Integrative Physio Exam II

Components of a neuron

• Dendrites

• Cell Body (Soma)

• Axon Hillock

• Axon

• Axon Terminals

Dendrites

Branching projections that receive incoming signals

Cell Body (Some)

Contains the nucleus, organelles, and metabolic machinery

Axon Hillock

The trigger zone where graded potentials are integrated to initiate action potentials

Axon

A long projections that conducts action potentials away from the cell body

Axon Terminals

Endings that release neurotransmitters into the synapse

Functional classes of neurons

• Sensory (Afferent) Neurons

• Motor (Efferent) Neurons

• Interneurons

Sensory (Afferent) Neurons

Transmit sensory information from peripheral receptors to the CNS

Motor (Efferent) Neurons

Transmit sensory information from peripheral receptors to the CNS

Interneurons

Process signals locally and integrate information completely within the CNS

Myelinated Neurons

Myelinated neurons are wrapped in a lipid-rich glial sheath that insulates the axon allowing action potentials to propagate rapidly via saltatory conduction

Unmyelinated Neurons

Unmyelinated neurons lack the insulation and must conduct action potentials continuously down the entire membrane length resulting in a much slower propagation speed.

Types of glial cells in CNS

• Astrocytes

• Oligodendrocytes

• Microglia

• Ependymal Cells

Astrocytes

• Provide structural support

• Regulate extracellular K+ and neurotransmitter levels

• Maintain blood-brain barrier

Oligodendrocytes

Form myelin sheaths around multiple axons in CNS

Microglia

Act as resident immune cells and phagocytes to clear cellular debris and pathogens

Ependymal Cells

• Line the brain ventricles and spinal canal

• Assist in the production and circulation of cerebrospinal fluid

Types of glial cells in PNS

• Schwann Cells

• Satellite Cells

Schwann Cells

• Form myelin sheaths around a single axon segment in PNS

• Assist in nerve regeneration

Satellite Cells

• Surround and cushion neuron cell bodies within peripheral ganglia

• Provide structural and metabolic support

Resting Membrane Potential

A stable baseline electrical voltage across the membrane of an unstimulated cell (Around -70 mV)

Graded Potential

Local, short distance changes in membrane potential whose magnitude varies directly with stimulus strength and diminishes over distance.

Action Potential

A large, rapid, all or none electrical impulse that travels long distances down the axon without diminishing in strength.

Events of Action Potential

• Depolarization

• Repolarization

• Hyperpolarization

Depolarization

Voltage gates Na+ channels open allowing Na+ ions to rush into the cells making the inside positive

Repolarization

Voltage gated Na+ channels inactivate while voltage gated K+ channels open letting K+ rush out to restore a negative interior

Hyperpolarization

K+ channels remain open briefly after reaching resting potential causing the membrane potential to dip below baseline (< -70 mV)

Absolute Refractory Period of an Action Potential

A period during which no new action potential can be fired regardless of stimulus strength because the voltage gated Na+ channels are inactivated.

Relative Refractory Period of an Action Potential

A period during which no new action potential can be fired but only a suprathreshold (much stronger) stimulus because the membrane is hyperpolarized and K+ channels are open.

Influence on neuronal excitability of higher potassium levels in the extracellular fluid

It decreases the concentration gradient driving K+ out which depolarizes the resting membrane potential (making it less negative) closer to threshold causing hyperexcitability

Influence on neuronal excitability of lower potassium levels in the extracellular fluid

It increases the gradient driving K+ out which hyperpolarizes the resting membrane potential (making it more negative) further from threshold reducing neuronal excitability

Nodes of Ranvier

Unmyelinated gaps along a myelinated axon packed with high densities of voltage gated Na+ channels

Saltatory Conduction

Action potentials jump from node to node allowing rapid propagation and high metabolic efficiency

Continuous Conduction

Slower propagation occurring step by step along the entire continuous membrane of unmyelinated fibers

Neurotransmitter Release Steps

1. Action potential propagates to the axon terminal

2. Depolarization opens voltage gated Ca²⁺ channels

3. Ca²⁺ flows down its concentration gradient into the intracellular fluid of the terminal

4. Calcium triggers synaptic vesicles to dock and undergo exocytosis

5. Neurotransmitters are released into the synaptic cleft, diffuse across, and bind to receptors on the postsynaptic cell

Sensory Neurons Function

Carry afferent signals from peripheral receptors to CNS

Motor Neurons Function

Carry efferent signals from CNS to effectors (muscles/glands)

Reflex

Involuntary, rapid, predictable motor response to an obligatory stimulus

Four Properties of a Stimulus needed for Sensory Coding

1. Modality: Specific type of energy or stimulus (thermal, chemical, mechanical) coded by labeled lines

2. Location: Site of origin coded by specific receptive fields and populations of active receptors

3. Intensity: Strength of stimulus coded by action potential frequency and the recruitment of additional receptors

4. Duration: How long the stimulus lasts coded by tonic (slow-adapting) or phasic (fast-adapting) firing patterns

Receptive Field

Area monitored by a single sensory neuron

Large Receptive Field

Cover a broad area with fewer neurons per unit area resulting in low spatial resolution and poor two point discrimination (ex: the back)

Small Receptive Field

Cover a small area with dense innervation, resulting in high spatial resolution and fine two point discrimination (ex: fingertips)

Classes of Somatosensory Nerve Fibers

1. A-Beta Fibers

2. A-Delta Fibers

3. C Fibers

A-Beta Fibers

Large diameter, heavily myelinated fibers for touch, vibration, and pressure

A-Delta Fibers

Medium diameter, myelinated fibers that transmit fast, sharp pain, and temperature

C Fibers

Small diameter, unmyelinated fibers that transmit slow, dull, aching pain, and temperature

Gate Control Theory of Pain

Theory that non-painful tactile signals carried by large A-Beta touch fibers stimulate inhibitory interneurons in the spinal cord which can close the gate and inhibit the transmission of pain signals sent by C fibers to higher brain centers

A neurological "gate" in the spinal cord's dorsal horn regulates whether pain signals are allowed to reach the brain

Referred Pain

Pain originating from internal visceral organs that is incorrectly perceived as coming from a specific somatic body surface location because visceral and somatic sensory fibers converge on the same secondary ascending neurons in the spinal cord.

Autonomic Nervous System (ANS) Function

• Regulates involuntary, unconscious body functions to maintain internal homeostasis

  • Controlling cardiac muscle, smooth muscle, and glands

Note: 3rd one

3rd one

Components of ANS

• Preganglionic motor neurons

• Postganglionic motor neurons

That span the CNS and peripheral effectors

Sympathetic Branch of ANS

• Fight or Flight

• Thoracolumbar Outflow

• Short preganglionic neurons release Acetylcholine (Ach) onto nicotinic receptors

• Long postganglionic neurons release norepinephrine onto adrenergic receptors

Parasympathetic Branch of ANS

• Rest and Digest

• Craniosacral Outflow

• Long preganglionic neurons release Acetylcholine (Ach)

• Short postganglionic neurons release Acetylcholine (Ach) onto muscarinic receptors

Autonomic Pathway to the Adrenal Medulla and Hormone Released

Preganglionic sympathetic fibers travel directly from CNS to adrenal medulla

• Without synapsing in a peripheral chain ganglion

Upon stimulation via Acetylcholine (Ach) specialized chromaffin cells secrete Epinephrine (~80%) and Norepinephrine (~20%) directly into the bloodstream as circulating hormones

Somatic Nervous System

Uses a single neuron chain from the CNS to voluntary skeletal muscle and is always excitatory

Autonomic Nervous System (ANS)

Uses a two neuron chain (pre and post ganglionic) to target involuntary visceral effectors and can be either excitatory or inhibitory depending on receptor types

Types of Muscle

• Skeletal Muscle

• Cardiac Muscle

• Smooth Muscle

Skeletal Muscle

• Striated

• Multinucleated

• Voluntary

• Attached to bones

Cardiac Muscle

• Striated

• Branching

• Involuntary

• Connected by intercalated discs with gap junctions found in the heart

Smooth Muscle

• Non-striated

• Spindle Shaped

• Involuntary

• Found in the walls of hollow internal organs and blood vessels

Organization of Skeletal Muscle

Muscle → Fascicle → Muscle Fiber (Cell) → Myofibrils → Myofilaments (Actin & Myosin)

Sarcomere Arrangement

Functional contractile unit spanning from Z line to Z line.

• Contains thin actin filaments anchored at Z lines

• Thick myosin filaments in the center

• Includes

  • A band (length of myosin)

  • I band (Actin only)

  • H zone (myosin only without overlapping actin)

Contraction Cycle Steps

1. Resting/Energized State: ATP splits into ADP and P_i cocking the myosin head

2. Cross Bridge Formation: Ca²⁺ binds troponin, shifting tropomyosin to expose actin binding sites myosin binds actin

3. Power Stroke: Release of P_i and ADP causes the myosin head to pivot, sliding actin toward the M line

4. Detachment: Binding of a new ATP molecule causes myosin to release actin

Causes Rigor Mortis

Occurs after death when

• Cellular respiration ceases

• ATP is depleted

• Myosin heads remain permanently locked onto actin filaments

Events of Contraction in a Skeletal Muscle Fiber

• Action potential arrives

• ACh released at neuromuscular junction

• Muscle fiber action potential travels down T-tubules

• Ca²⁺ is released from the sarcoplasmic reticulum

• Cross bridge cycling occurs

Events of Relaxation in a Skeletal Muscle Fiber

• Acetylcholinesterase breaks down ACh

• Ca²⁺ is actively pumped back into the sarcoplasmic reticulum

• Tropomyosin moves back to cover actin binding sites

• Muscle relaxes

ATP Production through Creatine Kinase

Rapidly transfers a phosphate group from creatine phosphate to ADP to immediately regenerate ATP during the initial seconds of intense exercise

ATP Production through Substrate Level Phosphorylation (Glycolysis)

Rapidly breaks down glucose anaerobically to yield ATP and lactic acid without requiring oxygen

3 Types of Muscle

• Slow Oxidative (Type I)

• Fast Oxidative (Type II a)

• Fast Glycolytic (Type IIb/IIx)

Slow Oxidative (Type I)

• Slow contraction speed

• High fatigue resistance

• High myoglobin content (red)

• Aerobic Respiration

Fast Oxidative (Type IIa)

• Fast contraction speed

• Intermediate fatigue resistance

• High aerobic/anaerobic capacity

Fast Glycolytic (Type IIb/IIx)

• Fast contraction speed

• Low fatigue resistance

• White color

• Anaerobic gylcolysis for rapid, powerful bursts

Somatic Reflexes

Rapid, involuntary motor responses to stimuli

Monosynaptic Reflexes

A single synapse between a sensory afferent and motor efferent neuron with no interneuron

Polysynaptic Reflexes

Involve one or more interneurons between the sensory and motor neurons creating processing delays

Stretch Reflex

Monosynaptic reflex where muscle stretching leads to reflex contraction of the same muscle to maintain posture and length

Tendon Reflex

Polysynaptic reflex where high muscle tension triggers tendon organ activation causing reflex inhibition/relaxation to protect the muscle and tendon from tearing

Propioceptors

Specialized receptors that monitor body position, muscle length, and tension

Muscle Spindles Function

Detect changes in muscle length and rate of stretch

Golgi Tendon Organs (GTOs)

Detect changes in muscle tension

Alpha/Gamma Co-activation

When alpha motor neurons stimulate extrafusal fibers to contract gamma motor neurons simultaneously stimulate intrafusal fibers inside muscle spindles.

• Keeps the spindle taut and sensitive to length changes throughout the contraction

Crossed Extensor Reflexes

A spinal reflex coupled with a withdrawal reflex.

When a painful stimulus causes withdrawal of one limb the contralateral (opposite) limb is simultaneously extended to maintain balance and support body weight