Neurophysiology

AUTONOMIC NERVOUS SYSTEM

What are the two structural divisions of the human nervous system?

1. Central nervous system
   
2. Peripheral nervous system

What are the two functional divisions of the human nervous system?

1. Somatic nervous system
   
2. Autonomic nervous system (ANS)

What is the function of the somatic nervous system?

Innervates skeletal muscle; largely under voluntary control

What is the function of the ANS?

Maintenance of homeostasis through involuntary coordination of glandular, cardiac, and smooth muscle activity throughout the body

What are the constituent parts of the ANS?

Enteric nervous system, sympathetic nervous system (SNS), and parasympathetic nervous system (PNS)

Generally speaking, what is the function of each part of the ANS?

Enteric nervous system

Coordination of various gut functions (motility, secretion, etc) (see Chap. 6)

Sympathetic nervous system

“Fight or flight” response. Coordinates the body’s response to stressors.

Parasympathetic nervous system

“Rest and digest” response. Coordinates the process of energy conservation and replenishment.

Where is the anatomic origin of SNS?

Intermediolateral cell column of the spinal cord between segments T1 to L3 (e.g., thoracolumbar)

Where is the anatomic origin of the PNS?

Nuclei of cranial nerves (CN) III, VII, IX, and X and spinal cord segments S2 to S4 (e.g., craniosacral spinal cord)

What type(s) of neurotransmitter(s) and receptors are present in the preganglionic fibers of the PNS and SNS?

PNS and SNS preganglionic neurons use ACh as the neurotransmitter and nicotinic receptors for transmission

Is the neurotransmitter/receptor combination used at the preganglionic synapse the same as that used at the neuromuscular junction (NMJ)?

No, while these receptors are nAChR (nicotinic Acetylcholine receptors) they differ from those at the NMJ. The nicotinic subunits here have a different subunit makeup allowing for differences in pharmacologic influences

Describe the axon length for the PNS and SNS in the following locations:

PNS

   SNS

Preganglionic nerve axon

Long

   Short

Postganglionic nerve axon

Short

   Long

Anatomically, where do most preganglionic sympathetic fibers synapse?

Either at the paravertebral chain ganglion, or at collateral ganglia that follow large vessels in the abdomen (celiac, superior mesenteric, etc)

Anatomically, where do most preganglionic parasympathetic fibers synapse?

In microscopic ganglia associated with the target organs

What receptor types are primarily used at the effector organs of the PNS and SNS? What are their respective neurotransmitters?

PNS: all receptors are muscarinic and ACh is the neurotransmitter

SNS: α₁, α₂,, β₁, or β₂; the primary neurotransmitter is norepinephrine (NE)

There is an exception to the above rule regarding the SNS. What is it?

Sweat glands have muscarinic receptors. The associated postganglionic SNS neurons release ACh.

What is unique about the adrenal medulla?

It is a specialized SNS ganglion where preganglionic fibers synapse directly with the effector organ (chromaffin cells)

What does SNS stimulation of chromaffin cells induce?

Secretion of epinephrine (Epi; 80%) and NE (20%) into the circulation

What is the purpose of releasing adrenal hormones into the circulation?

These substances activate organs that receive little innervation (fat cells, hepatocytes etc) allowing them to assist in the stress response

What are the SNS receptor types in the following locations and what are their effects?

Heart

SA nodeα

β₁: increased pacemaker activity

AV node

β₁: increased conduction velocity

Myocardium

β₁: increased contractility

Vascular smooth muscle

Skin and splanchnic circuits

α₁ and α₂: constriction

Skeletal muscle and pulmonary circuits

β₂: dilation

Peripheral veins

Eye

Ciliary muscle

β₁: relaxes muscle (for far vision)

Radial muscle

α₁: muscle contraction → dilates pupil

Bladder

Detrusor muscle

β₂: relaxes

Bladder sphincter

α₁: constricts

Bronchioles

Bronchial muscle

β₂: dilates smooth muscle

Bronchial glands

α₁: inhibits secretion

β₂: stimulates secretion

Gastrointestinal tract

Sphincters

α₁: constricts

Secretion

α₂: inhibition

Motility

α₁, α₂, β₂: decrease

Kidney

β₁: increase renin secretion

Male sex organs

α₁: ejaculation

Sweat glands

Muscarinic: increase sweat production

Adipose tissue

β₁, β3: increase lipolysis

For which of the above SNS locations is there no corresponding PNS innervation?

Vascular smooth muscle, kidney, sweat glands, fat cells, and liver

What is the mechanism of action for the following receptor types?

α₁

Inositol 1,4,5-triphosphate (IP₃) formation and increased intracellular [Ca²⁺]

α₂

Adenylate cyclase inhibition and decreased cyclic adenosine monophosphate (cAMP)

β₁

Adenylate cyclase activation and increased cAMP

β₂

Adenylate cyclase activation and increased cAMP

Nicotinic

Ion channel for Na⁺ and K⁺

Muscarinic

Heart (sinoatrial [SA] node): adenylate cyclase inhibition

Smooth muscle and glands: IP₃ formation and increased intracellular [Ca⁺]

What is the effect on the ANS of the following pharmacologic agents?

ACh

Nicotinic and muscarinic agonist

Albuterol

β₂ Agonist

Atropine

Muscarinic antagonist

Butoxamine

β₂ Antagonist

Carbachol

Nicotinic and muscarinic agonist

Clonidine

α₂ Agonist

Curare

Nicotinic antagonist

Dobutamine

β₁ Agonist

Hexamethonium

Nicotinic antagonist (ganglion only)

Isoproterenol

β ₁ and β₂ agonist

Metoprolol (at therapeutic doses)

β₁ Antagonist

Muscarine

Muscarinic agonist

Nicotine

Nicotinic agonist

NE

α₁ and β₁ agonist

Phenoxybenzamine

α₁₂ Antagonist

Phentolamine

α₁₂ Antagonist

Phenylephrine

α₁ Agonist

Prazosin

α₁ Antagonist

Propranolol

β₁ and β₂ antagonist

Yohimbine

α₂ Antagonist

What autonomic centers are located in the following areas?

Medulla

Respiratory; swallowing, coughing, and vomiting; and vasomotor

Pons

Pneumotaxic

Midbrain

Micturition

Hypothalamus

Regulation of food and liquid intake and temperature regulation

SENSORY SYSTEMS

What four qualities of sensation must be encoded for effective transmission?

1. Modality
   
2. Location
   
3. Intensity
   
4. Duration

What is a sensory receptor?

A specialized cell that transduces physical environmental stimuli into neural signals

What types of cells are usually sensory receptors?

Specialized epithelial and neuronal cells

What is meant by “receptive field”?

Area on the body that changes the firing rate of its sensory neuron when stimulated

What is the field called if it increases the firing rate?

Excitatory

What is the field called if it decreases the firing rate?

Inhibitory

A sensory receptor must transmit its findings back to the CNS for interpretation. How does the CNS differentiate between an AP (action potential) from a retinal cell and an AP from a somatosensory cell?

The receptors are simple transducers, each converting a different type of energy into electrical impulses. These impulses follow defined pathways into the CNS and relay on specific nuclei. These pathways define how sensory perception is recognized.

For an example of this, think about rubbing your eyes. That pressure is always perceived as flashes of light.

How are afferent neurons of the sensory systems classified?

By diameter (roman numerals I-IV) and conduction velocity (A and C)

Give the relative size of the following sensory neuron classifications:

I

Largest

II

Medium

III

Small

IV

Smallest

Give the relative conduction velocity of the following sensory neuron classifications:

Aα

Fastest

Aβ

Medium

Aδ

Medium

C

Slowest

Describe the series of events that occur in sensory transduction.

Figure 2.1 Sensory transduction sequence.

What effect does intensity of the stimulus have on the receptor potential generated?

Larger stimuli create larger graded potentials (e.g., receptor potential)

What direction does the current usually flow when sensory receptor channels open?

Positive inward flow, depolarizing the cell

What is an exception to this flow direction?

Photoreceptors: stimulation decreases inward current and hyperpolarizes the membrane

As seen in Fig. 2.1, how does the body encode intensity changes in sensory stimuli?

Primarily with frequency coding; greater intensity leads to a higher frequency. Also with population coding; a greater number of receptors will be triggered by a stimulus of increasing intensity.

What is sensory adaptation?

The dynamic change in the frequency of triggered AP’s with a static stimuli; this occurs at the level of the receptor

What types of adaptation do sensory receptors exhibit?

Tonic (slowly adapting) and phasic (rapidly adapting)

Which type of sensory adaptation detects the beginning of (onset) and the end of (offset) of a stimulus?

Phasic

What type of sensory adaptation responds consistently to prolonged stimuli?

Tonic

What type of sensory adaptation detects steady stimuli?

Tonic

What happens to action potential frequency in phasic receptors with constant stimulation?

Decreases

Label the three types of adaptation seen below:

Figure 2.2 Adaptation.

1. No Adaptation, also called tonic adaptation
   
2. Slowly adapting sensory neurons
   
3. Rapidly adapting sensory neurons, also called phasic adaptation

What is the location of the following structures?

First-order neurons

Dorsal root ganglia

Second-order neurons

Spinal cord or brain stem (depending on system)

Third-order neurons

Thalamus (relay nuclei)

What goes on beyond the relay nucleus?

Extensively overlapping cortical circuitry that interprets external stimuli and relays information throughout the cortex

Somatosensory System

What sensations are perceived by the somatosensory system?

Touch, movement, and position, as well as temperature, and pain

What two anatomical pathways does the somatosensory system use?

1. Dorsal-column system
   
2. Anterolateral system

What sensations are detected by the following systems?

Dorsal-column system

Touch, vibration, and proprioception

Anterolateral system

Temperature and pain

What path does sensory information take in the dorsal-column system?

Receptors with cell bodies in the
dorsal root ganglia receive stimulus (first order)
↓
Signal ascends to the nucleus gracilis
and nucleus cuneatus in medulla
(still first order)
↓
Signal crosses the midline and
ascends to enter contralateral
thalamus (second order)
↓
Signal ascends to the somatosensory
cortex (third order)

What path does sensory information take in the anterolateral system?

Receptors with cell bodies in the
dorsal root ganglia receive stimulus
(first order)
↓
Signal crosses the midline and enters
the anterolateral quadrant of the
spinal cord (second order)
↓
Signal ascends to contralateral thalamus
(still second order)
↓
Signal sent to somatosensory cortex
(third order)

What are the major differences between the two systems with respect to the anatomy of the ascending information?

Dorsal columns don’t cross midline until the medulla; anterolateral system crosses at the level of the peripheral nerve

Ascending information in dorsal columns carried by first-order neuron; in the anterolateral system, it is carried by the second-order neuron

What are the tracts of Lissauer?

These are found at the dorsal root where the anterolateral system enters the spinal cord. They allow for fibers to move vertically one or two segments before synapsing.

What is another name for the sensory cortex?

Sensory homunculus

How is the homunculus arranged?

Upside down (face most lateral, with feet and genitals inside the central sulcus)

Why are the face, hands, and genital regions of the homunculus so large?

They possess the highest density of nerve fibers

What are the types of mechanoreceptors that detect touch and pressure?

Meissner corpuscle

Merkel disk

Pacinian corpuscle

Ruffini corpuscle

What sensation is encoded by the following mechanoreceptors?

Meissner corpuscle

Velocity

Merkel disk

Pressure-small receptive field

Pacinian corpuscle

Vibration

Ruffini corpuscle

Pressure-large receptive field

Which types of mechanoreceptors demonstrate phasic adaptation?

Meissner and Pacinian corpuscles

Which types of mechanoreceptors demonstrate tonic adaptation?

Merkel disks and Ruffini corpuscles

What is nociception?

Detection and perception of noxious stimuli (e.g., pain)

What types of receptors detect pain?

No specialized receptors; pain is detected by free nerve endings

What triggers pain receptors?

Any excessive application of energy (chemical, thermal, or mechanical)

Where are pain receptors located?

Skin, muscle, and viscera

What two pathways does visceral pain stimulate?

1. Visceral afferent fibers ascend with sympathetic nerves
   
2. Referred to the skin in a dermatomal fashion

Explain referred pain:

Afferent pain fibers serving the viscera enter the spinal cord at particular levels. Since the body is not used to visceral pain, it confuses those stimuli with dermatomal pain from the same spinal root.

What fibers carry fast pain signals?

Group III

What fibers carry slow pain signals?

C fibers

Opiates inhibit the release of what neurotransmitter for nociception?

Substance P

VISION

In the image below, label the numbered portions of the eye.

Figure 2.3 The eye.

1. Cornea
   
2. Iris
   
3. Lens
   
4. Ciliary body
   
5. Aqueous humor
   
6. Vitreous humor
   
7. Retina
   
8. Optic nerve
   
9. Central vessels of the retina
   
10. Fovea

What is the function of the lens?

Focuses light onto the retina

What is the condition called when the curvature of the lens is not uniform?

Astigmatism

What structure produces aqueous humor?

Ciliary epithelium covering the surface of the ciliary body

How does aqueous humor drain?

Via the canal of Schlemm

What conditions are described by the following?

Lens focuses light onto the retina

Emmetropia (this is normal)

Lens focuses light in front of the retina

Myopia (nearsighted)

Lens focuses light behind the retina

Hyperopia (farsighted)

What is accommodation?

Focusing of light by the lens

How does accommodation occur?

The smooth muscle of the ciliary body contracts to focus the lens

What is presbyopia?

Loss of accommodation due to stiffening of the lens

Identify the labeled cell types of the retina in the image below.

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