Unit 5: Nervous System

5.1 Nervous System Basics

• 5.1.1 Central and Peripheral Nervous Systems
  • Central Nervous System (CNS)
  • Consists of the brain and spinal cord.
  • Functions as the command center for the body, where impulses are integrated, interpreted, and dictated.
  • Peripheral Nervous System (PNS)
  • Comprises nerves that connect the CNS to the rest of the body.
  • Contains ganglia, clusters of neuron cell bodies.
  • Divided into two functional divisions: Efferent (motor) and Afferent (sensory).
  • Efferent Division: Sends impulses to effectors (muscles, glands).
  • Afferent Division: Receives sensory input from receptors in sensory organs.
  • Subdivided into two systems:
    • Somatic Nervous System: Control over skeletal muscles, sensory functions include touch, pain, temperature, hearing, equilibrium.
    • Autonomic Nervous System: Controls involuntary functions such as digestion, glands, and reflex actions.

5.1.2 Neuron Structure

• Neuron Anatomy
  • Neuron: Fundamental unit of the nervous system responsible for generating and conducting impulses.
  • Structural features:
    • Cell Body (Soma): Contains organelles and protein fibers crucial for information processing.
    • Dendrites: Branch-like structures that receive signals and transmit them toward the cell body, with a large surface area for signal intake.
    • Axon: Long, singular process that transmits nerve impulses away from the cell body.
  • Axon Hillock: Cone-shaped region connecting to the axon where action potentials are initiated.
  • Schwann Cells: Glial cells forming myelin sheath around axons in PNS.
  • Myelin Sheath: Insulates nerves, composed of protein and fats, enhancing impulse conduction.
  • Nodes of Ranvier: Gaps in the myelin sheath allowing for ion exchange and rapid signal transmission.

5.1.3 Types of Nervous Cells

• Neuroglia: Supportive cells in the nervous system; 6 types exist (4 in CNS, 2 in PNS).
  • Astrocytes: Star-shaped and most abundant; regulate blood flow and neurotransmitter levels, form the blood-brain barrier.
  • Microglia: Smallest glial cells; act as phagocytes, consume pathogens.
  • Ependymal Cells: Line the brain's ventricles; help circulate cerebrospinal fluid (CSF).
  • Oligodendrocytes: Form myelin sheaths around CNS axons.
  • Satellite Cells: Surround neuron bodies in ganglia of PNS.
  • Schwann Cells: Form myelin sheaths around PNS axons.

5.1.4 Regeneration of Neurons

• Regeneration in PNS: Schwann cells facilitate regeneration of damaged axons by guiding regrowth to the original connection point and remyelinating the axon.
• Regeneration in CNS: Generally limited; oligodendrocytes do not promote regeneration due to lack of neurilemma.
• Key Processes:
  • Recruitment of macrophages to clear debris from injury sites.
  • Schwann cells proliferate and form a protective tube guiding regrowth.

5.1.5 Action Potential and Nerve Signaling

• Membrane Potential: The difference in ion concentration across the neuronal membrane contributes to the resting potential.

  • Inside the cell is negative due to large negatively charged ions (PO₄³⁻ & SO₄²⁻); sodium ions (Na⁺) are in a higher concentration outside.
  • Sodium-Potassium pump maintains resting potential and promotes excitability.

• Phases of Action Potential:

  • Resting State: Na⁺ channels are mostly closed; the inside of the neuron is negatively charged.
  • Depolarization: Na⁺ channels open, Na⁺ enters, making the interior positive.
  • Repolarization: Na⁺ channels close and K⁺ channels open, allowing K⁺ to exit, restoring the negative charge.
  • The action potential propagates along the axon via local currents reaching successive Nodes of Ranvier, leading to rapid impulse conduction in myelinated axons.

5.1.6 The Synapse

• Definition: Junction between two communicating neurons.
• Synaptic Transmission: The process of impulse transfer across the synaptic cleft via neurotransmitters released from the presynaptic neuron.
  • Components:
  • Presynaptic Neuron: Neuron sending the impulse; contains synaptic vesicles holding neurotransmitters.
  • Postsynaptic Neuron: Neuron receiving the impulse; contains receptors for neurotransmitters.
• Process:
  • When a nerve impulse reaches the synaptic knob, neurotransmitters are released into the synaptic cleft and diffuse to bind receptors on the postsynaptic neuron's membrane.

5.2 Central Nervous System

5.2.1 Gray vs White Matter

• Gray Matter:
  • Located on the brain's exterior (~40% of the brain).
  • Contains dendrites, cell bodies, and synaptic connections.
  • Responsible for processing information.
  • In spinal cord, forms a butterfly-shaped core.
• White Matter:
  • Found interiorly in brain and outer layer of spinal cord (~60% of the brain).
  • Composed mainly of myelinated axons, responsible for communication between different brain regions.
  • Allows for memory recall, emotional responses, and learning.

5.2.2 Anatomy of the Brain

• Meninges: Three protective membranes covering the brain:

  1. Dura Mater: Outermost, tough layer containing blood vessels.
  2. Arachnoid Mater: Middle layer, web-like, enclosing CSF.
  3. Pia Mater: Innermost layer, thin and containing blood vessels tightly adhering to the brain's surface.

• Major Portions of the Brain:

  • Brainstem
  • Cerebellum
  • Diencephalon
  • Cerebrum

• The brain operates through ~86 billion multipolar neurons and oversees multiple functions including sensation, movement, memory, reasoning, and regulation of visceral activities.

5.2.3 Cerebral Cortex

• The thin layer of gray matter on the cerebrum’s surface, it contains 75% of the neurons in the nervous system, and is crucial for higher mental functions and sensory integration.
  • Divided into four lobes:
  • Frontal, Parietal, Temporal, Occipital.

5.2.4 The Spinal Cord

• Extends from the medulla oblongata to approximately T12, tapering into the cauda equina, which contains nerve roots.
  • Internal structure:
  • White matter is outer; gray matter is internal, primarily consisting of cell bodies, dorsal (sensory) and ventral (motor) horns.

5.2.5 Cerebrospinal Fluid (CSF)

• Circulates between the layers of the meninges, protecting the CNS and facilitating nutrient transfer.

Spinal Nerves

• There are pairs of spinal nerves at each vertebral level, forming part of the PNS, which is composed of nerves and ganglia outside the CNS.
• Classification of Nerves:
  • Mixed Nerves: Carry both sensory and motor fibers.
  • Afferent Nerves: Convey impulses towards the CNS.
  • Efferent Nerves: Carry impulses away from the CNS.

Autonomic Nervous System

• Divided into two branches:
  • Sympathetic Division: Responsible for fight or flight responses.
  • Parasympathetic Division: Manages restful states and homeostasis.

Development Aspects of the Nervous System

• The nervous system forms during early embryonic development, with the hypothalamus being one of the last areas to mature.

• Postnatally, no new neurons are formed, but growth and synapse development continue, allowing for increased complexity.

• This comprehensive study guide encapsulates the extensive and multifaceted information regarding the nervous system, covering anatomical details, physiological functions, and significant processes, adhering to an exhaustive format suitable for academic learning.