Untitled Quiz

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary function of inhibitory neuronal pools like the basal ganglia?

Increase sensory sensitivity

Facilitate rapid reflex responses

Prevent overexcitation in sensory pathways (correct)

Enhance muscle contraction strength

Which physiological type of somatic sense is primarily activated by mechanical displacement?

Deep Sensation

Mechanoreceptive Sense (correct)

Thermoreceptive Sense

Visceral Sensation

What is synaptic fatigue primarily responsible for?

Increasing receptor sensitivity in overused pathways

Weakening synaptic transmission with prolonged excitation (correct)

Decreasing sensitivity in underused pathways

Blocking signal transmission to the cortex

Which type of tactile receptors are highly sensitive and rapid-adapting, found in the fingertips?

Meissner's Corpuscles Signup and view all the answers

What is the primary role of the somatosensory cortex's Layer IV?

Act as the primary entry point for sensory signals Signup and view all the answers

Which mechanism is involved in providing information on muscle tendon tension?

Golgi Tendon-Like Receptors Signup and view all the answers

What distinguishes the fast pain pathway from slow pain pathways?

Fast pain feels sharp and is felt almost immediately Signup and view all the answers

Where do sensory signals enter the spinal cord?

Through the dorsal roots Signup and view all the answers

Which characteristic is noted in the dorsal column-medial lemniscal system?

High temporal and spatial fidelity Signup and view all the answers

What clinical relevance does understanding dermatomes provide?

Helps localize spinal nerve issues Signup and view all the answers

How is referred pain perceived?

In a location distant from the actual source Signup and view all the answers

What phenomenon explains why visceral pain is often hard to localize?

Dual pain pathways complicating localization Signup and view all the answers

Which of the following accurately describes static position sense?

Conscious perception of body orientation Signup and view all the answers

Which area is considered sensitive and has extensive pain innervation?

Bronchi Signup and view all the answers

What type of pain is primarily elicited by mechanical or thermal stimuli?

Fast-sharp pain Signup and view all the answers

What is the function of the neospinothalamic tract?

To transmit fast-sharp pain signals Signup and view all the answers

Which neurotransmitter is primarily involved in the pain suppression mechanism?

Serotonin Signup and view all the answers

What effect does CO2 have on cerebral blood flow?

It induces vasodilation Signup and view all the answers

Which structure is primarily responsible for regulating visceral functions like gastrointestinal activity?

Brain stem reticular substance Signup and view all the answers

Which type of receptors are found at the synapses between preganglionic and postganglionic neurons?

Nicotinic receptors Signup and view all the answers

What is the primary role of the periaqueductal gray in the pain inhibition pathway?

Stimulating enkephalin release Signup and view all the answers

Which of the following is a characteristic of visceral pain compared to parietal pain?

It is often diffuse Signup and view all the answers

What happens to sympathetic tone after cutting a sympathetic nerve?

It decreases followed by intrinsic compensation Signup and view all the answers

What is the primary action of beta receptors in the autonomic nervous system?

Increased heart activity Signup and view all the answers

Which factor does not influence cerebral blood flow?

Blood glucose levels Signup and view all the answers

How does peripheral tactile stimulation provide pain relief?

Enhances Aδ fiber activity Signup and view all the answers

Which type of fibers transmit signals related to cold sensations?

Type Aδ fibers Signup and view all the answers

What is the primary mechanism through which the neuron sends output signals?

Axon Signup and view all the answers

What can happen to nerve impulses as they transmit information between neurons?

They can be integrated with other impulses. Signup and view all the answers

Which of the following accurately describes the synapse's function?

Ensure one-way signal flow. Signup and view all the answers

Which statement about neurotransmitter receptor types is correct?

Metabotropic receptors act through second messengers. Signup and view all the answers

What distinguishes Type A fibers from Type C fibers?

Type A fibers are myelinated and conduct signals quickly. Signup and view all the answers

What type of receptor primarily detects changes in temperature?

Thermoreceptors Signup and view all the answers

Which statement correctly describes the action of inhibitory postsynaptic potential (IPSP)?

IPSPs open chloride channels leading to hyperpolarization. Signup and view all the answers

What is the role of calcium influx in neurotransmitter release?

It triggers neurotransmitter release from vesicles. Signup and view all the answers

Which region is primarily involved in refining coordination and balance?

Cerebellum Signup and view all the answers

What type of summation involves multiple presynaptic terminals firing simultaneously?

Spatial Summation Signup and view all the answers

What function does the thalamus serve in the sensory information pathway?

It acts as a sensory relay station. Signup and view all the answers

Which mechanism is employed by inhibitory feedback circuits in the nervous system?

Inhibiting excitatory neurons at various stages. Signup and view all the answers

What is the resting membrane potential of a typical neuron?

$-65$ mV Signup and view all the answers

What is the primary role of astrocytes in cerebral physiology?

Releasing vasoactive metabolites for vasodilation Signup and view all the answers

Which physiological process occurs when oxygen supply is insufficient in the brain?

Vasodilation to restore blood flow Signup and view all the answers

Where in the brain is the distribution of blood capillaries highest?

Gray matter Signup and view all the answers

What mechanism helps protect cerebral capillaries from high blood pressure?

Arteriolar constriction Signup and view all the answers

What is the average normal pressure of cerebrospinal fluid (CSF)?

130 mm of water Signup and view all the answers

How much cerebrospinal fluid is produced daily?

500 mL Signup and view all the answers

What occurs during chronic hypertension in cerebral blood vessels?

Hypertrophic remodeling Signup and view all the answers

Which fluid characteristic ensures the brain can float within the cranial vault?

Equal specific gravity of brain and CSF Signup and view all the answers

Which ions are primarily involved in the active transport mechanisms of the choroid plexus?

Sodium ions Signup and view all the answers

Which of the following statements about the blood-brain barrier (BBB) is true?

It facilitates the transport of glucose and hormones. Signup and view all the answers

Which condition can disrupt the autoregulatory mechanisms of cerebral blood flow?

Chronic hypertension Signup and view all the answers

Which of the following components is involved in the flow pathway of cerebrospinal fluid?

Cisterna magna Signup and view all the answers

What is a potential consequence of impaired autoregulation in cerebral blood flow?

Increased risk of brain injuries Signup and view all the answers

Which characteristic of brain capillaries makes them less 'leaky' compared to other tissues?

Presence of tight junctions Signup and view all the answers

Study Notes

Organization of the Nervous System

The central nervous system (CNS) contains approximately 80 to 100 billion neurons.
A typical neuron receives signals through synapses on the dendrites and soma and sends output signals via a single axon.
Signals pass in a one-way direction across synapses, from the axon of one neuron to the dendrites of the subsequent neuron.
Information is carried as nerve impulses (action potentials) through sequential neurons.
Nerve impulses can be blocked, transformed, or integrated with other impulses for complex patterns.

Synapses

A synapse is the junction between neurons, allowing signal transfer and controlling the direction of signal flow.
Synapses can transmit signals readily or offer resistance.
Facilitatory signals enhance transmission, while inhibitory signals reduce or block transmission.

Physiologic Anatomy of the Neuron

The soma is the cell body of the neuron.
The axon extends from the soma to subsequent nerves, carrying output signals.
Dendrites branch out from the soma (up to 200,000 presynaptic terminals).

Synaptic Transmission

Presynaptic terminals form synapses with other neurons and can be excitatory (stimulating) or inhibitory (blocking).
Depolarization opens voltage-gated calcium channels, triggering neurotransmitter release from vesicles.

Effect on Neurotransmitter on Postsynaptic Terminals

Ionotropic receptors directly open ion channels.
Metabotropic receptors activate second messengers to alter cell functions.

Transmitter Actions

Cation channels allow sodium (Na+), sometimes potassium (K+) or calcium (Ca2+) influx; they are excitatory.
Anion channels primarily allow chloride (Cl-) ions influx; they are inhibitory.
Channel selectivity is determined by the size, shape, and charge of the channel.

Second Messenger Systems

G proteins bind GTP and separate into active components.
Active G proteins can open ion channels, activate cAMP or cGMP, activate enzymes for specific cellular functions, and trigger gene transcription leading to long-term changes.

Excitation and Inhibition

Opening sodium channels allows positive ions to enter, increasing membrane potential toward threshold.
Reduced chloride or potassium conductance makes the inside of the cell more positive.
Metabolic changes can increase excitatory receptors or decrease inhibitory receptors on the membrane.
Opening chloride channels allows negative ions to flow in, making the cell interior more negative.
Increased potassium conductance allows positive ions to exit, making the cell interior more negative.
Enzyme activation alters cellular functions to boost inhibition and reduce excitation.

Types of Synapses

Chemical synapses are predominant in the brain and CNS.
Chemical synapses release neurotransmitters to excite, inhibit, or modify the postsynaptic neuron.
Chemical synapses have one-way conduction.
Examples of neurotransmitters include acetylcholine, norepinephrine, GABA, serotonin, and glutamate.
Electrical synapses connect cytoplasms of adjacent cells via gap junctions.
Electrical synapses allow the free movement of ions and, therefore, electrical charge.
Electrical synapses have bidirectional transmission.
Electrical synapses are common in cardiac and smooth muscle.

Synaptic Transmitters

Small-molecule, rapidly acting transmitters trigger acute responses and are synthesized in the cytosol of presynaptic terminals and stored in recycled vesicles.
Neuropeptides, slowly acting transmitters induce prolonged effects and are synthesized in the cell body and transported to nerve terminals.
Co-transmission involves the storage and release of both small-molecule and neuropeptide transmitters by neurons.

Resting Membrane Potential

Resting membrane potential in neurons is approximately -65 mV.
The ion concentrations are high Na+ outside, low inside; high K+ inside, low outside; Cl- high outside, low inside due to the negative membrane potential.

Neuronal Excitation

Excitatory postsynaptic potential (EPSP) raises membrane potential (e.g., -65 mV to -45 mV).
The threshold for action potential is approximately -45 mV.
Action potentials initiate at the axon hillock where voltage-gated Na+ channels are dense.
Spatial summation involves activating increasing numbers of parallel fibers.
Temporal summation increases the frequency of action potentials within a single fiber.
Dendritic transmission primarily occurs via electronic conduction, with closer synapses to the soma having a greater effect.

Neuronal Inhibition

Presynaptic inhibition involves inhibitory transmitters acting on presynaptic terminals, reducing the excitatory effect.
Inhibitory postsynaptic potential (IPSP) arises from the opening of chloride channels and increased potassium conductance.
IPSPs counteract EPSPs, partially or fully canceling each other.
Hyperpolarization moves membrane potential further from threshold.

Special Characteristics of Synaptic Transmission

Synaptic fatigue occurs when the presynaptic terminals become depleted of transmitters.
Synaptic facilitation involves repeated signal transmission, strengthening synapses and making them more capable of transmitting similar signals.
Post-tetanic potentiation maintains an elevated state of excitability for several minutes.
Long-term potentiation is a form of facilitated transmission that lasts for a long period, possibly weeks, months, or even years.

Sensation

The nervous system initiates most activities in response to sensory experiences.
Sensory input can lead to immediate responses or be stored as memories for future reactions.
Sensory information from receptors travels through peripheral nerves to the CNS.
Information is relayed to multiple areas including the spinal cord, reticular substance, cerebellum, thalamus, and cerebral cortex.

Motor Function

Skeletal muscle contraction, smooth muscle contraction, and secretion of substances are regulated by the nervous system.
Muscles and glands are effectors, carrying out responses based on nerve signals.
Spinal cord, reticular substance, basal ganglia, cerebellum, and motor cortex are involved in controlling skeletal muscle contraction.

Major Divisions of the Central Nervous System

The spinal cord level manages reflexive and automatic responses, including walking, withdrawal reflexes, postural reflexes, and autonomic reflexes.
The lower brain (subcortical) level manages subconscious and autonomic functions, including emotional responses and basic life functions.
The higher brain (cortical) level acts as a memory storehouse and is essential for thought processes and precise control over lower brain functions.

Integrative Function of the Nervous System

One of the primary functions of the nervous system is to process sensory input to elicit appropriate mental and motor responses.
Most sensory information is discarded as irrelevant or unimportant.
Significant sensory inputs are directed to specific integrative and motor regions of the brain.
The process of filtering, channeling, and directing sensory information to evoke targeted responses is known as integrative function.

Storage of Information

Most sensory information is stored in the cerebral cortex, forming the basis of memory.
Repeated signal transmission strengthens synapses, making them more capable of transmitting similar signals in the future, enabling memories to be recalled even without sensory input.

Sensory Receptors and Neuronal Circuits

Mechanoreceptors detect mechanical compression or stretching in tissues.
Thermoreceptors sense temperature changes.
Nociceptors (pain receptors detect physical or chemical damage in tissues.
Electromagnetic receptors respond to light on the retina enabling vision.
Chemoreceptors detect chemical changes (taste, smell, O2 and CO2 levels, osmolality, etc.).

Differential Sensitivity

Some receptors have a low threshold and are activated by weak stimuli, while others require stronger stimuli.

Receptor Potentials

Receptors generate graded potentials that vary in magnitude depending on stimulus intensity.

Adaptation of Receptors

Most receptors adapt to continuous stimuli, reducing the frequency of action potentials.
Rapidly adapting receptors respond to changes in stimuli, while slowly adapting receptors respond to sustained stimuli.

Nerve Fibers

Type A fibers are large to medium myelinated fibers with high conduction velocity.
Type C fibers are small, unmyelinated fibers with low conduction velocity.

Summation

Spatial summation involves activating increasing numbers of parallel fibers, enabling stronger signals to recruit additional fibers.
Temporal summation increases the frequency of action potentials within a single fiber, allowing higher stimulus intensity to result in more frequent impulses.

Neuronal Pools

Neuronal pools are composed of varying numbers of neurons, each processing signals uniquely.
The stimulatory field is the area within a pool influenced by an incoming fiber, with the strongest influence on nearby neurons.
Divergence amplifies signal spread to an increasing number of neurons.
Convergence involves multiple inputs combining on a single neuron.

Stimulation and Inhibition

A single excitatory input rarely causes an action potential; summation of inputs is often required.
The discharge zone is the area where neurons are strongly stimulated and reach threshold.
The facilitated zone is where neurons are influenced but do not reach threshold.
The inhibitory zone dampens neuronal activity.
Reciprocal inhibition circuits enable coordination of antagonistic muscle pairs.

Afterdischarge

Afterdischarge is a prolonged output discharge from a neuron that continues after the initial signal ends.
Afterdischarge can last from milliseconds to several minutes.
Afterdischarge can be caused by postsynaptic potential, allowing a single brief input to produce a prolonged effect.

Continuous and Rhythmical Output in Neuronal Circuits

Intrinsic neuronal discharge involves neurons that emit impulses continuously.
Reverberatory circuits generate continuous impulses with input signals that can increase or decrease output.
Rhythmical signals are produced by reverberating circuits that create cyclical patterns, such as respiratory rhythm, heart rate, vascular tone, and digestive activity.

Stabilization Mechanisms in the Nervous System

Inhibitory circuits help stabilize neuronal activity.
Inhibitory feedback circuits inhibit excitatory neurons at the input or intermediate stages.

Inhibitory Neuronal Pools

Inhibitory neuronal pools help prevent sensory overexcitation.
The basal ganglia is a key example, controlling muscle activity.

Synaptic Fatigue

Prolonged neuronal stimulation leads to synaptic fatigue, reducing signal transmission.
This moderates overused pathways and enhances sensitivity in underused ones.
Long-term regulation involves downregulation of receptors with overactivity and upregulation with underactivity.

Somatic Senses

Somatic senses are related to the body and its environment.

Physiological Types of Somatic Senses

Mechanoreceptive senses detect mechanical displacement in the body, including tactile and position senses.
Thermoreceptive senses detect heat and cold.
Pain sense is triggered by tissue damage.

Other Classifications of Somatic Senses

Exteroreceptive sensations originate from the body surface.
Proprioceptive sensations relate to body position, muscle tension, and balance.
Visceral sensations arise from internal organs.
Deep sensations originate from deep tissues, including deep pressure, pain, and vibration.

Tactile Receptors

Touch, pressure, and vibration are sensed by tactile receptors.
Touch is detected near the skin surface, pressure in deeper tissues, and vibration through rapid repetitive signals.

Types of Tactile Receptors

Free nerve endings detect touch and pressure.
Meissner's corpuscles are highly sensitive, rapid-adapting, and found in fingertips and lips.
Merkel's discs are slow-adapting, grouped in touch domes, and help localize touch and texture.
Hair end-organs detect movement and initial contact, adapting quickly.
Ruffini's endings are slow-adapting, detect continuous deformation, and are found in skin and joint capsules.
Pacinian corpuscles are rapid-adapting and detect vibration and quick changes in pressure.

Nerve Fiber Types

Type Aβ fibers transmit rapidly (30-70 m/s) and are used by most specialized receptors.
Type Aδ and C fibers transmit slower and carry signals from free nerve endings.

Position Senses (Proprioception)

Position senses are related to the body's position and movement.
Static position sense is the conscious perception of body orientation and part positions.
Dynamic position sense (kinesthesia) is the awareness of movement speed and direction.

Mechanisms of Position Sensing

Joint angulation is determined by receptors providing angle and movement information.
Muscle spindles sense midrange joint angles and muscle stretch.
Deep receptors, like Pacinian corpuscles and Ruffini's endings, are important for extreme joint angles and detect ligament and deep tissue stretch.
Golgi tendon-like receptors provide information on muscle tension.

Somatosensory Cortex

The cerebral cortex consists of six layers of neurons, each with a specific role in sensory processing.
Layer IV is the primary entry point for sensory signals.
Layers I and II receive diffuse input for excitability control.
Layers II and III send signals across hemispheres through the corpus callosum.
Layers V and VI project to deeper brain areas, influencing signal transmission and thalamic activity.
Vertical columns specialize in specific sensory modalities within the somatosensory cortex.
Somatosensory area I (postcentral gyrus) is organized by body region, with higher density of receptors in the hands, lips, and face.
Somatosensory area II involves less precise localization.

Somatosensory Association Areas

Located in Brodmann's areas 5 and 7, they interpret and integrate sensory information from somatosensory area I.
This allows for deeper understanding of sensory input.

Effects of Damage to Somatosensory Association Areas

Amorphosynthesis results in difficulty recognizing complex objects or forms by touch and decreased awareness of the opposite side of the body.

Dermatomes

A dermatome is a specific area of skin innervated by a single spinal nerve.
These areas overlap significantly.

Clinical Relevance of Dermatomes

Dermatomes help diagnose nerve damage and sensory loss.
Specific patterns of sensory changes can localize spinal nerve or spinal cord issues.

Sensory Pathways to the Brain

Sensory signals travel to the spinal cord through the dorsal roots.
Two main pathways are the dorsal column-medial lemniscal system and the anterolateral system.

Functional Differences Between Sensory Pathways

The dorsal column-medial lemniscal system transmits information rapidly and precisely.
The anterolateral system transmits information less rapidly and less precisely.

Dorsal Column-Medial Lemniscal System

Fibers enter the spinal cord and divide into medial and lateral branches.
The medial branch ascends through the dorsal columns to the brain.
The lateral branch synapses locally in the dorsal horn, contributing to reflexes, dorsal columns, and spinocerebellar tracts.
Fibers ascend through the dorsal columns to the cuneate and gracile nuclei in the medulla, where they synapse.
Second-order neurons cross to the opposite side in the medulla and ascend via the medial lemnisci to the thalamus.
Third-order neurons project to the ventrobasal complex in the thalamus.
Final projection is to the somatosensory area I in the postcentral gyrus.
Spatial orientation is maintained throughout the pathway, with lower body fibers central and higher body fibers lateral.
Contralateral representation occurs in the thalamus.

Signal Transmission and Analysis in the Dorsal Column Pathway

Divergence occurs at each synaptic stage, increasing the number of neurons activated by stronger stimuli.
Two-point discrimination is tested to assess sensitivity to tactile discrimination (e.g., fingertips have fine discrimination).
Lateral inhibition enhances contrast by blocking lateral spread of excitatory signals.
Vibratory signal transmission is detected by Pacinian corpuscles for high frequencies and Meissner's corpuscles for lower frequencies.
Intensity is encoded by both the number of activated fibers and the impulse rate in each fiber, allowing for a wide range of stimulus intensities.

Anterolateral System

Carries pain, temperature, and crude touch information.
Less precise and spatially organized than the dorsal column pathway.

Pain

Pain is a complex experience involving physiological and psychological components.
Fast pain is felt within 0.1 seconds and is sharp, pricking, or electric, limited to superficial tissues.
Slow pain begins after 1 second and is burning, aching, throbbing, or chronic, occurring in both superficial and deep tissues.

Referred Pain

Pain perceived at a location distant from the actual source, often due to shared pathways between visceral and superficial pain fibers.
This is clinically important for diagnosing visceral ailments.

Visceral Pain

Pain originating from internal organs is often poorly localized due to lack of direct experience and dual pain pathways.
True visceral pain travels via autonomic nerves and is often referred to surface areas based on embryonic origin.
Parietal pain travels via local spinal nerves and is localized directly over the affected area.
Visceral pain is a critical indicator of inflammation, infection, or disease in internal organs.
Causes include ischemia, chemical damage, smooth muscle spasms, and overdistention.

Characteristics of Visceral Pain

Distinctive characteristics:
- Localized damage may not cause severe pain.
- Diffuse stimulation of pain fibers, like ischemia, can cause intense pain.
- Intermittent cramping pain can occur with smooth muscle spasms.

Insensitive and Sensitive Areas

Liver parenchyma and lung alveoli are insensitive to pain.
Liver capsule, bile ducts, bronchi, and parietal pleura are sensitive to pain.

Parietal Pain

Parietal surfaces include parietal peritoneum, pleura, and pericardium.
These surfaces are richly innervated with pain fibers, similar to skin.
When disease affects a viscus and spreads to parietal surfaces, pain is felt more sharply.
Parietal pain is well-localized, unlike diffuse visceral pain.

Fast-Sharp and Slow-Chronic Pain Pathways

Fast-sharp pain is elicited by mechanical or thermal stimuli and transmitted via type Aδ fibers at 6-30 m/sec.
Slow-chronic pain is primarily elicited by chemical stimuli (sometimes mechanical/thermal) and transmitted via type C fibers at 0.5-2 m/sec.
Double Pain Sensation: Immediate fast-sharp pain followed by delayed slow pain, prompting sustained relief.

Pain Signal Processing in the Spinal Cord

Pain fibers enter the spinal cord via dorsal spinal roots.
Fast-sharp and slow-chronic pain signals are processed separately, with relay neurons in the dorsal horns.

Dual Pathways for Transmission of Pain Signals

Neospinothalamic Tract: Transmits fast-sharp pain signals.
Paleospinothalamic Tract: Transmits slow-chronic pain signals.

Brain Processing and Arousal

Slow pain signals terminate in the reticular areas of the brain stem and intralaminar nuclei of the thalamus.
This causes strong arousal, keeping the brain alert, explaining difficulty sleeping during severe pain.

Role of Cerebral Cortex

Complete removal of somatic sensory cortex does not eliminate pain perception.
Lower brain centers (brain stem and thalamus) are essential for conscious pain perception.
The cortex helps in interpreting pain quality.

Clinical Intervention: Cordotomy

Cutting pain-conducting tracts (anterolateral quadrant) in the spinal cord to relieve intractable pain, typically for lower body pain.

Pain Suppression (Analgesia) System

Components: Periaqueductal gray and periventricular areas in the mesencephalon and pons, Raphe magnus nucleus and nucleus reticularis paragigantocellularis in the medulla, Dorsal horn inhibitory complex in the spinal cord.
Pain Inhibition Pathway: Signals from periaqueductal gray → raphe magnus nucleus → dorsal horns of spinal cord.
Serotonin release in the spinal cord stimulates enkephalin secretion, inhibiting pain signals.

Neurotransmitters and Natural Opiates

Enkephalin and Serotonin: Key neurotransmitters for pain suppression.
Natural opiate-like substances: β-endorphin, met-enkephalin, leu-enkephalin, and dynorphin.

Clinical and Practical Applications

Peripheral Tactile Stimulation: Rubbing skin near painful areas activates Aβ fibers, providing pain relief through lateral inhibition.
Electrical Stimulation Techniques: Electrodes on the skin, spinal cord, or brain regions (e.g., thalamus, periventricular, periaqueductal areas) for pain control.

Thermal Sensations

Gradations: Freezing cold, cold, cool, indifferent, warm, hot, burning hot.
Extreme temperatures can stimulate pain receptors, creating sensations of freezing cold or burning hot.

Types of Thermal Receptors

Cold Receptors: Type Aδ fibers, thinly myelinated, transmit signals at 20 m/sec. Some type C fibers may also function as cold receptors, suggesting a role of free nerve endings.
Warmth Receptors: Likely free nerve endings, transmit signals via unmyelinated type C fibers at 0.4 to 2 m/sec.
Pain Receptors: Activated by extreme cold and heat.

Thermal Receptor Adaptation

Cold receptors adapt significantly but not fully, responding more strongly to temperature changes than to constant exposure.

Transmission Pathways of Thermal Signals

Mechanism of Receptor Stimulation: Thermal receptors likely respond to temperature-driven metabolic rate changes that affect intracellular chemical reactions. Temperature change rather than direct effect on nerve endings likely causes stimulation.
Transmission of Thermal Signals: Thermal signals follow pathways similar to pain pathways.
Upon entering the spinal cord, they travel a few segments in Lissauer's tract and terminate in laminae I, II, and III.
Ascend in the anterolateral tract to the reticular formation and ventrobasal complex of the thalamus.
Some signals reach the somatic sensory cortex, though cortical removal reduces but doesn't abolish temperature discrimination.

The Autonomic Nervous System (ANS)

Regulates visceral functions such as arterial pressure, gastrointestinal activity, urinary bladder control, sweating, and body temperature.
Rapid and intense adjustments to internal organ functions.

Control Centers

Brain Stem Reticular Substance: Controls arterial pressure, heart rate, glandular secretion, gastrointestinal peristalsis, and urinary bladder contraction.
Hypothalamus: Influences brain stem centers and autonomic functions. Regulates body temperature, salivation, gastrointestinal activity, and bladder emptying.

Effect of Brain Stem Transection

Above midpontine level: Basal control of arterial pressure remains, but higher modulation is lost.
Below medulla: Arterial pressure drops significantly.

ANS Control

ANS is activated mainly by the brain stem and hypothalamus.
Cerebral cortex (especially limbic cortex) can influence autonomic control by sending signals to lower centers.
Visceral Reflexes: Sensory signals from visceral organs trigger reflex responses in autonomic ganglia, brain stem, or hypothalamus. Reflexes produce subconscious control of visceral organs.

Subdivisions

Sympathetic Nervous System (SNS): Prepares the body for "fight or flight" responses.
Parasympathetic Nervous System (PNS): Controls "rest and digest" functions, promoting relaxation and conservation of energy.

Organization of the SNS

Two Main Structures:
- Paravertebral sympathetic chains: Interconnected ganglia beside the spinal column.
- Prevertebral ganglia: Includes celiac, superior mesenteric, aorticorenal, inferior mesenteric, and hypogastric ganglia.
Sympathetic Nerve Origin: Sympathetic nerve fibers originate between T1 and L2 segments of the spinal cord. These fibers first enter the sympathetic chain before traveling to target organs.

Sympathetic Neuron Types

Preganglionic Neurons: Originate in the intermediolateral horn of the spinal cord.
Postganglionic Neurons: Synapse in ganglia and travel to target organs.

Types of Autonomic Receptors

Cholinergic Receptors:
- Muscarinic: Found on all effector cells targeted by postganglionic cholinergic neurons; uses G proteins.
- Nicotinic: Located in autonomic ganglia at synapses between preganglionic and postganglionic neurons; ligand-gated ion channels.
Adrenergic Receptors:
- Alpha Receptors: Alpha1 and Alpha2 receptors, linked to different G proteins.
- Beta Receptors: Divided into Beta1, Beta2, and Beta3, each with distinct effects.
- Norepinephrine excites primarily alpha receptors; epinephrine excites both alpha and beta receptors.

ANS Neurotransmitters

Acetylcholine Synthesis: Produced in terminal endings and varicosities. Rapid breakdown by acetylcholinesterase into acetate and choline after release.
Norepinephrine Synthesis: Begins in the axon terminal and finishes in secretory vesicles. Conversion Process: Tyrosine → Dopa → Dopamine → Norepinephrine → Epinephrine.
Removal: Reuptake into nerve endings (50-80%), Diffusion into surrounding fluids, Enzyme destruction (e.g., monoamine oxidase).

ANS Receptors

Receptors on effector cells bind acetylcholine, norepinephrine, or epinephrine, causing conformational changes in receptor proteins.
These changes excite or inhibit the cell through ion channel changes (altering membrane permeability) or enzyme activation (activating/inactivating enzymes inside the cell).

ANS Tone

Both sympathetic and parasympathetic systems are continually active, known as sympathetic tone and parasympathetic tone.
This allows a single nervous system to both increase and decrease the activity of a stimulated organ.
Sympathetic Tone: Keeps systemic arterioles constricted to about one-half their maximum diameter. Increased with heightened sympathetic stimulation or dilated with decreased stimulation.
Parasympathetic Tone: Background tone in the gastrointestinal tract. Surgical removal of parasympathetic supply can lead to serious and prolonged gastric and intestinal atony.

Denervation Hypersensitivity

Loss of tone occurs immediately after cutting a sympathetic or parasympathetic nerve, followed by intrinsic compensation returning function almost to normal.
After nerve destruction, the organ becomes more sensitive to injected norepinephrine or acetylcholine. Example: Blood flow in the forearm increases markedly post-sympathetic tone loss, then shows enhanced responsiveness to norepinephrine due to increased receptor numbers.

Mass Discharge of the SNS

Activates large portions of the sympathetic nervous system simultaneously.

Effects on Body

Increased arterial pressure, increased blood flow to active muscles and decreased blood flow to non-essential organs, increased cellular metabolism rates, increased blood glucose concentration, increased glycolysis, increased muscle strength, increased muscle activity, increased blood coagulation rate.

Sympathetic Stress Response

Provides extra activation of the body during mental or physical stress, commonly referred to as the fight-or-flight reaction.

Autonomic Reflexes

Sympathetic nerves to the adrenal medullae release epinephrine and norepinephrine into the bloodstream.
These hormones affect various organs similarly to direct sympathetic stimulation but last 5 to 10 times longer.

Dual Mechanism of Action

Both hormones are released simultaneously with sympathetic activation, providing a safety factor; one mechanism compensates for the other if needed.
They can also stimulate structures not directly innervated by sympathetic fibers.

Physiological Effects

Norepinephrine: Causes vasoconstriction of most blood vessels, increases heart activity, inhibits gastrointestinal activity, dilates pupils.
Epinephrine: Similar effects as norepinephrine but greater cardiac stimulation, causes weak vasoconstriction in muscle blood vessels, increases metabolic rate.

Cerebral Blood Flow, Cerebrospinal Fluid, and Cerebral Metabolism

Cerebral Blood Flow: Normal blood flow averages 50 to 65 mL / 100 g of brain tissue/min, totaling 750 to 900 mL/min for the entire brain. The brain receives 15% of the resting cardiac output despite constituting only 2% of body weight.

Arterial Supply

The brain is supplied by four large arteries: two carotid arteries and two vertebral arteries. These arteries merge to form the circle of Willis at the base of the brain.

Branching of Arteries

Arteries from the circle of Willis travel along the brain surface. They give rise to pial arteries that branch into smaller vessels called penetrating arteries and arterioles.

Penetrating Vessels

The penetrating vessels are separated from brain tissue by the Virchow-Robin space, an extension of the subarachnoid space. These vessels dive into the brain tissue, forming intracerebral arterioles that branch into capillaries for nutrient and gas exchange.

Regulation of Cerebral Blood Flow

Key metabolic factors influencing blood flow: CO2 concentration, Hydrogen ion (H+) concentration, O2 concentration, substances released from astrocytes.

Response to CO2

Increased CO2 leads to vasodilation; a 70% increase in arterial CO2 can double cerebral blood flow.

Cerebral Blood Flow Regulation

CO2 in blood forms carbonic acid, which dissociates into H+, causing vessel dilation.
O2 utilization remains stable at 3.5 mL O2/100 g of brain tissue/min.
Insufficient O2 supply results in vasodilation to restore blood flow.
Astrocytes release vasoactive metabolites that mediate local vasodilation in response to neuronal activity.

Cerebral Microcirculation

Capillary density in the brain is highest where metabolic needs are greatest.
Gray matter has a metabolic rate four times higher than white matter, leading to a higher capillary count and blood flow.
Brain capillaries are less leaky compared to those in other tissues.
Glial feet from surrounding glial cells provide physical support and prevent overstretching of capillaries under high pressure.
Arterioles leading to brain capillaries thicken in response to high blood pressure, maintaining constriction to protect capillaries.
Breakdown of these protective mechanisms can lead to serious brain edema, potentially resulting in coma and death.

Autoregulation of Cerebral Blood Flow

Arterial pressure fluctuates widely during daily activities.
Cerebral blood flow is autoregulated between arterial pressure limits of 60 and 150 mm Hg.
Chronic hypertension leads to hypertrophic remodeling of cerebral blood vessels, shifting the autoregulatory curve to higher blood pressures.
This adaptation partially protects against high blood pressure but increases vulnerability to ischemia if pressure drops too rapidly.
Impaired autoregulation makes cerebral blood flow more dependent on arterial pressure.
Preeclampsia can disrupt autoregulation, causing pressure-dependent increases in cerebral blood flow, edema, and seizures.
In old age, atherosclerosis, and various brain disorders, autoregulation impairment increases the risk of brain injury related to blood pressure fluctuations.

Cerebrospinal Fluid

The cranial vault has a capacity of approximately 1600 to 1700 mL, with 150 mL occupied by CSF.
CSF is present in the ventricles of the brain, cisterns around the brain, and the subarachnoid space surrounding the brain and spinal cord.
All chambers containing CSF are interconnected, maintaining a surprisingly constant pressure.

Function of Cerebrospinal Fluid

CSF cushions the brain within its solid vault.
The brain and CSF have similar specific gravity, allowing the brain to float in the fluid.

Formation and Flow of Cerebrospinal Fluid

CSF is formed at a rate of about 500 mL/day, with two-thirds originating from the choroid plexus in the ventricles.
The choroid plexus is a cauliflower-like growth of blood vessels covered by epithelial cells, actively transporting sodium ions to drive fluid secretion into the ventricles.
CSF has an osmotic pressure approximately equal to plasma, with similar sodium concentration, 15% greater chloride concentration, 40% less potassium concentration, and 30% less glucose concentration compared to plasma.
CSF flows from the lateral ventricles to the third ventricle, down the aqueduct of Sylvius to the fourth ventricle.
It exits the fourth ventricle through foramina into the cisterna magna, then into the subarachnoid space.
Arachnoidal villi are microscopic projections that allow CSF to flow into the venous sinuses, visible as granulations protruding from the sinuses.

Cerebrospinal Fluid Pressure

Normal pressure averages 130 mm of water (10 mm Hg), ranging from 65 mm to 195 mm of water in healthy individuals.
Arachnoidal villi function as valves for CSF flow.
CSF pressure must be about 1.5 mm Hg greater than venous blood pressure for flow to occur.

Blood-Brain Barrier

The blood-brain barrier exists between blood vessels and brain parenchyma.
It facilitates transport of certain hormones (e.g., leptin) into the hypothalamus.
It's highly permeable to glucose, hormones, CO2, O2, and most lipid-soluble substances, and slightly permeable to electrolytes.
It's almost impermeable to plasma proteins and most non-lipid-soluble large organic molecules.

Blood-Cerebrospinal Fluid Barrier

It has similar permeability characteristics to the BBB.
Tight junctions between endothelial cells limit permeability.

Brain Metabolism and Energy Supply

The resting brain accounts for 15% of total body metabolism despite being only 2% of body mass.
Metabolism per unit mass is about 7.5 times that of non-nervous tissues.
Neurons rely primarily on glucose for energy, with a limited capacity to store glycogen.
The brain can utilize ketone bodies as an alternative energy source during prolonged fasting or in conditions like diabetes.
Astrocytes play a crucial role in supplying neurons with lactate generated from glucose during high neuronal activity.
The brain can also use lactate as an energy source.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.