Nervous Tissue & System Subdivisions

Nervous Tissue Function

• The nervous system controls, regulates and communicates throughout the body.
• Neurons (nerve cells) send electrical/chemical signals cell to cell.

Subdivisions of Nervous System

• Central Nervous System (CNS): brain and spinal cord
• Peripheral Nervous System (PNS): all nerves and ganglia outside the CNS
• Nerve: bundle of nerve fibers (axons) in fibrous connective tissue.
• Ganglion: knot-like swelling in a nerve where neuron cell bodies of PNS are concentrated
• Sensory (afferent) division: signals from receptors (sense organs) to CNS
  • Visceral Sensory Division: delivers signals from viscera (heart, lungs, stomach, urinary bladder)
  • Somatic Sensory Division: delivers signals from skin, muscles, bones, and joints
• Motor (efferent) division: signals from CNS to effectors (glands and muscles)
  • Somatic Motor Division: signals to skeletal muscles, resulting in voluntary contraction/automatic reflexes
  • Visceral motor division (Autonomic Nervous System or ANS): signals to glands, cardiac/smooth muscle without voluntary control, called visceral reflexes.
    • Sympathetic Division: stimulates and prepares the body for action
    • Parasympathetic Division: has a calming effect.
    • Enteric plexus: nerves within digestive tract wall for digestive coordination/communication.

Neuron Categories

• Sensory (afferent) neurons: detect stimuli and transmit information toward CNS
  • Sensory Information + located not in CNS
• Interneurons: receive signals from other neurons, process data, and make decisions.
  • Lie entirely within CNS; most common functional type of neuron (about 90%)
• Motor (efferent) neurons: send signals out to muscles and gland cells (the effectors)

Neuron Parts and Structure

• Cell body: also called neurosoma or soma, contains nucleus/organelles.
• Neurites: Extensions reaching out to other cells
  • Dendrites: primary sites for receiving signals from other neurons
  • Axon or nerve fiber: long, cylindrical extension specialized for rapid conduction
    • Axon terminal: bulbus end of axon which forms a synapse with next cell
• Multipolar: one axon, multiple dendrites, most common in body
• Bipolar: one axon, one dendrite, found in retina/olfactory cells.
• Unipolar: axon leads away from cell body and splits in two; carries sensory info
• Anaxonic: many dendrites, but no axon; only in brain, retina, and adrenal gland

Glial Cells and Functions

• CNS:
  • Oligodendrocytes: form myelin sheaths in CNS
  • Ependymal cells: line brain cavities, secrete and circulate CSF
  • Microglia: macrophages that engulf debris and defend against pathogens.
  • Astrocytes: most abundant, with diverse functions like a framework for tissue, stimulate blood-brain barrier, adjust blood flow, secrete growth factors, influence synaptic signaling, and regulate tissue fluid

PNS

• Scwhann cells, or neurolemmocytes: envelop axon of PNS, form myelin sheath, and assist in regeneration of damaged fibers
• Satellite cells: surround nerve cell bodies in ganglia of PNS; provide insulation and regulate chemical enviornment

Myelin Sheath and Importance

• Myelin Sheath: spiral layers of insulation around an axon.
• Myelination: production of the myelin sheath
  • Schwann cells create it in PNS and oligodendrocytes create it in CNS Sheath consists of 20% protein and 80% lipid of the plasma membrane Begins during fetal development and continues rapidly through infancy, and is complete by late adolescence.

PNS vs CNS Myelination

• PNS: Schwann Cells wrap entire cell around the axon, One Schwann cell can only myleninate one axon
• CNS: Oligodendrocytes wrap appendages around axon; a single oligodendrocyte can myleninate many axons

Saltatory Conduction & Conduction Speed

• Saltatory conduction: nerve signal "leaps" between Nodes of Ranvier (myelin sheath gaps).
• Nerve signal speed depends on myelin and axon diameter.
• Myelin speeds up signal conduction
• Larger axons have more surface area and conduct signals more rapidly

Neuron Electrical Potential

• Voltage or electrical potential: the difference in concentration of charged particles between two points, measured in volts.
• Current- flow of charged particles from one point to another
• Polarization- something with electrical potential (voltage)
• Membrane potential- difference in electrical potential between interior and exterior of the cell
• Membrane Potentials:
  • Living cells have electrical potential (polarized) and can produce current Electrical currents creates flow of ions such as Na+/K+ through gated channels in the plasma membrane Gates open/close (ligand gated, voltage gated, etc.) This allows cells to turn electrical currents "on and off"

Resting Membrane Potential

• Resting Membrane Potential: the difference in charge across the plasma membrane at rest and typically measures -70mV.
• The resting membrane potential stems from:
  • Ions diffusing down their concentration gradient in leaky channels
  • Large protein anions inside the cell
  • The sodium-potassium (Na+/K+) pump
• Potassium (K+):
  • K+ wants to be -90mV; K+ wants to leave cell
  • K+ Channels VERY leaky
  • K+ has the largest influence on RMP!
• Sodium (Na+)
  • Na+ wants to be +60mV; Wants to enter cell
  • Channels are kind of leaky, and some Na+ enters cells
  • Makes the RMP a bit more positive
• Na+/K+ Pump
  • Kicks out 3 Na+ from cell and allows 2 K+ to enter cell, using 1 ATP
  • Helps maintain concentration gradients
• RMP is the net effect of K+ leaving the cell, Na+ entering the cell, and the Na+/K+ pump which continually pumps Na+ out and K+ in

Local Potential

• Local Potential: a temporary, short-range change in voltage
• Local Potential Characteristics:
  • Graded: magnitude varies with stimulus strength
  • Decremental: weaker the farther it spreads from the point of stimulation
  • Reversible: membrane voltage quickly returns to normal resting if stimulation ceases.
  • Excitatory or inhibitory: can make a neuron MORE or LESS likely to fire an action potential
  • Depolarization occurs, making the membrane potential less negative and closer to the threshold
  • Inhibitory Signals: local potentials result in hyperpolarization; the membrane potential becomes more negative and further from the threshold

Action Potential Generation

• If the excitatory signal reaches threshold, creating an action potential is fired
• Action potential- rapid up-and-down change in voltage from coordinated opening and closing of voltage-gated ion channels

Action Potential Characteristics

• All or none law- if threshold reached, neuron fires up to maximum voltage; if threshold not reached, it does not fire
• Nondecrimental- do not get weaker with distance
• Irreversible- once started, an action potential travels all the way down the axon; cannot be stopped

Action Potential Steps

• Local potential depolarizes membrane
• A threshold is reached (~55mV) and voltage gated channels open
• The neuron "fires the action potential. Na+ channels open quickly, more and more open.
• Voltage peaks at 35 mV. Around 0 mV, Na+ channels are inactivated and begin to close. K+ channels still opening slowly
• K+ are now wide open. K+ leaving the cell repolarizes the membrane
• Because K+ still open, membrane hyperpolarizes
• Na+ diffuses back into cell. Membrane back to RMP

Refractory Period

• Refractory Period: period of resistance to stimulation
• Absolute Refractory Period: no stimulus of any strength can trigger an AP, because Na+channels are inactivated
• Relative Refractory Period: only a very strong stimulus can trigger an AP, because Na+ channels can open, but K+ channels typically override

Message Transmission Between Neurons

• Electrical synapse: adjacent cells signal electrically through gap junctions
• Chemical Synapse: adjacent cells send signals with neurotransmitters across a synaptic cleft in Neuromuscular Junctions
  • Chemical synapses are the most common type of synapse
  • There are over 100 different neurotransmitters and they belong to different classifications
  • Neurotransmitter receptor binding determines its effect
  • A given neurotransmitter does not have the same effect everywhere in the body
• Synapse: point where an axon terminal meets the next cell (another neuron, gland cell, muscle cell)
• Presynaptic Neuron: neuron transmitting the signal and releasing the neurotransmitters
• Postsynaptic neuron: the neuron on the other side of the synaptic cleft that responds to the neurotransmitters

Synaptic Transmission Steps

• Action potential arrives at the end of axon of presynaptic neuron
• Presynaptic neuron releases neurotransmitter
• The neurotransmitter binds to receptors on the postsynaptic neuron
• The binding can have either excitatory or inhibitory effects
• The communication ends

What happens to neurotransmitters?

• When the communication ends, the presynaptic cell stops releasing neurotransmitter, clearing it from the synaptic cleft
• Neurotransmitter degradation: neurotransmitter is broken down by enzymes in synaptic cleft
• Reuptake: neurotransmitter is reabsorbed into axon terminal.
• Diffusion: neurotransmitter diffuses away from synapse

Neural Integration

• Neural Integration: the ability to process, store, and recall information, and use it to make decisions
• Chemical synapses allow for decision making
• Brain cells are highly connected, allowing for complex integration, like pyramidal cells of the cerebral cortex, which have about 40,000 contacts with other neurons
• Trade off: synaptic delays in chemical transmission makes information travel slower
• Excitatory Postsynaptic Potential (EPSP)- voltage becomes more positive and closer to threshold
• Inhibitory postsynaptic potential (IPSP)- voltage becomes more negative and further from threshold
• Summation: adding up postsynaptic potentials, determining the response
• Response depends on whether net input is excitatory or inhibitory.
• Balance between EPSPs and IPSPs enables nervous system "decisions."

Temporal + Spatial Summation

• Temporal summation: One synapse generates EPSPs quickly, and each is generated as the previous one fades allowing EPSPs to add up over time to a threshold that triggers an action potential.
• Spatial summation: EPSPs come from several different synapses and add up to threshold at an axon hillock, requiring simultaneous input from multiple presynaptic neurons for the postsynaptic neuron to fire.

Presynaptic Facilitation / Inhibition

• Presynaptic facilitation: one presynaptic neuron enhances another, increasing synaptic transmission, increasing likelihood of neuron firing an action potential.
• Presynaptic inhibition: one presynaptic neuron suppresses another, reducing/halting unwanted transmission, decreasing likelihood of neuron firing an action potential.

Spinal Cord Function

• Conducts nerve fibres to carry sensory/motor information up and down (information highway)
• Integrates spinal neurons to receive input, integrate, and execute output.
• Locomotion occurs with repetitions coordinated for walking.
• Reflexes- involuntary responses to stimuli are vital to posture, coordination, and protection

Ascending and Descending Tracts

• Ascending tracts: sensory information up to the brain: proprioception, deep touch, vibration, light touch, itch, temp, nociception, pressure, tickle, pain
• Descending tracts: motor information down from the brain; fine motor signals, gross motor signals, reflex turning head, posture and balance, and body response to tilt.
• Decussation- tracts that cross over from one side of the body to the other

Reflexes

• Reflexes: quick, involuntary, and stereotyped reactions of glands or muscle to stimulation
• Requires stimulation, is quick, is involuntary, and will be stereotyped

Reflex Arc

• Somatic receptors in skin, muscles, or tendons
• Afferent nerve fibers carry information to posterior horn of spinal cord/brainstem
• An integrating center, a point of synaptic contact between neurons in gray matter of spinal cord or brainstem
• Efferent nerve fibers carry motor impulses to muscles
• Effectors: muscles or glands that carry out the response

Reflex Types

• Flexor or withdrawal reflex: is the quick contraction of flexor muscle and relaxation of extensor muscles, resulting in withdrawal from injury.
• Stretch or myostatic reflex: muscle is suddenly stretched reacts by contrating and involves muscle spindles, stretch receptors embedded in the muscle
• Tendon reflex occurs when there is excessive tension on the tendon, the muscle does not contract as strongly as to not tear it and involves golgi tendon organs, proprioceptors located in tendons

Brain Lobes Anatomy

• Cerebrum - Frontal, occipital, parietal, temporal
• Cerebellum _ Brainstem - Midbrain, pons, and medulla oblongata

Higher Brain Functions

• Cognition: the range of mental processes to acquire and use knowledge, including sensory perception, thought, reasoning, judgement, memory, imagination, and intuition
  • Widely distributed regions of cerebral cortex (75% of brain tissue) are involved
  • Almost all lobes of cerebrum involved
• Memory (and amnesia)- special aspect of cognition including new info learning, retrieval, and elimination of info (forgetting)
  • Amnesia- is the inability to recall past events/form new memories
  • Associated areas- hippocampus: organizes cognitive info into unified long-term memory but does not hold the memory itself
• Emotion: anger, fear, aggression, pleasure, pain, love, arousal, motivation
  • Prefrontal cortex: seat of judgement, intent, and control over expression of emotions
  • Amygdala: receives sensory input and mediates raw emotional responses, especially fear
• Sensation: touch, pressure, stretch, movement, heat, cold pain
  • Primary somatosensory cortex (postcentral gyrus): awareness of stimulation
  • Somatosensory association areas: makes cognitive sense of stimulus
• Motor Control: plan, program, and execute skeletal muscle contraction
  • Motor association area (premotor area): intention and planning for movement
  • Primary motor area (precentral gyrus): execution of motor program, sends signals to muscles via spinal cord
  • Basal ganglia: onset and cessation of movement
  • Cerebellum: motor coordination
• Language: reading, writing, speaking, gesturing, sign language, and understanding words
  • Aphasia: language deficit whose classified type is affected by area and outcome - Wernicke area (posterior speech area): recognition of spoken and written language - Broca area (motor language area): generates motor program for speech to primary motor cortex

Sensory + Motor Homunculus

• Diagram of cortical region size correlated to areas it devotes to body region
• Sensory homunculus: shows amount of cortex dedicated to sensation
• Motor homunculus: shows amount of cortex dedicated to motor control

Autonomic Nervous System (ANS)

• Motor nervous system that controls glands, cardiac muscle, and smooth muscle and does NOT innervate skeletal muscle
• Responsible for visceral reflexes and operates regulating: HR, BP, Respiratory Airflow, Digestion, Defecation, Urination, Sexual Function, Energy Metabolism, Body Temperature, Pupillary Diameter

ANS Divisions

• Parasympathetic Division: reduces energy expenditure increases digestion and waste elimination; "Rest & Digest” - Location: craniosacral, with long preganglionic fibers and short postganglionic fibers - Main neurotransmitter released to targets: acetylcholine (Ach) - Effects: Local and more specific than the SNS
• Sympathetic Division: prepares body for action: exercise, trauma, arousal, competition, anger, fear, calling it the "Fight or flight". - Location: thoracolumbar, with short preganglionic fibers and long postganglionic fibers. - Main neurotransmitter released to targets: norepinephrine (NE) - Effects: widespread, general, and lasting longer than the PNS, and increasing stress hormones

Sympathetic/Parasympathetic Effects

     - Pupillary Size: Dilates (SNS) and Constricts (PNS)
     - Saliva: Thick Mucous secretion (SNS) and Thin (PNS)
      - Heart Rate: Increases (SNS) and Decreases (PNS)
     - Airflow to lungs: Bronchodilation (SNS) and Bronchoconstriction (PNS)
    - Glucose Production: Increases (SNS) and Decreases (PNS)
    - Digestion: Decreases (SNS) and Increases (PNS)
    - Release of epi/ne: Increases (SNS) and Decreases (PNS)
    - Urination: Increases (SNS) and Decreases (PNS)
   - Blood Vessels: Vasoconstriction (SNS) and Vasodilation (PNS)

Autonomic Tone Balance

The body shows constant activity from both ANS divisions at the same rate, indicating activity. Autonomic tone- normal background rate of activity that represents the balance of the two systems according to the body's needs

Autonomic Effects on Organs

• Sympathetic/parasympathetic fibers secrete different neurotransmitters depending on target cells (Para= ACh, Sympa = NE*)
• Receptors on target cells have different types

Autonomic Pathways

• It travels from spinal cord -> ganglion -> target
• Preganglionic fiber: first neuron in pathway. Cell body in the brainstem/spinal cord and its axon extends to autonomic ganglion and releases Acetylcholine (Ach) Postganglionic fiber: second (unmyelinated) neuron with its cell body in autonomic ganglion connecting to target cell.
  • Postganglionic sympathetic fiber : secretes NE - Postganglionic parasympathetic fiber: secretes ACh

Cholinergic + Adrenergic Receptors

• Cholinergic Receptor: binds acetylcholine (Ach) - Muscarinic receptor- found on cardiac muscle, smooth muscle, glands - Nicotinic receptor- found on autonomic ganglia, neuromuscular junction, adrenal medulla
• Adrenergic Receptor: binds norepinephrine (NE) - Alpha-adrenergic receptor- found on sympathetic effectors - Beta-adrenergic receptor- found on sympathetic effectors

Sense Organs

• Receptor: detects a stimulus
• Sense Organs: combines nerve tissue with other tissues to enhance stimulus response
• Fundamental Purpose: transduction converts stimulus energy (light, heat, touch, sound, etc.) into nerve signals

Sensation + Perception

• Sensation: sensory receptor detects stimulus, creates local electrical change
• Perception: conscious experience and interpretation of a stimulus. - Not all sensations get perceived as they are filtered before reaching the cortex

Receptor Transmission

• Modality- type of stimulus or the perception
• Location- where a stimulus is located
• Intensity- strength of stimulus
• Duration- how long the stimulus lasts

Receptive Fields

• Receptive Field- area within which a sensory neuron detects stimuli. Its size determines resolution.
• Neurons in fingertips have the smallest receptive fields allowing for fine two-point touch discrimination

Special Senses

• Gustation or taste: perceives molecules dissolved in water - Stimulus is tastants (chemical), receptors are taste cells (chemoreceptors) - Pathway: tastant > taste cell > sensory neuron > cranial nerves (VII, IX, X) > primary gustatory cortex and signals digestion that food is coming
• Olfaction or smell: sensitive to airborne chemicals in the nasal cavity - Stimulus are odorants (chemical), receptors are olfactory cells (chemoreceptors), and they each have their own for a chemical - odorant > olfactory cell > olfactory bulb (CN I) > primary olfactory cortex and sends in limbic system for memory/emotions
• Vision or light vision perception: objects environment emits/reflects - Stimulus: electromagnetic radiation (light) and the receptor: rods and cones (photoreceptors). - light > photoreceptors > sensory neurons > optic nerve (CN II) > optic tract > primary visual cortex to identify their location.
• Hearing or vibration perception: it vibrates vibrating air molecules - Stimulus: vibration in the air , receptor: hair cells in cochlea (mechanoreceptors) which is amplified in a fluid that activates - vibration > outer ear > middle ear > inner ear (cochlea) > hair cells > vestibulocochlear nerve (CN VIII) > primary auditory cortex
• Equilibrium or balance and movement: orientation of the body - Stimulus: acceleration (change in velocity) and the receptor: hair cells in vestibular apparatus (mechanoreceptors) full of fluid enclosed that moves cells when they accelerate. - Acceleration > hair cells in vestibular apparatus > vestibulocochlear nerve (CN VIII) > vestibular cortex and cerebellum

Endocrine System

• Glands, tissues, and cells that secrete hormones.
• Hormones are chemical messengers in the bloodstream that stimulate physiological responses in other tissues/organs.

Nervous vs Endocrine Systems

Target Cell

• They have receptors for a specific hormone to responds to it.
• Hormone released in blood and target cell might be far away as cells are specific and saturated

Hormone Secretion Intervals

• Time Intervals
• Daily (Circadian Rhythm): melatonin + cortisol
• Monthly Rhythm: ovarian hormones
• Response to Stimuli
• Neural-Neurons
• Hormonal- other hormones
• Humoral- stuff in blood

Half-Life

• Metabolic clearance rate (MCR)- rate of hormone removal from the blood
• Half-life: time required to clear 50% of hormone from the blood

Hormone Classes

• Monoamines:
  • Single amino acids
  • Catecholamines (dopamine, epinephrine, norepinephrine), thyroid hormone
• Peptides:
  • Chain of amino acids
  • Most common type of hormone
  • Insulin is large peptide hormone
• Steroids:
  • Derived from cholesterol (lipids)
  • Sex steroids (estrogen, testosterone), cortisol
  • NEED TRANSPORT PROTEIN unless it's RECEPTOR INSIDE CELL

Transport Proteins

• Hydrophilic proteins from the liver binding for transport
• They can be found either bound, or unbound and free

Hormone Receptors

• Located on surface or in the cell
• On surface triggering fast responses from signaling, vs intracellular creating cell changes.
• Second messenger molecules are used to relay receptors surface signals to molecules in cell to activate intracellular and metabolic and enzymes triggering large effects.

Endocrine Organs

• Hypothalamus
• Anterior pituitary gland
• Thyroid gland
• Parathyroid gland
• Adrenal gland
• Pancreas

Hypothalamus + Pituitary

• Hormone release triggered by other hormones and their location
• The Hypothalamus regulates homeostatic functions, carried out by pituitary gland as triggered by Thyrotropin releasing hormone (TRH which creates thyroid stimulating hormone (TSH)).
• Pituitary anterior is linked by blood vessels and release _ Thyroid stimulates for thyroid hormones _ ACTH promotes cortisol _ Growth hormone for mitosis
• Pituitary posterior linked by nerves

Growth Hormone

• Promotes tissue growth with widespread bodily childhood adolescence _ Gigantism occurs as before epiphyseal plates are depleted causing accelerated growth _ Pituitary with dwarfism from stunted childhood growth _ Acromegaly later causes thickened bones

Thyroid Function

• The large gland that surround trachea and Releases: Thyroid Hormone (TH4), increasing metabolic rate, heat production, appetite, and alertness around body and regulated feedback.
• Hormone-Cascade:
• Hypothalamus Releases TRH > Anterior Pituitary Releases TSH > Thyroid Releases TH
• Hypothyroidism- inadequate with the decreased metabolism weight fatigue by tumor or lesion
• Called Hashimoto's
• Hyperthyroidism- excessive with increased metabolism sweating.
• Called Graves

Parathyroid

• Four small glands in the thyroid gland

• Releases parathyroid hormone (PTH) increases calcium levels

• Hypoparathyroidism in rapid calcium decline, is fatal

• Hyperparathyroidism: excess PTH in bones malformation

Adrenal

• Large glands
• With it's medualla and cortex is the response to stress
• Its release and effect is done through the Hypothalamus releasing CRH > Anterior Pituitary Releasing ACTH to trigger release of cortisol. Which shows Axis dysregulation with issues to hormone cascades.

Illnesses

• Addison's Disease where hyposecretion and side effects appear such as that treated can fatal ones untreated
• Cushing Syndrome side effects show with excess cortisol.

Insulin + Glucagon Action

• An opposite effects -Secreted after meals which stimulates absorb stores outside blood
• Glucagon increases released during meal glucose _ Glucose which results when its secreted in bloodstream

Diabetes Mellitus

• Diabetes mellitus (DM)- is from hypo/hypersecretion disruption
• With classical signs which include: _ Hyper _ Poly _Phagia

Diabetes Differences

• Type I: - Caused by Insulin _5 with cellIrreversible _Insulin
• Type II - Resulted byInsulin _90 canCell _Can Insulin