Anatomy & Physiology Study Notes: Neurological System and Autonomic Nervous System

Neurological System

Information Transfer in Neurons

The nervous system transmits information between nerve cells using both electrical and chemical signals. This process is essential for communication within the body and for coordinating responses to stimuli.

• Electrical charges travel within neurons, allowing rapid transmission of signals.

• Chemicals (neurotransmitters) bridge the gap between neurons and effector cells, such as muscle fibers or glands.

• Myelinated axons provide insulation, increasing the speed of signal conduction.

• Neural networks can process signals in both temporal (time) and spatial (location) domains, allowing for complex integration and response.

Key Definitions

Understanding the terminology related to neuronal signaling is crucial for grasping how the nervous system functions.

• Depolarisation: A change in the membrane potential towards zero or positive values, making the inside of the cell less negative.

• Repolarisation: The return of the membrane potential to its original negative value after depolarisation.

• Hyperpolarisation: An "overshoot" of repolarisation, making the membrane potential more negative than its resting value.

• Threshold: The level of depolarisation required to trigger an action potential (AP); depends on stimulus strength.

Membrane Potentials in Neurons

Neurons exhibit different types of membrane potentials that underlie their signaling capabilities.

• Resting membrane potential: Established by potassium (K+) leakage out of the cell and the sodium-potassium (Na+/K+) pump.

• Graded potential: Localised depolarisation or hyperpolarisation; short-lived and travels only a short distance.

• Action potential: Depolarisation triggered by sodium (Na+) influx; follows the "all or nothing" rule.

• EPSP (Excitatory Postsynaptic Potential): Graded depolarisation at the postsynaptic membrane that can lead to an AP.

• IPSP (Inhibitory Postsynaptic Potential): Graded hyperpolarisation at the postsynaptic membrane that decreases the likelihood of an AP.

Types of Ion Channels

Ion channels regulate the movement of ions across the neuronal membrane, influencing membrane potential and signal transmission.

• Leakage channels: Non-gated channels that are always open, allowing Na+ to move into the cell and K+ to move out.

• Voltage-gated channels: Open or close in response to changes in membrane potential.

• Chemically gated channels: Open in response to the binding of a chemical, such as a neurotransmitter.

Autonomic Nervous System (ANS)

Divisions of the ANS

The autonomic nervous system regulates involuntary functions and is divided into two main branches: the sympathetic and parasympathetic divisions.

• Sympathetic division: Prepares the body for "fight or flight" responses; increases heart rate, dilates airways, and mobilizes energy stores.

• Parasympathetic division: Promotes "rest and digest" activities; decreases heart rate, stimulates digestion, and conserves energy.

Neurotransmitters and Receptors

Neurotransmitters are chemicals released by neurons to communicate with other cells. Their effects depend on the type of receptor they bind to.

• Acetylcholine (ACh): Used by both sympathetic (preganglionic) and parasympathetic neurons.

• Norepinephrine (NE): Used by sympathetic postganglionic neurons.

• Cholinergic receptors: Bind ACh; include nicotinic (on all postganglionic neurons) and muscarinic (on parasympathetic effector organs and sweat glands).

• Adrenergic receptors: Bind NE; include alpha (α1, α2) and beta (β1, β2, β3) subtypes.

Functional Effects of ANS Divisions

The sympathetic and parasympathetic divisions often have opposing effects on target organs, maintaining homeostasis.

Organ/System	Sympathetic Effect	Parasympathetic Effect
Heart Rate	Increases	Decreases
Airways	Dilates	Constricts
Digestive Activity	Decreases	Increases
Metabolism	Increases	Decreases
Urine Output	Decreases	Increases
Sweat Glands	Activates	No effect

Summary of Key Equations

• Resting Membrane Potential: Determined by the Nernst equation for K+ and Na+ ions: $E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$ Additional info: This equation describes the equilibrium potential for a given ion across the membrane.

• Action Potential: All-or-nothing response when threshold is reached: $\text{If } V_{m} \geq V_{\text{threshold}}, \text{AP is generated}$

Examples and Applications

• Example: When you touch a hot surface, sensory neurons transmit the signal to the spinal cord, which then activates motor neurons to withdraw your hand.

• Application: Understanding neurotransmitter function is essential for pharmacology, as many drugs target specific receptors to treat conditions like hypertension or asthma.

Additional info: These notes expand on brief points from the original materials, providing academic context and definitions for clarity.