CNS Physiology Study Notes

• Study notes on central nervous system (CNS) physiology

Central Nervous System Components

• The CNS includes the motor cortex, caudate nucleus, putamen, globus pallidus, red nucleus, tectum, reticular formation, pons, vestibular nucleus, and medulla oblongata
• It also consists of the thalamus, hypothalamus, superior colliculus, inferior colliculus, and cerebellar nuclei

Brain Regions and Development

• Telencephalon develops into cerebral cortex, basal ganglia, hippocampus, and amygdala, and constitutes the forebrain
• Diencephalon develops into the thalamus and hypothalamus, part of the forebrain
• Mesencephalon becomes the midbrain, including the tectum and tegmentum
• Metencephalon develops into the pons and cerebellum, and is part of the hindbrain
• Myelencephalon develops into the medulla, also part of the hindbrain

Nervous System Functions

• Coordinates activities of other systems with the endocrine system through senses and responses to maintain homeostasis via sensory and motor functions
• Stores experiences as memory and establishes patterns of response based on prior learning

Functional Levels of the CNS

• The sensory-somatic peripheral nervous system mediates communication between the external environment and the CNS, while the autonomic peripheral nervous system mediates communication between the internal environment and the CNS
• Spinal Cord Level: Acts as a conduit for signals between the body's periphery and the brain, and houses reflex control centers
• Lower Brain Level (Subcortical Level): Controls subconscious activities like arterial pressure, respiration, equilibrium, and emotional patterns in the medulla, pons, epencephalon, hypothalamus, thalamus, cerebellum, and basal ganglia
• Higher Brain Level (Cortical Level): Converts lower CNS functions into precise operations; the cerebral cortex acts as a large memory storehouse and is essential for thought processes in association with lower CNS centers

Neuronal Pools

• Each possess unique organizational characteristics to process signals
• CNS consists of separate neuronal pools, each with afferent (incoming) and efferent (outgoing) nerve fibers
• Input signals can excite, inhibit, or facilitate pool neurons
• Neuronal pools process signals sequentially (serial) or simultaneously (parallel), amplify signals (amplification; divergence), and may generate prolonged output discharges (after-discharge)

After-Discharge Mechanisms

• Synaptic after-discharge: Long-acting synaptic transmitter substances discharge on postsynaptic neuron surfaces
• Parallel circuit after-discharge: Input signals spread through a series of neurons, with impulses converging on an output neuron
• Reverberatory circuit after-discharge: Excited neurons in a pool re-excite themselves. These systems are seen during respiration, where the medulla's inspiratory neuronal pool is active for about 2 seconds per respiratory cycle, and may also be involved in maintaining wakefulness

Stabilization of Neuronal Circuits

• Inhibitory mechanisms inhibit signal transmission and prevent unrestrained re-excitation
• Some neuronal pools in basal ganglia exert broad inhibitory control over motor control systems

Inhibitory Mechanisms in the CNS

• Presynaptic Inhibition: Inhibition occurs at the presynaptic neuron before the signal reaches the synapse
• Postsynaptic Inhibition: Inhibition occurs due to IPSP generation at the postsynaptic membrane or synaptic fatigue
• Anatomical Inhibition: Can occur via lateral or recurrent inhibition within the CNS

Presynaptic Inhibition Mechanisms

• Chloride and Potassium Ion Channels: Inhibitory neurons release GABA, opening Cl- and K+ channels at the presynaptic terminal, leading to local hyperpolarization and reduced action potential voltage
• Calcium Channels: Some inhibitory neurons release neurotransmitters like enkephalin that block Ca2+ channels

Adjustment of Pathway Sensitivity

• The nervous system adjusts sensitivity by using two mechanisms
• Fatigue Mechanism: Overused pathways become fatigued, reducing sensitivity; underused pathways rest and increase sensitivity for short-term adjustment
• Receptor Regulation: Synaptic receptors are downgraded with overuse and upgraded with underuse for long-term adjustment

Somatosensory Functions

• Somatosensory system is defined as the sensory system associated with different parts of the human body
• Input to the NS is provided by the sensory receptors that detect sensory stimuli
• Sensory receptors are specialized neurons that transduce environmental signals into action potentials

Sensory Receptor Types

• Mechanoreceptors: Detect mechanical deformation, examples are tactile sensations hearing, equilibrium, position sense
• Thermoreceptors: Detect temperature changes to hot and cold
• Pain Receptors (Nociceptors): Detect tissue damage resulting from physical injury or chemical damage
• Electromagnetic Receptors (Photoreceptors): Detect light on the retina
• Chemoreceptors: Detect taste, smell, O2, CO2, and chemistry of body fluids, as well as osmolality, and pressure

Senses Classification

• Somatic Senses: Sensations from skin, muscles, tendons, and joints with specific receptors for touch, pressure, tickling, itch, vibration, stereognosis, position, pain, and temperature
• Special Senses: Complex sensations require specialized sense organs for vision, smell, taste, hearing, and equilibrium (rotational and linear acceleration)
• Visceral Senses: Detect internal environment conditions: osmolarity, pH, body fluids chemistry, and pressure

General Properties of Receptors

• Sensitivity: Each receptor is designed to be highly sensitive to one type of stimulus (or particular type of energy)
• Specificity: Nerve fibers transmit only one modality of sensation
• The CNS determines the sensation type and site based on the fiber's destination
• Generator Potential: A receptor produces a receptor potential varies depending on its type
• The brain can recognize the intensity of the stimulus that is transmitted to it by:
  • Variation in the frequency of the action potential generated by the activity in a given receptor (called temporal summation, or frequency coding)
  • Variation in the number of receptor activated (called spatial summation or population coding).

Receptor Adaptation

• Involves a progressive reduction of receptor response over time with constant stimulation
• Tonic receptors: adapt slowly and continue to transmit impulses for extended periods
• Phasic receptors: adapt rapidly and are stimulated only upon stimulus intensity changes

Sensory Unit

• A single sensory axon with its branches forms the sensory unit
• As stimulus intensity increases, recruitment of sensory units occur
• One type of receptor can overlap the sensory units of other types in the skin

Tactile Sensations

• Encapsulated mechanoreceptors are specialized to receive tactile information: Meissner's & Pacinian corpuscles, Merkel's disks, and Ruffini's corpuscles
• Touch and pressure: conveyed by rapidly adapting and slowly adapting receptors
• Located most numerously on the skin of fingers and lips and hairless skin, and conveyed by type A nerve fibers
• Itch and tickle: caused by mild stimulation, conveyed by type C nerve fibers
• Vibratory sensation: detected by all tactile receptors, conveyed by type A fibers
• Stereognosis: touch that is essential for perception of form, shape, and spatial nature of objects in the palm and fingertips

Position or Proprioceptive Sense

• Conveys sensations about physical state via type A nerve fibers including position and movement
• Includes sensory input from tendons, muscles, joint capsules, ligaments, skin, deep tissues, and equilibrium and can be divided into static (orientation) and dynamic (kinesthesia) subtypes

Pain Sensation

• A protective experience with actual or potential tissue damage
• Acute Pain: sharp, results from tissue damage and is fast
• Chronic Pain: burning/throbbing, related to persistent issues and unidentifiable
• Those born without the sense of pain tend to injure themselves

Acute Pain

• Occurs within 0.1 sec after a pain stimulus
• In skin and can be highly localized (superficial)
• Transmitted through type Ad pain fibers and blocked by moderate compression
• Glutamate may be the neurotransmitter
• 1st order neurons terminate mainly in lamina I then excite second order neurons, eventually terminating in the thalamus
• Evokes a withdrawal reflex and sympathetic response

Chronic Pain

• Occurs more slowly and is felt in skin and internal tissue (deep)
• Transmitted through type C pain blocked by local anesthetics
• Substance P is the neurotransmitter, and its release is inhibited by opioids
• 1st order neurons terminate in lamina II and III, then 2nd order in reticular formation and hypothalamus
• Produces nausea, sweating, blood pressure lowering, and reduced muscle tone

Types of Pain Receptors

• Nociceptors include mechanosensitive, thermosensitive and chemosensitive receptors
• Chemo-sensitive respond to chemicals released by injured sites such as substance P, lactic aid and acids

Referred Pain

• Felt in an area remote from the causal tissue
• Visceral pain fibers synapse with second order neurons shared with skin pain fibers
• Pain areas follow dermatomal, brain interpretation, and facilitation rules

Visceral Pain

• Transmitted via type C nerve fibers in sympathetic or parasympathetic nerves
• Viscera have few pain receptors resulting in poor localization
• Pain is caused by diffuse stimulations of nerve endings due to ischemia, damage, smooth spasms or stretching of ligaments

Central Inhibition of Pain

• Is affected by psychological factors like mood and emotional state
• Varies with genetic makeup, culture, age, gender, and controllability of pain signals via analgesia system, as well as stimulation of large sensory fibers

Analgesia System

• Composed of periaqueductal gray neurons, raphe magnus nucleus neurons, and pain inhibitory complexes in the spinal cord
• Involves secretion of enkephalin to inhibit pain signal transmission
• Opiate receptors in analgesia system interact with morphine-like neurotransmitters found in the brain

Peripheral Sensory Fiber Stimulation

• Minimizes pain via gate control theory and by releasing neurotransmitters like serotonin

Thermal Sensations

• Via cold and warmth receptors in the skin
• Cold receptors respond best to temperatures slightly below body temperature, warmth receptors respond best to temperatures slightly above
• Thermal receptors respond notably to temperature changes

Sensory Pathways

• Dorsal Column: Carries fine touch/pressure, vibration, stereognosis, and conscious proprioception
• Anterolateral Pathways: Carries crude touch/pressure, pain, thermal, tickle/itch, and sexual sensations

Spinal Cord Lesions

• Posterior Column Lesions: Loss of ipsilateral position/vibration
• Spinothalamic Tract Lesions: Impaired pain/temperature appreciation on the contralateral side
• Lesions in the Brain stem: all sensation contralateral side

Spinocerebellar Pathways

• Carry unconscious proprioceptive information
• Damage to these will damage specific kinesthetic sensations

NOTE

• Proprioceptive sensations are carried by either the Lemniscal pathway or spinocerebral tract

Cerebral Cortex Layers

• Cortical neurons are arranged in six layers, each performing unique functions
• Layer IV receives incoming sensory signals, while layers I and II receive diffuse input from lower brain centers
• Layers II and III send axons to related cortical portions
• Layers V and VI project to deeper and more distant nervous system parts

Corpus Callosum Role

• Integrates activity of the two cerebral hemispheres
• Cross-connections permit each brain hemisphere to be updated activities of other, allowing integrative brain function across both hemispheres

Cerebral Cortex

• Sensory info interpreted in three areas
  • Primary sensory
  • Sensory associative
  • Wernicke's
• Each has spatial location, only analyze simple sensations and inform to brain the sensory signal that arrived at cortex
• Cortical lesions result inability of discretely localize sensations, and judge stimulus characteristics
• Bordered by sensory association areas for higher level processing
• Somatic sensory association areas located in parietal cortex and play an imp