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Neurologic System Overview

Week Two: Neurologic System Overview

I. Introduction

  • Welcome to week two of the course.
  • Focus of this week: neurological system covering structure and function.

II. Overview of the Nervous System

A. Main Divisions

  • Two main groups:
    1. Central Nervous System (CNS)
    • Composed of the brain and spinal cord.
    • Primary purpose: organize and analyze information.
    • Nervous impulses travel to/from the brain via the spinal cord.
    1. Peripheral Nervous System (PNS)
    • Follows commands from the CNS.
    • Divided into cranial and spinal nerves.

B. Sensory and Motor Pathways

  • Sensory Nerves: Interpret stimuli and send information to CNS via spinal nerves.
  • Efferent Nerves:
    • Transmit commands from CNS to effector organs (muscles, visceral organs, etc.).
    • Descending pathway from the spinal cord.

III. Subdivisions of the Peripheral Nervous System

A. Somatic Nervous System

  • Includes motor and sensory pathways.
  • Regulates voluntary motor control of skeletal muscle.

B. Autonomic Nervous System

  • Regulates involuntary control of internal organs.

C. Subgroups of the Autonomic Nervous System

  1. Sympathetic Nervous System: Fight or flight response.
  2. Parasympathetic Nervous System: Rest and digest response.

IV. Visual Representation of the Nervous System

A. Hierarchical Structure

  • CNS and PNS depicted from top (brain) down (effector organ).
  • PNS further subdivided into motor divisions, which are somatic and autonomic.

B. Sensory Pathways

  • Afferent fibers send impulses to CNS for interpretation.

V. Neurons: Structure and Function

A. Basic Structure of Neurons

  • Components: Cell body, dendrites, axons.

B. Types of Neurons

  1. Unipolar Neurons: One process from the cell body.
    • Short process splits into an axon and dendrite; typically sensory neurons.
  2. Bipolar Neurons: Two processes (one axon, one dendrite); found in retina of the eye.
  3. Multipolar Neurons: Multiple processes; most common; involved in motor and association functions.

C. Support Cells: Neuroglial Cells

  • Five to ten times more numerous than neurons.
  1. Astrocytes: Fill spaces between neurons, surround blood vessels.
  2. Microglial Cells: Remove debris from CNS.
  3. Ependymal Cells: Line the ventricles and cerebrospinal fluid (CSF) cavities.
  4. Oligodendrocytes: Produce myelin within CNS.
  5. Schwann Cells: Produce myelin in PNS.

VI. Myelin and Nerve Function

A. Myelination

  • Myelinated axons have a myelin sheath which insulates and speeds impulse propagation.
  • Nodes of Ranvier: gaps in myelin sheath where action potentials jump, increasing conduction velocity via saltatory conduction.

B. Axonal Characteristics

  • Diameter affects speed: larger diameters lead to faster transmission.

C. Neuronal Communication

  1. Divergent Communication: One neuron communicates with several others.
  2. Convergent Communication: Multiple neurons connect to one.

VII. Action Potential and Synapses

A. Action Potential Generation

  • Occurs when sufficient stimulus reaches the threshold.
  • All-or-nothing response: if stimulus is weak, neuron remains unexcited.

B. Synaptic Transmission

  • Synapses: regions where signals are transmitted between adjacent neurons.
  • Synaptic boutons store neurotransmitters.

C. Neurotransmitters

  1. Norepinephrine/Adrenaline: Released from sympathetic fibers; stimulates alpha adrenergic receptors for vasoconstriction.
  2. Acetylcholine: Used at neuromuscular junctions to activate muscles.
  3. Dopamine: Regulates sleep cycle.
  4. Serotonin: Contributes to well-being, cognition, learning, memory, bowel function.

D. Postsynaptic Potentials

  1. Excitatory Postsynaptic Potential (EPSP): Causes influx of positive ions, making neuron likely to fire.
  2. Inhibitory Postsynaptic Potential (IPSP): Causes influx of negative ions, making neuron less likely to fire.

VIII. Summary of Key Functional Concepts in Neurons

A. Summation

  • Combined effects of EPSPs and IPSPs determine if action potential is generated.
  • Temporal Summation: Repeated inputs closely timed.
  • Spatial Summation: Multiple inputs from different sources.

B. Facilitation

  • Created potentials are easier to achieve if impulses follow closely.

IX. Brain Structure: Major Divisions

A. Forebrain

  1. Telencephalon:
    • Cerebral cortex, basal ganglia, corpus callosum, limbic system.
    • Gray matter consists of neuronal cell bodies; white matter consists of axons.
  2. Diencephalon:
    • Includes thalamus, hypothalamus, pituitary gland, pineal gland.

B. Midbrain (Mesencephalon)

  • Involves motor movement and sensory processing, particularly auditory and visual.

C. Hindbrain (Metencephalon)

  1. Cerebellum: Involved in fine motor control, balance, and coordination.
  2. Pons: Relays information between cerebellum and brainstem.
  3. Medulla Oblongata: Responsible for reflex activities like heart rate, respiration, blood pressure.

X. Spinal Cord Structure and Function

A. Anatomy

  • Lies within vertebral column and connects brain and body.
  • Ends at conus medullaris; contains cauda equina for nerve roots.
  • Gray Matter: Contains cell bodies, dendrites, and axon terminals.
  • White Matter: Myelinated axons connecting gray matter.

B. Reflex Arcs

  • Neural pathways controlling reflex actions, allowing fast responses without brain involvement.
  • Afferent neurons transmit sensory input; efferent neurons cause muscle contraction.

XI. Peripheral Nervous System

A. Spinal Nerves

  • 31 pairs; names correspond with vertebral level of exit.
  • Mixed Nerves: Contain sensory and motor fibers.

B. Autonomic Nervous System

  1. Sympathetic: Fight or flight function; originates from thoracolumbar region.
  2. Parasympathetic: Rest and digest function; originates from craniosacral region.

XII. Neurotransmitters and Receptors

A. Sympathetic vs. Parasympathetic

  1. Sympathetic:
    • Preganglionic fibers release acetylcholine; postganglionic fibers release norepinephrine.
  2. Parasympathetic: Both pre and postganglionic fibers release acetylcholine.

B. Types of Adrenergic Receptors

  1. Alpha (α) Receptors:
    • Alpha 1: Excitation/stimulation.
    • Alpha 2: Relaxation/inhibition.
  2. Beta (β) Receptors:
    • Beta 1: Increases heart rate/contractility.
    • Beta 2: Facilitates actions in other contexts.

XIII. Nervous System Protection

A. Cranium and Meninges

  1. Cranium: Composed of 8 bones, provides protection.
  2. Meninges: Three protective membranes around the brain and spinal cord (dura mater, arachnoid, pia mater).

B. Cerebrospinal Fluid (CSF)

  • Clear, colorless fluid produced by choroid plexus, circulates through brain ventricles, and protects the brain.

XIV. Vascular Supply to the Brain

A. Blood Supply

  • Brain receives 20% of cardiac output, primarily from internal carotid arteries and vertebral arteries.

B. Circle of Willis

  • Arterial circle at the base of the brain allowing collateral circulation to compensate for blood flow reduction.

C. Blood Brain Barrier (BBB)

  • Cellular structures regulating substances entering the brain from the blood, maintaining selective permeability.

XV. Aging and the Nervous System

A. Structural Changes

  • Neuron reduction, changes in CSF, brain weight decrease, fibrosis of meninges, etc.

B. Functional Changes

  • Diminished sensory functions, memory impairments, increased BBB permeability with age.

XVI. Diagnostic Tests for Nervous System Function

A. Imaging and Analysis Tools

  • X-rays, CT, MRI, PET scans, cerebral angiography, EEG, lumbar puncture for CSF analysis, etc.

XVII. Conclusion

  • Recap of various topics discussed related to the structure, function, and significance of the neurological system in overall health.