Cellular Physiology: Neuron and Muscle Structure & Function

Cellular Physiology of the Neuron and Muscle

Overview of Synapses

Synapses are specialized junctions that facilitate communication between neurons or between neurons and effector cells. They play a critical role in the transmission of signals within the nervous system.

• Definition: A synapse is a site where a neuron communicates with another cell (neuron or effector).

• Types of Synapses:

  • Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

  • Electrical Synapses: Use gap junctions to allow direct electrical communication between cells.

• Presynaptic vs. Postsynaptic Neuron: The presynaptic neuron sends the signal, while the postsynaptic neuron receives it.

• Synaptic Cleft: The fluid-filled space (30-50 nm wide) separating the presynaptic and postsynaptic membranes.

Chemical Synapses: Structure and Function

Chemical synapses are the most common type in the nervous system and are specialized for the release and binding of neurotransmitters.

• Components:

  • Axon Terminal: Contains synaptic vesicles filled with neurotransmitter.

  • Receptor Region: Located on the postsynaptic membrane, often on dendrites or cell body.

• Function: Neurotransmitters released from the presynaptic neuron bind to receptors on the postsynaptic neuron, opening or closing ion channels and influencing membrane permeability and potential.

Information Transfer Across Chemical Synapses

The process of signal transmission at chemical synapses involves several steps:

1. An action potential arrives at the axon terminal of the presynaptic neuron.

2. Voltage-gated Ca2+ channels open, allowing Ca2+ to enter the terminal.

3. Ca2+ influx triggers neurotransmitter release into the synaptic cleft.

4. Neurotransmitter diffuses across the cleft and binds to postsynaptic receptors.

5. Binding opens ion channels, generating graded potentials in the postsynaptic cell.

Termination of Neurotransmitter Effects

Neurotransmitter action must be terminated to prevent continuous stimulation. This occurs by:

• Reuptake by astrocytes or the presynaptic terminal

• Degradation by enzymes

• Diffusion away from the synaptic cleft

Synaptic Delay

Synaptic delay refers to the time required for neurotransmitter release, diffusion, and receptor binding. This delay (0.3–5.0 ms) is the rate-limiting step in neural transmission, making synaptic transmission slower than action potential propagation along axons.

Electrical Synapses

Electrical synapses are less common and involve direct cytoplasmic connections via gap junctions, allowing rapid, bidirectional or unidirectional communication. They are found in regions of the brain responsible for stereotyped movements, in the hippocampus, and are most abundant in embryonic nervous tissue.

Synaptic Potentials

Postsynaptic Potentials: EPSP and IPSP

Neurotransmitter receptors generate graded potentials in the postsynaptic neuron, which vary in strength based on neurotransmitter amount and duration in the cleft.

• EPSP (Excitatory Postsynaptic Potential):

  • Neurotransmitter binding opens chemically gated channels for Na+ and K+.

  • Na+ influx exceeds K+ efflux, causing local depolarization.

  • If strong enough, EPSPs can trigger an action potential at the axon hillock.

• IPSP (Inhibitory Postsynaptic Potential):

  • Neurotransmitter binding opens channels for K+ (out) or Cl- (in), causing hyperpolarization.

  • Moves the membrane potential further from threshold, reducing the likelihood of an action potential.

Integration and Modification of Synaptic Events

Postsynaptic neurons integrate multiple inputs. A single EPSP cannot induce an action potential, but EPSPs and IPSPs can summate:

• Temporal Summation: Multiple impulses from one neuron in rapid succession add together.

• Spatial Summation: Simultaneous stimulation by multiple neurons; EPSPs from different locations add together.

• Action potential is generated only if EPSPs predominate and reach threshold.

Physiology of Muscle

Overview of Muscle Tissue

Muscle tissue comprises nearly half of the body's mass and is specialized for converting chemical energy into directed mechanical work, producing force and movement.

• Muscle Fiber: Elongated cells referred to as muscle fibers.

• Functions:

  • Produce movement

  • Maintain posture and body position

  • Stabilize joints

  • Generate heat (e.g., shivering)

  • Control organ volume, form valves, regulate pupil size

Functional Characteristics of Muscular Tissue

• Excitability (Responsiveness): Ability to receive and respond to stimuli, usually chemical (neurotransmitter, hormone, pH).

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to resting length after being stretched.

Muscle Fiber Microanatomy

Skeletal muscle fibers are large, cylindrical cells with multiple oval nuclei. Their structure is specialized for contraction.

• Cell Membrane: Called the sarcolemma.

• Size: Diameter 10–100 μm; length can be several centimeters.

• Syncytium: Each muscle fiber is a syncytium (multinucleated cell).

• Sarcoplasm: Contains abundant glycogen and myoglobin for energy storage and oxygen binding.

• Myofibrils: Numerous contractile units within the fiber.

• Sarcoplasmic Reticulum: Extensive network for calcium storage and release.

• T-tubules: Invaginations of the sarcolemma that help transmit action potentials into the fiber.

Example: Excitation-Contraction Coupling

Excitation-contraction coupling is the process by which an action potential in the sarcolemma leads to muscle contraction via calcium release from the sarcoplasmic reticulum.

Summary Table: Types of Synapses

Type	Mechanism	Speed	Location
Chemical Synapse	Neurotransmitter release and receptor binding	Slower (synaptic delay)	Most of CNS and PNS
Electrical Synapse	Direct ion flow via gap junctions	Very rapid	Brain regions, embryonic tissue

Additional info:

• Excitation-contraction coupling involves the interaction of actin and myosin filaments, regulated by calcium ions and ATP.

• Action potential propagation along the sarcolemma is essential for muscle contraction.