Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS)

• Part of the Peripheral Nervous System (PNS) that functions autonomously and continuously without conscious effort.
• Controls visceral activities by regulating smooth muscles, cardiac muscles, and glands.
• Maintains homeostasis by overseeing heart rate, blood pressure, breathing rate, and body temperature.
• Responds to emotional stress and prepares the body for strenuous physical activity.
• Regulated by reflexes initiated by sensory signals from receptors in the viscera and skin.

Autonomic Motor Pathway

• Consists of two autonomic motor neurons in a series with a visceral effector.
• The preganglionic neuron extends from the Central Nervous System (CNS) to an autonomic ganglion.
• The postganglionic neuron extends from the autonomic ganglion to the effector organs, like, smooth muscle, cardiac muscle, or a gland.
• A ganglion is a cluster of neuronal cell bodies in the PNS.
• The dendrites of a preganglionic neuron are found in the CNS.

Organization of Parasympathetic and Sympathetic Nervous Systems

• The parasympathetic nervous system originates in the brain stem and sacral region of the spinal cord.
• The sympathetic nervous system originates in the thoracic and lumbar regions of the spinal cord.

Neuroeffector Junction

• Synapse between an autonomic postganglionic neuron and a visceral effector that differs from neuron-to-neuron synapses.
• Axon terminals of postganglionic neurons lack synaptic end bulbs, and instead have swollen regions called varicosities containing synaptic vesicles with neurotransmitters.
• Receptors for neurotransmitters are located across the entire surface of the effector.
• Autonomic postganglionic neurons release neurotransmitters from varicosities.

Signal Transmission at a Neuroeffector Junction

• Action potential arrives at the varicosity
• Voltage gated Ca2+ channels open and Ca2+ enters the varicosity
• Ca2+ trigger the release of neurotransmitters from synaptic vesicles
• Neurotransmitters bind to receptors on the plasma membrane of the effector cell
• Binding of neurotransmitter to receptor causes either excitation or inhibition of the effector cell, via G protein interaction

Neurotransmitters and Receptors of the ANS

• The ANS utilizes two types of neurotransmitters and receptors: Cholinergic and Adrenergic.

Cholinergic

• Acetylcholine is the neurotransmitter
• Acetylcholine is released by preganglionic neurons of both systems.
• Acetylcholine is released by postganglionic neurons of the parasympathetic nervous system
• Receptors for acteylcholine include nicotinic and muscarinic

Nicotinic receptors

• Found on postganglionic neuron cell bodies

Muscarinic receptors

• Found on target tissue of the parasympathetic nervous system.

Adrenergic

• Norepinephrine and epinephrine are the Neurotransmitters
• Norepinephrine is released by postganglionic cells of the sympathetic nervous system
• Epinephrine is released by chromaffin cells of the adrenal medulla
• Alpha and Beta receptors are used for norepinephrine and epinephrine

Functions of the Autonomic Nervous System

• Include autonomic tone
• Regulated by the hypothalamus

Autonomic Tone

• Body organs receive constant innervation by both branches of the ANS.
• One branch will increase activity while the other decreases to achieve the desired effect.

ANS Functions: Parasympathetic Effects

• "Rest and digest" activities use this system.
• It conserves energy and promotes the breakdown and absorption of food.
• Remembered with the acronym SLUDD: Salivation, Lacrimation, Urination, Digestion, Defecation

ANS Functions: Sympathetic Effects

• Fight or flight activities use this system.
• Supports physical activity and ATP production.
• Remembered as "E" functions: excitement, emergency, exercise, and embarrassment.
• Effects are diffuse, affecting many organs, and are longer-lasting than parasympathetic effects.

Autonomic Reflexes

• Help maintain homeostasis
• Components include: sensory receptor, sensory neuron, integrating center, autonomic motor neurons, and visceral effector.

Autonomic Control Centers

• Found in the brain and spinal cord
• Controls include: cardiovascular, respiratory, and other centers.

Sensory Systems and Homeostasis

• Contribute to homeostasis by detecting, conducting, and processing information about the external or internal environment.
• Consist of: sensory receptors, neural pathways, and parts of the CNS that process information.
• Major sensory systems include: somatic sensory, visual, olfactory, gustatory, auditory, and vestibular (equilibrium) systems.

Process of Sensation

• Involves four steps
  1. Stimulation of the sensory receptor.
  2. Transduction of the stimulus.
  3. Generation of action potentials.
  4. Integration of sensory input.

Types of Sensory Receptors

• Mechanoreceptors detect mechanical stimuli like deformation, stretching, or bending.
• Thermoreceptors detect changes in temperature.
• Photoreceptors detect light striking the retina of the eye.
• Chemoreceptors detect chemicals in the mouth, nose, and body fluids.
• Nociceptors respond to painful stimuli resulting from physical or chemical damage.

Properties of Sensory Receptors

• Sensory receptors can be:
  • Peripheral endings of sensory neurons
  • Free nerve endings
  • Encapsulated nerve endings
• A separate cell

Receptive Fields

• Area that gives response when stimulated
• Can be used by different types of neurons, like somatic sensory neurons, visual neurons, olfactory receptors, auditory neurons, or taste neurons
• Sensory neurons can have separate or overlapping receptive fields

Modality and Labeled Lines

• Modality: a single type of sensation
• Labeled line: all receptors are sensitive to a specific modality

Stimulus Intensity

• Coding of stimulus intensity can occur by - the frequency of action potentials generated - the number of sensory receptors activated

Adaptation of Receptors:

• Receptors can be slowly adapting or rapidly adapting

Slowly adapting receptors

• Adapt slowly and continue to trigger action potentials as long as the stimulus persists
• The receptor potential slowly decreases

Rapidly adapting receptors

• Adapt quickly and stop triggering action potentials even with a persistent stimulus
• Receptor potential rapidly decreases

Sensory Pathways to the Brain

• Consist of a sensory receptor, first-order neuron, second-order neuron and third-order neuron
• First-order neurons synapse in the brainstem
• Second-order neurons send signal to the thalamus
• Third-order neurons send response to the cerebral cortex

Tactile Sensations

• Encompass a variety of sensations: touch, pressure, vibration, itch, and tickle.

Thermal Sensations

• Detected by thermoreceptors.
• Cold receptors are activated by temperatures between 10°C-35°C (50-95°F).
• Warm receptors are activated by temperatures between 30-45°C (86-113°F), and are fewer than cold receptors.

Pain Sensations

• Protect the body from damaging stimuli.
• Types of nociceptors respond to: mechanical, thermal, or polymodal stimuli.

Pain Pathways

• Skin nociceptors activate a first-order neuron
• The signal is sent to the spinal cord, activating a second-order neuron
• The signal is sent to the brain

Proprioceptive Sensations

• Provide information about muscle and joint position.

Olfactory Epithelium

• Includes a olfactory tract, bulb, epithelium and nerve to detect odors.

Olfactory Transduction

• Olfactory receptor proteins bind an odorant molecule
• Binding causes Golf protein to bind the receptor
• G-olf stimulates adenylyl cyclase
• The stimulated adenylyl cyclase turns ATP to cAMP
• Increased cAMP opens a cation channel, allowign Ca2+ and Na+ into the cell
• The channels create a depolarizing receptor potential, triggering an action potential

The Olfactory Pathway

• Odors travel down glomeruli from the olfactory bulb, through the olfactory tract and synapses at the base of the frontal lobe

Taste Buds:

• Microscopic structures in the tongue that contain the receptors for taste.

Tactile transduction (Taste)

• Salty tastants can cause salt entry into the cell through sodium channels, which causes depolarization, calcium vvoltage gated channels to open, and neurotransmitter release
• Sour tastants cause H+ entry through H+ channels, caussing depolarization, calcium vvoltage gated channels to open, and neurotransmitter release
• Sweet, bitter and umami tastants release neurotransmitters (gustducin) that lead to increased calcium in the endo-plasmic reticulum, and then depolarization occurs because sodium and calcium channels open

The Gustatory Pathway

• Extends from taste receptors to the gustatory cortex via the medulla and the thalamus.
• Includes signals from gustatory nucleus, facial, glossopharyngeal, and vagus nerves

Accessory Structures of the Eye

• Protect, lubricate, and move the eye.
• Include eyebrows, eyelids, lacrimal glands, extrinsic eye muscles, and the nasal cavity.

Functional Components of the Eye:

• Includes the anterior and posterior chambers, as well as other parts of the eye, to promote sight.

Structures and Function of the Retina

• Consists of a neural layer and a pigmented layer of cells

Image Formation in the Eye

• Occurs through refraction by the cornea and lens

Adjustment of Image on the Retina

• Achieved through accomodation, or changing in curvature of the lens

Problems with Vision

• Can stem from corneal deficiencies
• The eyes muscles losing sight over time
• Malformation of the eye's shape

Photoreceptors

• Two types of photoreceptors required for normal vision: rods and cones
• Rods are highly sensitive and operate at low light levels
• Cones have low sensitivity and operate best at high light levels and provides color vision.

The Visual Pathway

• Begins at the retina and extends to the visual areas of the brain.

How Eyes Receive Visual Fields

• Both eyes receive input from the left and right visual fields.

Ear Components

• Include the outer, middle and inner ear bones and components

Sound Waves

• Generated from a vibrating object
• Pitch is determined by frequency
• Intensity is determined by amplitude, which is measured in decibels.

Transmission of Sound Waves

• They bounce off of the tympanic membrane
• The vibration runs through the bones of the middle ear
• It travels to the cochlea and out the round window
• At the cochlear duct, the signal is transferred to perilymph and can move the vestibular membrane, ultimately sending a signal out

Sound Transduction

• Transmitted by hair cells.
• Inner hair cells are responsible for sound transduction.
• Outer hair cells enhance the sensitivity of inner hair cells.

Sound Discrimination on the Basilar Membrane:

• Pitch depends on which basilar membrane region vibrates
• Loudness depends on how much of the basilar membrane vibrates

Auditory Pathway

• Starts in the cochlear of the ear, runs through the thalamus, and synapses at the primary auditory cortex

Equilibrium and Balance

• Equilibrium relies of otolithic organs in the utricle and saccule
• They are used to detect linear acceleration/deceleration, and head tilt

Semicircular Ducts

• Detect rotational acceleration deceleration when drag of the endolymph causes cupula and its embedded hairs to bend in opposite direction

Equilibrium Pathway

• Runs from the vestibular of the ear, to the Thalmus and the vestibular cortex of the brain