Central Nervous System & Brain Anatomy

• The study notes cover the physiology of the central nervous system, sensory receptors, somatosensory function, and more

Brain Anatomy

• Motor cortex, caudate nucleus, putamen, and globus pallidus are key structures
• Red nucleus, tectum, reticular formation, pons, and vestibular nucleus have distinct roles
• Medulla oblongata connects to the spinal cord
• Thalamus and hypothalamus are central relay and regulatory centers
• Superior and inferior colliculi are involved in sensory processing
• Cerebellar nuclei connect to the cerebellum

Brain Regions and Development

• Telencephalon includes the cerebral cortex, basal ganglia, hippocampus, and amygdala
• Diencephalon contains the thalamus and hypothalamus
• The mesencephalon is the midbrain
• Metencephalon consists of the pons and cerebellum
• Myelencephalon is the medulla oblongata
• Forebrain comprises the telencephalon and diencephalon
• Hindbrain includes the metencephalon and myelencephalon

Nervous System Organization

• Central nervous system (CNS) comprises the brain and spinal cord
• Peripheral nervous system connects the CNS to the rest of the body
• Autonomic nervous system controls involuntary functions via sympathetic and parasympathetic branches
• Somatic nervous system controls voluntary movements and sensory information

Nervous System Functions

• The nervous system coordinates activities through sensory input and motor responses
• Along with the endocrine system it maintains homeostasis
• The nervous system forms memory, and response patterns based on prior experiences
• The sensory-somatic nervous system relays external environment info to the CNS
• The autonomic nervous system relays internal environment info to the CNS
• The CNS has three functional levels: spinal cord, lower brain, and higher brain (cortical)

Spinal Cord Function

• The spinal cord acts as a conduit for signals between the periphery and the brain
• It houses reflex control centers controlled by higher brain levels

Lower Brain Function

• The lower brain controls subconscious bodily activities such as arterial pressure, respiration, and equilibrium
• Activities in the lower brain also include feeding reflexes and emotional patterns

Cortical Brain Functions

• The cerebral cortex converts lower CNS functions into precise operations
• It stores memory and is crucial for association with lower CNS centers

Neuronal Pools

• Neuronal pools, or nuclei/centers, are collections of intercommunicated neurons
• Each neuronal pool has special characteristics of organization that cause it to process signals uniquely
• Examples of neuronal pools include the basal ganglia, thalamic nuclei, and cerebellum
• Thousands of separate neuronal pools make up the CNS
• Each neuronal pool receives input (afferent) and sends output (efferent) via fiber tracts
• Input signals can excite, inhibit, or facilitate neuronal pool neurons
• The neuronal pool may pass signals sequentially (serial processing), simultaneously (parallel processing) amplifies input signals

Signal Transmission

• Signals can be transmitted to one or more directions (divergence) in neuronal pools
• A pool may send excitatory signals in one direction and inhibitory signals elsewhere
• Neuronal centers summate converging multiple incoming signals
• Prolonged output discharge (after-discharge) happens even after input signal ends

Mechanisms of Signal Discharge

• Synaptic after-discharge occurs when excitatory synapses discharge long-acting transmitter substances on postsynaptic neurons
• Parallel circuits for after-discharge involve prolonged converging impulses on an output neuron
• Reverberatory circuits for after-discharge occur when excited neurons re-excite themselves
• Inspiration during respiration exemplifies reverberatory circuits
• Continuous reverberation within the brainstem might sustain wakefulness

Inhibitory Mechanisms in Neuronal Circuits

• Stabilization of neuronal circuits prevents continuous re-excitation and uncontrolled signals
• Inhibitory mechanisms involve presynaptic and postsynaptic inhibition
• Presynaptic inhibition involves opening Cl and K ion channels or blocking Ca channels at the presynaptic terminal

Presynaptic Inhibition

• Inhibitory neurons release GABA which opens Cl and K ion channels causing hyperpolarization
• This reduces the voltage of incoming action potentials and transmitter release
• Some neurons secrete enkephalin blocking Ca channels, and transmitter release.

Postsynaptic Inhibition

• Generation of IPSPs at the postsynaptic membrane
• Synaptic fatigue reducing the signal Progressive weakening during prolonged excitation
• Refractory periods

Anatomical Inhibition

• Lateral inhibition involves collateral fibers synapsing with inhibitory neurons to sharpen signal pathways
• Recurrent inhibition utilizes collateral terminals to excite inhibitory interneurons, inhibiting the initial excitatory neuron

Adjustment of Pathway Sensitivity

• Fatigue mechanism provides short-term adjustment, reducing sensitivity of overused pathways
• Downgrading and upgrading of synaptic receptors provide long-term adjustment

Sensory Receptors

• Somatosensory system includes sensory receptors associated with different body parts
• Sensory receptors transduce environmental signals into neural signals (action potentials)

Types of Sensory Receptors by Stimulus

• Mechanoreceptors detect mechanical deformation, including tactile sensations, hearing, equilibrium, and position
• Thermoreceptors detect temperature changes (cold and warmth)
• Nociceptors detect tissue damage (physical or chemical)
• Electromagnetic receptors (photoreceptors) detect light
• Chemoreceptors detect taste, smell, O2 & CO2 concentrations, and osmolality

Clinical Classification of Senses

• Somatic senses arise from skin, muscles, tendons and joints, responding to specific stimuli.
• Somatic senses include tactile, position, pain, and thermal sensations
• Special senses include vision, smell, taste, hearing, and equilibrium
• Visceral senses perceive the internal environment, such as osmolarity, pH and pressure

General Properties of Sensory Receptors

• Receptors have high sensitivity to a specific stimulus (or energy type)
• Adequate stimulus refers to the stimulus with the lowest threshold for detection
• Nerve fibers are specialized to transmit one modality of sensation (labeled line)
• Sensation type and site depend on the CNS point to which the fiber leads

Receptor Potentials

• Receptors generate receptor potentials (generator potentials) varying with the receptor type
• Second messenger systems can be activated, or ion permeability can be altered
• The local graded change in transmembrane potential is considered the receptor potential
• Receptor potential causes depolarization, except in photoreceptors, where it causes hyperpolarization

Stimulus Intensity and Impulse Generation

• Action potentials are elicited as the receptor potential reaches a threshold
• Impulse rate is proportional to the stimulus intensity
• Directly proportional to lower intensities - less steep when intensities are higher
• The brain recognizes intensity via temporal summation (frequency coding) and spatial summation coding

Adaptation of Sensory Receptors

• Adaptation or desensitization: receptor response progressively declines receptor to continuous stimulation
• Adaptation time varies by receptor type -- Tonic receptors adapt slowly and incompletely -- Phasic receptors adapt rapidly and completely

Sensory Unit Concept

• A single sensory axon with its branches defines the sensory unit
• Response occurs in the receptive field Increased stimulus intensity leads to sensory Recruitment ( of sensory unit )
• Sensory units overlap Increased stimulation can influence adjacent sensory units

Tactile Sensations

• Mechanoreceptors specialized to receive tactile information -- Touch, pressure, itch, tickle, vibration, all are part of this sensation family -- Meissner's corpuscles, Pacinian corpuscles, Merkel's disks, and Ruffini's corpuscles are key mechanoreceptors

Tactile Subsensations:

• Touch and pressure: most numerous where sensitivity is high (finger tips) ----Mediated by type A nerves as
• Itch and tickle: Mild stimulation -- mediated by type C nerves ----Scratching: blocks transmission - lateral inhibition
• Vibratory: Mediated by type A nerves
• Stereognosis: Tactile sensors in the fingertips -- the sense of touch that is essential for perception of form, shape

Position Sense

• Sensations of orientation in the body in space
• Physical state of the body is carried by type A nerves Static (conscious) and Dynamic (kinesthesia)

Pain

• Pain: An unpleasant experience related to actual or potential tissue damage ---Acute (sharp) and chronic (burning, aching.)

Acute and Chronic pain qualities

• Acute results from tissue damage ---A sharp prick
• Chronic Persists after tissue damage repair occurs

Other Pain Qualities

• Referred Pain: Body remote from tissue injury hurts Dermatomal rule: Pain usually referred to associated development areas Facilitation effect: Visceral stimulus lowers pain threshold Visceral Pain: Mediated by Type C nerves

Visceral Somatic Sensory Qualities

• The viscera lack pain receptors generally Severe stimuli: include ischemia of visceral tissue, smooth muscle spasm, distension
• Terminating gray region neurons, then send message along spinothalamic tract to thalamus

Pain Control Mechanisms of The Brain

• Mediators: psychological vs neurological

Spinal control of pain pathways

• Stimulation blocks passage of incoming pain signals to brain
• Large tactile receptor fibers are used for control of signals

Thermal Sensations

• Two types

• --1: Cold sensitive receptors

• --2: Warmth sensitive receptors

Both types of receptors are active in body Signals all enter the spinal column- enter into brain stem by various routes Dorsal column enters the spinal cord for entry to medulla

Sensations in Spinal Cord

Signals carried, and split routes Dorsal route: Fine touch and pressure, vibratory, stereognosis, proprioception Anterolateral system: crude touch and pressure, general signals of pain, itching