Nerve and Muscle Physiology: An Overview – Study Notes

Nervous System Organization

Central and Peripheral Nervous Systems

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all neural tissue outside the CNS.

• CNS: Integrates and processes information.

• PNS: Transmits sensory and motor signals between the CNS and the rest of the body.

• Divisions of PNS:

  • Sensory Division: Carries sensory information to the CNS.

  • Efferent Division: Carries motor commands from the CNS to effectors. Subdivided into:

    • Somatic: Controls voluntary movements (skeletal muscle).

    • Autonomic: Controls involuntary functions (smooth muscle, cardiac muscle, glands). Includes Sympathetic and Parasympathetic branches.

• Enteric Nervous System: Regulates gastrointestinal function.

Sympathetic vs. Parasympathetic Pathways

These branches of the autonomic nervous system maintain homeostasis through dynamic balance:

• Sympathetic: "Fight-or-flight" responses; increases heart rate, dilates pupils.

• Parasympathetic: "Rest-and-digest" responses; slows heart rate, stimulates digestion.

Cells of the Nervous System

Neurons

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

• Parts of a Neuron:

  • Dendrites: Receive signals.

  • Cell Body (Soma): Contains nucleus and organelles.

  • Axon: Transmits electrical impulses.

  • Axon Terminals: Release neurotransmitters at synapses.

• Types of Neurons:

  • Sensory Neurons: Transmit sensory information.

  • Interneurons: Integrate signals within the CNS.

  • Efferent Neurons: Carry motor commands to effectors.

Glial Cells

Glial cells support and protect neurons. They are essential for homeostasis, myelination, and immune defense.

Glial Cell Type	Location	Main Function
Ependymal cells	CNS	Produce cerebrospinal fluid
Astrocytes	CNS	Maintain blood-brain barrier, regulate K+ levels
Microglia	CNS	Immune defense (scavengers)
Oligodendrocytes	CNS	Form myelin sheaths
Schwann cells	PNS	Form myelin sheaths
Satellite cells	PNS	Support cell bodies

Graded Action Potentials

Properties and Conduction

Graded potentials are changes in membrane potential that vary in size and decrease in strength as they spread from the point of origin. They occur in dendrites and cell bodies.

• Subthreshold Graded Potential: Does not reach threshold; no action potential generated.

• Suprathreshold Graded Potential: Reaches threshold at the trigger zone; action potential is generated.

Action Potentials

Action potentials are rapid, uniform electrical signals that travel along axons. They are all-or-none events.

• Resting Potential: The baseline membrane potential, typically around -70 mV.

• Threshold Potential: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Key Ion Movements:

  • Na+ influx causes depolarization.

  • K+ efflux causes repolarization.

Equation:

$I = g(V - E)$

Where $I$ is ionic current, $g$ is conductance, $V$ is membrane potential, and $E$ is equilibrium potential.

Synapses and Neurotransmission

Synaptic Transmission

Synapses are junctions where neurons communicate via neurotransmitters.

• Presynaptic Neuron: Releases neurotransmitter.

• Postsynaptic Neuron: Receives neurotransmitter and responds.

• Neurotransmitter Release: Triggered by Ca2+ influx.

Neurotransmitters & Receptors

Neurotransmitters bind to specific receptors, causing excitatory or inhibitory effects.

Chemical	Receptor	Type	Location
Acetylcholine	Nicotinic, Muscarinic	Ion channel, GPCR	CNS, PNS
Norepinephrine	Adrenergic	GPCR	CNS, PNS
Glutamate	AMPA, NMDA	Ion channel	CNS
GABA	GABAA, GABAB	Ion channel, GPCR	CNS

Integration and Summation

Temporal and Spatial Summation

Neurons integrate multiple inputs to determine whether to fire an action potential.

• Temporal Summation: Multiple signals from one neuron over time.

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

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increases likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreases likelihood of action potential.

Autonomic Nervous System

Pathways and Synapses

The autonomic nervous system controls involuntary functions via two-neuron pathways:

• Preganglionic Neuron: Originates in CNS, synapses in autonomic ganglion.

• Postganglionic Neuron: Projects to target tissue.

Sympathetic vs. Parasympathetic Synapses

Division	Neurotransmitter	Receptor
Sympathetic	Norepinephrine	Adrenergic
Parasympathetic	Acetylcholine	Muscarinic cholinergic

Sensory Neurons and Pathways

Types of Sensory Receptors

Type	Stimulus
Chemoceptors	Chemicals
Photoreceptors	Light
Thermoreceptors	Temperature
Mechanoreceptors	Pressure, vibration
Nociceptors	Pain (noxious stimuli)

Pain Pathways and Modulation

Gate-Control Model

Pain perception is modulated by the interaction of sensory and inhibitory neurons in the spinal cord.

• Neuropathic Pain: Chronic pain due to nerve damage (e.g., diabetes, chemotherapy).

• Analgesic Drugs:

  • Aspirin/NSAIDs: Reduce inflammation and pain.

  • Opiates: Block pain transmission in CNS.

  • Gabapentin/Pregabalin: Block calcium channels, reduce neurotransmitter release.

  • Amitriptyline: Blocks reuptake of serotonin and norepinephrine, increasing their synaptic concentration.

Examples and Applications

• Neuromuscular Junction (NMJ): Specialized synapse between motor neuron and skeletal muscle fiber; uses acetylcholine as neurotransmitter.

• Clinical Application: Understanding neurotransmitter pathways is essential for pharmacological management of pain and neurological disorders.

Additional info: These notes expand on the original slides by providing definitions, context, and tables for clarity and completeness.