General Physiology: Neurons and Action Potential

Neurons

• Neurons are specialized nerve cells responsible for transmitting impulses.
• Cell body (soma) contains the nucleus and is the central part of the neuron.
• Neurons have two processes:
  • Dendrites, which carry impulses toward the soma.
  • Axons, which carry impulses away from the soma.

Types of Neurons

• Unipolar: One extension from the cell body.
• Bipolar: Two extensions from the cell body.
• Multipolar: Multiple dendrites and one axon.

Neuron Functions

• Sensory (afferent): Transmit impulses from receptors to the CNS.
• Motor (efferent): Relay impulses from the CNS to effectors (muscles or glands).
• Association (interneurons): Connect sensory and motor neurons.

Supporting Cells (CNS)

• Oligodendrocytes: Form myelin sheaths around CNS axons.
• Microglia: Migrate to phagocytose foreign materials.
• Astrocytes: Regulate the external environment of neurons in the CNS.
• Ependymal cells: Line brain ventricles and the spinal cord's central canal.

Supporting Cells (PNS)

• Schwann cells: Form myelin sheaths around peripheral axons.
• Satellite cells: Support neuron cell bodies within ganglia of the PNS.

Nervous System Structure

• CNS (Central Nervous System): Comprises the brain and spinal cord.
• PNS (Peripheral Nervous System): Includes somatic (motor sensory) and autonomic (involuntary) nervous systems.

Terminologies

• Nerve: Bundle of axons in the PNS containing sensory and motor fibers.
• Ganglion: Grouping of neuron cell bodies outside the CNS.
• Nucleus: Grouping of neuron cell bodies within the CNS.
• Tract: Grouping of axons that interconnect CNS regions.

Resting Membrane Potential

• A constant membrane potential in excitable and non-excitable cells when at rest.
• Neurons typically have a resting membrane potential of -70mV.
• The interior of the cell is negatively charged compared to the outside.

Action Potential

• Involves rapid changes in membrane potential (approximately 100mV).
• Generated by depolarization of excitable membranes due to voltage-gated Na+ channels.
• Involves phases of depolarization, repolarization, and hyperpolarization.

Propagation Factors

• Degree of myelination affects impulse transmission speed.
• Larger axon diameters enhance conduction velocities.
• Temperature influences action potential propagation.

Refractory Periods

• Absolute Refractory Period: No action potential can occur during this time regardless of stimulus.
• Relative Refractory Period: A stronger-than-normal stimulus can initiate a new action potential.

Central Nervous System Organization

• Cerebrum: The largest brain part (80% of brain mass) with lobes controlling various functions (voluntary movement, personality, speech).
• Cerebral Cortex:
  • Contains sensory, motor, and association areas.
  • Gray matter consists of cellular bodies, while white matter includes axon tracts.

Lobes of the Cerebrum

• Frontal Lobe: Involved in voluntary motor control, personality, and intellect.
• Parietal Lobe: Processes somatosensory information and language.
• Temporal Lobe: Handles auditory information and memory.
• Occipital Lobe: Focuses on visual processing and perception.

Limbic System

• Known as the "emotional brain," it plays a key role in emotions and memory.
• Key components include the cingulate gyrus, amygdala, hippocampus, and septal nuclei.

Diencephalon

• Central core of the brain housing sensory and motor processing nuclei.
• Includes thalamus (sensory relay), hypothalamus (homeostasis), and epithalamus (includes pineal gland).

Brainstem Regions

• Midbrain: Contains nuclei for visual and auditory reflexes.
• Pons: Relays signals between different brain regions and assists in regulating respiration.
• Medulla Oblongata: Controls autonomic functions like heart rate and respiration, and houses vital centers.

Cerebellum

• Located at the back of the brain, involved in coordination and balance through input from proprioceptors.