Nervous System: Neuron and Action Potential (AP)

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron when it is not actively sending a signal. This potential is primarily established by the sodium-potassium pump (Na+/K+ ATPase) and the differential permeability of the membrane to ions.

• Na+/K+ Pump: Actively transports 3 Na+ ions out and 2 K+ ions into the cell, consuming ATP.

• Ion Distribution: More K+ inside, more Na+ outside; the inside of the neuron is negatively charged relative to the outside.

• Typical Value: Approximately -70 mV in neurons.

Equation:

(Nernst equation for K+)

Action Potentials

Action potentials are rapid, transient changes in membrane potential that propagate along axons to transmit signals.

• Initiation: Triggered when the membrane depolarizes to threshold, usually due to excitatory input.

• Propagation: The AP travels along the axon by opening voltage-gated Na+ and K+ channels sequentially.

• Refractory Periods: Ensure unidirectional propagation and limit firing frequency.

Synaptic Transmission

Neurons communicate at synapses, which can be electrical or chemical.

• Electrical Synapses: Direct ion flow via gap junctions; rapid but less common.

• Chemical Synapses: Neurotransmitter release from presynaptic neuron binds to receptors on postsynaptic cell, causing excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs).

• Excitatory vs. Inhibitory: EPSPs depolarize the membrane (e.g., glutamate), IPSPs hyperpolarize (e.g., GABA).

• Synaptic Delay: Chemical synapses are slower due to neurotransmitter diffusion and receptor binding.

Example: Acetylcholine at the neuromuscular junction is excitatory; GABA in the CNS is inhibitory.

Role of Myelin

Myelin increases the speed of AP transmission by insulating axons and enabling saltatory conduction between nodes of Ranvier.

• Nodes of Ranvier: Gaps in myelin where APs are regenerated.

• Clinical Relevance: Demyelinating diseases (e.g., multiple sclerosis) slow or block AP conduction.

Brain

Hypothalamus and Thalamus

The hypothalamus regulates homeostasis, endocrine function, and autonomic processes. The thalamus acts as a relay station for sensory and motor signals to the cerebral cortex.

• Hypothalamus: Controls body temperature, hunger, thirst, circadian rhythms, and links nervous and endocrine systems via the pituitary gland.

• Thalamus: Processes and relays sensory information (except olfaction) to the cortex.

Lateralization of Cortical Function

Lateralization refers to the specialization of certain functions in either the left or right cerebral hemisphere.

• Left Hemisphere: Language, logic, analytical tasks.

• Right Hemisphere: Spatial abilities, face recognition, music.

Cerebellum

The cerebellum coordinates voluntary movements, balance, and posture.

• Structure: Contains two hemispheres and a central vermis; highly folded cortex.

• Function: Integrates sensory input and fine-tunes motor activity.

Meninges, Cerebrospinal Fluid (CSF), and Blood-Brain Barrier

• Meninges: Three protective layers—dura mater, arachnoid mater, pia mater.

• CSF: Cushions the brain, circulates nutrients, removes waste.

• Blood-Brain Barrier: Selective barrier that protects the CNS from toxins and pathogens.

Spinal Cord

Somatic vs. Visceral Motor Neurons

Somatic motor neurons control voluntary skeletal muscles; visceral motor neurons (autonomic) control involuntary muscles and glands.

• Somatic Motor Neuron Cell Bodies: Located in the anterior horn of the spinal cord.

• Visceral Motor Neuron Cell Bodies: Located in the lateral horn (thoracic and upper lumbar regions) for sympathetic, and in brainstem/spinal cord for parasympathetic.

Autonomic Nervous System (ANS)

Parasympathetic vs. Sympathetic Nervous System

The ANS regulates involuntary functions. It has two main divisions:

Dermatomes

Dermatomes are areas of skin innervated by sensory fibers from a single spinal nerve root.

• Clinical Use: Helps localize neurological lesions.

Phrenic Nerve

The phrenic nerve arises from cervical spinal nerves (C3–C5) and innervates the diaphragm, essential for breathing.

Cholinergic and Adrenergic Receptors

• Cholinergic Receptors: Bind acetylcholine; include nicotinic (at ganglia and NMJ) and muscarinic (at target organs in parasympathetic system).

• Adrenergic Receptors: Bind norepinephrine/epinephrine; include alpha and beta subtypes (in sympathetic system).

ANS Effects on Organs

• Heart: Parasympathetic slows rate; sympathetic increases rate and force.

• Urinary Bladder: Parasympathetic promotes urination; sympathetic inhibits.

• Arterioles: Sympathetic causes vasoconstriction; parasympathetic has little effect.

Patellar Reflex

The patellar reflex is a monosynaptic stretch reflex that tests the integrity of the femoral nerve and spinal cord segments L2–L4.

Clinical Terms and Cranial Nerves

Review of clinical terms and cranial nerves is essential for understanding neurological assessment and diagnosis.

• Cranial Nerves: Twelve pairs, each with specific sensory, motor, or mixed functions.

• Example: The optic nerve (CN II) is purely sensory for vision; the vagus nerve (CN X) has mixed functions including parasympathetic output to thoracic and abdominal organs.

Vocabulary Review

Understanding key terms is crucial for mastering anatomy and physiology concepts.