anatomy

The class discussion focused on the structure and function of neurons and their role in impulse transmission within the nervous system.

Neurons: Basic units of the nervous system that transmit impulses.
Neuroglia: Support cells for neurons, playing crucial roles in maintaining homeostasis, forming myelin, and providing support and protection.
- Different types of neuroglia include:
  - Astrocytes
  - Oligodendrocytes
  - Microglia
  - Ependymal cells

The process begins when a neuron receives information, typically through neurotransmitters binding to channel proteins on dendrites.
Cellular Depolarization:
- When a neurotransmitter binds, it causes depolarization leading to an influx of sodium ions, until the membrane potential reaches -55 millivolts (threshold potential), triggering an action potential.
- Resting Membrane Potential:
  - At rest, the membrane potential of a neuron is approximately -70 millivolts.
  - This is referred to as the resting membrane potential.

Depolarization:
- Sodium ion channels open, sodium enters, leading the potential to rise to around +30 millivolts.
- Membrane potential becomes positive.
Repolarization:
- At +30 mV, sodium channels close and potassium channels open, allowing potassium ions to flow out, making the cell more negative.
- This phase returns the membrane potential toward resting level.
Hyperpolarization:
- The membrane potential temporarily dips below -70 mV, reaching about -80 to -90 mV due to slow closure of potassium channels.
- This state is known as hyperpolarization.
Refractory Periods:
- Following depolarization, there is a period during which the neuron cannot fire again (refractory period), ensuring the impulse travels in one direction (unidirectional flow).

Myelinated vs. Unmyelinated Axons:
- Myelinated axons conduct impulses rapidly due to insulation provided by the myelin sheath, allowing faster transmission of signals (saltatory conduction).
- In diseases such as multiple sclerosis, the myelin sheath is attacked, leading to slower impulse transmission as the action potential may not reach the synaptic knobs effectively, causing symptoms like muscle weakness and fatigue.

When action potentials reach synaptic knobs:
- Voltage-gated calcium channels open, allowing calcium ions to enter the cell.
- Calcium binds to proteins anchoring vesicles containing neurotransmitters, prompting them to fuse with the membrane and release their contents into the synaptic cleft.
The releasing neuron is referred to as the presynaptic cell, and the receiving neuron as the postsynaptic cell, where neurotransmitters initiate a new action potential.
Example of Neurotransmitters:
- Excitatory Neurotransmitters: Cause depolarization (e.g., acetylcholine, serotonin, dopamine).
  - Bind to sodium channels, making the membrane potential more positive.
- Inhibitory Neurotransmitters: Cause hyperpolarization (e.g., GABA).
  - Bind to potassium or chloride channels, leading to a more negative membrane potential.

After neurotransmitters have done their job, some are reabsorbed back into the presynaptic cell for recycling (e.g., serotonin through selective serotonin reuptake inhibitors, or SSRIs).

The nervous system processes multiple stimuli simultaneously, either converging or diverging signals to orchestrate an appropriate response.
Convergent Processing: Integration of multiple sensory inputs into a unified response.
Divergent Processing: Distribution of a signal to multiple effector pathways.

Central Nervous System (CNS): Composed of the brain and spinal cord.
Peripheral Nervous System (PNS): All other neural pathways, including cranial and spinal nerves.
- PNS is divided into afferent (sensory) and efferent (motor) pathways:
  - Efferent further divides into somatic (voluntary control) and autonomic nervous systems (involuntary control).

The brain is divided into various regions:
- Cerebrum: Responsible for higher functions like reasoning and sensory processing.
- Cerebellum: Coordinates voluntary movements and balance, fine motor skills.
- Diencephalon: Includes the thalamus and hypothalamus; regulates homeostasis and endocrine functions.
- Brain Stem: Controls basic life functions such as heart rate and breathing.

The CNS is protected by meninges, which consists of three layers:
- Dura mater (outermost, durable layer),
- Arachnoid mater (middle layer, web-like),
- Pia mater (innermost layer, direct contact with the CNS).
Cerebrospinal Fluid (CSF) is produced in the ventricles of the brain and circulates through the subarachnoid space, providing cushioning and nutrient transport.

Neurons do not regenerate, and neurodegenerative diseases decrease cognitive function as brain cells die off with age, leading to declines in memory, reflex responses, and sensory processing.
- Brain loses approximately 10% of its mass over a lifetime.
- Neurotransmitter levels decrease with age, reducing the action potential propagation rate by 5-10%, leading to fading memory and increased risk of falls.

General Senses: More broadly distributed throughout the body (e.g., touch).
Special Senses: Concentrated in the head (e.g., vision, hearing, taste, smell).
- Chemoreceptors play a key role in taste and smell.
- Vision involves light detection by rods (low light) and cones (color) in the retina.

This discussion unified foundational aspects of neuroanatomy and neurophysiology, detailing how neurons communicate through electrical signals and neurotransmission while providing an overview of the CNS and PNS anatomy, highlighting the necessity of proper myelination and neurotransmitter function, and concluding with the significant effects of aging on neurological health.