Study Notes: Neurological System – Anatomy & Physiology

Neurological System Overview

Introduction

The neurological system is responsible for the rapid transmission and processing of information throughout the body. It consists of the central nervous system (CNS) and peripheral nervous system (PNS), which coordinate sensory input, motor output, and higher cognitive functions.

Information Transfer in Neurons

Mechanisms of Signal Transmission

• Electrical charges travel within neurons as action potentials.

• Chemicals (neurotransmitters) bridge the gap between neurons and effector cells (e.g., muscle fibers, glands).

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

• Neural networks process signals both temporally (timing) and spatially (location), allowing for complex integration and response.

Example: The transmission of a pain signal from the skin to the brain involves electrical impulses along sensory neurons and chemical neurotransmission at synapses.

Membrane Potentials and Action Potentials

Key Definitions

• Depolarisation: Change in membrane potential towards zero or positive values.

• Repolarisation: Return to the original negative membrane potential.

• Hyperpolarisation: Membrane potential becomes more negative than the resting potential.

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

Types of Potentials in Neurons

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

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

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

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

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

Formula:

$V_{rest} = -70\ \text{mV}$ (typical resting membrane potential)

$\text{Na}^+/\text{K}^+\ \text{pump}:\ 3\ \text{Na}^+\ \text{out},\ 2\ \text{K}^+\ \text{in}$

Ion Channels in Neurons

Types of Channels

• Leakage channels: Non-gated; always open, allowing passive movement of Na+ and K+.

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

• Chemically gated channels: Open in response to binding of specific chemicals, such as neurotransmitters.

Example: Voltage-gated Na+ channels open during the initiation of an action potential.

Synaptic Transmission

Function of Synapses

• Synapses are junctions where neurons communicate with other neurons or effector cells.

• Neurotransmitters are released into the synaptic cleft in response to an AP, binding to receptors on the postsynaptic membrane.

• Graded potentials (EPSPs or IPSPs) are generated, influencing the likelihood of an AP in the postsynaptic cell.

Neurotransmitters

Definition and Classification

• Neurotransmitter: Chemical messenger released by neurons to transmit signals across a synapse.

• Classified by chemical structure and function (e.g., acetylcholine, noradrenaline).

Sensation and Perception

Receptors and Pathways

• Receptors: Specialized cells or nerve endings that detect specific stimuli.

• Types include mechanoreceptors (touch, pressure), thermoreceptors (temperature), photoreceptors (light), chemoreceptors (chemical changes), nociceptors (pain), exteroceptors (external environment), interoceptors (internal organs), and proprioceptors (body position).

• Receptors can be non-encapsulated (free nerve endings) or encapsulated (e.g., Meissner corpuscle).

• Special senses involve complex organs (e.g., eye, ear).

Example: Muscle spindles are mechanoreceptors that detect stretch in skeletal muscles.

Motor Activity

Pathways and Endings

• Motor activity involves brain areas, descending pathways, and motor endings.

• Neuromuscular junctions use acetylcholine (ACh) as the neurotransmitter for skeletal muscle contraction.

• Motor pathways include upper and lower motor neurons, with cell bodies in the primary motor cortex and spinal cord, respectively.

Major Sensory and Motor Pathways

Neuron Types and Pathways

• Multipolar neurons: Many dendrites, one axon; most common in CNS.

• Bipolar neurons: One dendrite, one axon; found in the eye and nose.

• Unipolar (pseudounipolar) neurons: One peripheral and one central branch; mainly sensory neurons in PNS.

• Functional types: Sensory (afferent), motor (efferent), interneurons (association).

Table: CNS vs PNS Terminology

Term	Definition
Nucleus	A collection of neuron cell bodies in the CNS
Ganglion	A collection of neuron cell bodies in the PNS
Tract	A bundle of axons in the CNS
Nerve	A bundle of axons in the PNS

Autonomic Nervous System (ANS)

Overview and Divisions

• The ANS regulates involuntary functions such as cardiac activity, gland secretion, and smooth muscle contraction.

• It consists of the sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.

Sympathetic Division

• Originates from thoracic and lumbar spinal cord.

• Short preganglionic and long postganglionic fibers.

• Neurotransmitters: Acetylcholine (ACh) and noradrenaline (NE).

• Stimulates adrenal gland to release adrenaline and noradrenaline for prolonged effects.

Parasympathetic Division

• Originates from cranial nerves and sacral spinal cord.

• Long preganglionic and short postganglionic fibers.

• Neurotransmitter: Acetylcholine (ACh).

Receptors in the ANS

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

• Adrenergic receptors: Bind noradrenaline/adrenaline; include alpha (α1, α2) and beta (β1, β2, β3).

Table: Effects of Sympathetic vs Parasympathetic Divisions

Function	Sympathetic	Parasympathetic
Mental alertness	↑	—
Metabolism	↑	↓
Digestion & urine output	↓	↑
Respiration	↑, dilate airways	Constrict airways
Heart rate & blood pressure	↑	↓
Sweat glands	Activate	—
Gut motility & blood flow	↓	↑
Urine output & defecation	↓	↑

Medications Affecting the ANS

• Agonists: Mimic neurotransmitter effects (e.g., sympathomimetics like adrenaline).

• Antagonists: Block neurotransmitter effects (e.g., β-blockers, muscarinic antagonists like atropine).

Reflexes

Types and Functions

• Reflexes are rapid, predictable motor responses to stimuli, often protective or postural.

• Types include stretch, tendon, cross-extensor, and superficial reflexes (abdominal, plantar).

• Reflex arc involves receptor, sensory neuron, integration center, motor neuron, and effector.

References

• Marieb, E.N. & Hoehn, K. (2023). Human Anatomy & Physiology, 12th ed.

• Huether, S.E., McCance, K.L., et al. (2022). Understanding Pathophysiology, 4th ANZ ed.

• Potter, J., Douglas, C., Rebeiro, G., & Waters, D. (2020). Fundamentals of Nursing, 6th ANZ ed.