Notes on Structure and Function of the Neurologic System

Overview of the Nervous System

  • Central Nervous System (CNS): Comprises the brain and spinal cord.

  • Peripheral Nervous System (PNS): Includes cranial nerves, spinal nerves, and pathways classified as:

    • Afferent (ascending): Pathways bringing sensory information to the CNS.

    • Efferent (descending): Pathways conducting motor commands from the CNS to effector organs.

PNS Functional Divisions

  • Somatic Nervous System: Controls voluntary motor functions, primarily skeletal muscle movements.

  • Autonomic Nervous System: Regulates involuntary functions to maintain internal homeostasis, divided further into:

    • Sympathetic Division: Mobilizes body systems during activity (e.g., ‘fight or flight’ response).

    • Parasympathetic Division: Conserves energy and maintains functions during rest (e.g., ‘rest and digest’).

Functional Classification of Neurons

  • Sensory Neurons: Transmit impulses from sensory receptors to the CNS.

  • Associational (Interneurons): Relay impulses between neurons.

  • Motor Neurons: Transmit impulses from the CNS to effectors such as muscles or glands.

Synapses

  • Definition: Junctions between adjacent neurons where impulses are transmitted via chemical and electrical conduction.

  • Components:

    • Presynaptic Neurons: Neurons sending the signal, located proximal to the synapse.

    • Postsynaptic Neurons: Neurons receiving the signal, located distal to the synapse.

    • Synaptic boutons: Ending of the presynaptic neuron.

    • Synaptic cleft: Gap between presynaptic and postsynaptic neurons.

Neurotransmitters

  • Role of Neurotransmitters: Chemicals affecting ion channels with two main types:

    • Excitatory Neurotransmitters: Generate excitatory postsynaptic potentials (EPSP).

    • Inhibitory Neurotransmitters: Generate inhibitory postsynaptic potentials (IPSP).

  • Summation Types:

    • Temporal Summation: Stimulus frequency affects the summation of EPSP.

    • Spatial Summation: Multiple stimuli from different neurons combine for action potential generation.

Predominant Neurotransmitters

  • Important neurotransmitters include:

    • Epinephrine (adrenaline)

    • Norepinephrine

    • Acetylcholine

    • Serotonin

    • Dopamine

    • Gamma-aminobutyric acid (GABA): Inhibitory.

    • Glutamate: Excitatory.

Structure of the Brain

  • Components:

    • Cerebrum: The largest part, divided into forebrain, midbrain, and hindbrain.

    • Brainstem: Comprising midbrain, medulla, and pons.

  • Lobes of the Brain:

    • Frontal Lobe: Involved in reasoning, motor function.

    • Parietal Lobe: Processes sensory information.

    • Temporal Lobe: Involved in memory and auditory perception.

    • Occipital Lobe: Handles visual processing.

Structure of the Spinal Cord

  • Location and Protection: Encased in the vertebral canal, providing a conduit between the brain and body.

  • Functions:

    • Connects with both somatic and autonomic reflexes.

    • Contains centers for motor pattern control.

    • Modulates sensory and motor information.

  • End Structure: Conus medullaris, where the nerve bundle known as cauda equina continues.

Protective Structures

  • Meninges: Three membranes enveloping the CNS:

    • Dura Mater: The outer tough layer.

    • Arachnoid Mater: Middle layer with a web-like structure.

    • Pia Mater: Inner layer closely adhering to the brain and spinal cord.

  • Cerebrospinal Fluid (CSF):

    • Acts as a cushion and provides buoyancy to the brain.

    • Normal volume is 125 to 150 ml in the ventricles.

    • Produced in the choroid plexuses and reabsorbed via arachnoid villi.

  • Vertebral Column: Protective structure comprising 33 vertebrae:

    • 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal.

Blood Supply in the Brain

  • Receives about 20% of cardiac output (800-1000 ml/min).

  • Maintains stable flow via autoregulation, primarily influenced by CO2 levels.

  • Major Vessels:

    • Internal carotid and vertebral arteries.

    • Circle of Willis: Provides alternate routes if major arteries obstructed.

Blood-Brain Barrier

  • Selectively restricts substances in blood from entering the brain, enforced by tight junctions between endothelial cells of capillaries.

  • Significance for drug therapy: Some drugs can cross the barrier while others cannot, affecting therapeutic effectiveness.

Peripheral Nervous System (PNS)

  • Comprises nerves arranged in fascicles, including:

    • Cranial Nerves: 12 pairs originating from the brain.

    • Spinal Nerves: 31 pairs correlating with vertebral levels; primarily mixed nerves containing both sensory and motor fibers.

Autonomic Nervous System (ANS)

  • Divided into sympathetic and parasympathetic systems.

  • Sympathetic System:

    • Mobilizes energy in stressful situations (fight or flight).

    • Preganglionic neurons arise from T1 to L2 regions.

  • Parasympathetic System:

    • Conserves energy and promotes restful functions (rest and digest).

    • Preganglionic neurons arise from cranial nerve nuclei and sacral region.

Aging and the Nervous System

  • Reduction in neuron count leading to decreased brain mass and function.

  • Accumulation of metabolic byproducts, including lipofuscin and neurofibrillary tangles, affecting neurologic responses and performance.

Pain Assessment and Types

  • Pain Definition: An unpleasant sensation unique to each person. "Pain is whatever the experiencing person says it is."

  • Nociception: The process of detecting harmful stimuli via nociceptors.

    • Types of Pain:

    • Acute Pain: Sharp and localized, arising from skin, joints, muscles (somatic) or organs (visceral).

    • Chronic Pain: Lasts longer than expected healing time, affecting psychological well-being.

    • Neuropathic Pain: Due to dysfunction in the nervous system, producing abnormal sensations.