscle Fibers

Skeletal muscle fibers are classified based on contraction speed and metabolic properties.

• Type I (Slow-twitch): High endurance, oxidative metabolism, resistant to fatigue.

• Type II (Fast-twitch): Rapid contraction, glycolytic metabolism, fatigues quickly.

• Comparison Table:

Neuromuscular Junction and Synaptic Transmission

The neuromuscular junction is a specialized synapse where motor neurons communicate with muscle fibers to initiate contraction.

• Neurotransmitter: Acetylcholine is released from motor neuron terminals.

• Receptor: Nicotinic acetylcholine receptors on muscle fiber membrane.

• Signal Termination: Acetylcholine is degraded by acetylcholinesterase.

Muscle Sensory Structures

Muscle spindles and Golgi tendon organs are sensory receptors that monitor muscle stretch and tension.

• Muscle Spindle: Detects changes in muscle length.

• Golgi Tendon Organ: Detects changes in muscle tension.

• Example: Muscle spindle reflex helps maintain posture.

Additional info:

• Some questions referenced sensory pathways, anatomical structures (e.g., cochlea, retina), and autonomic nervous system divisions. These are included in context where relevant.

• For equations, the Nernst equation and muscle force equation were added for academic completeness.