Lecture 8 - Peripheral Nervous System & Sensory Receptors Flashcards

Flashcards reviewing key concepts of the Peripheral Nervous System, spinal nerve anatomy, functional nerve fiber types, autonomic nervous system divisions, and sensory receptors based on lecture notes.

How many pairs of cranial and spinal nerves are present in dogs and cats compared to horses?

Dogs and cats have $12$ pairs of cranial nerves and $36$ pairs of spinal nerves, whereas horses have $42$ pairs of spinal nerves.

What three connective tissue layers envelope a peripheral nerve?

Epineurium (around the entire nerve) → perineurium (around individual nerve fascicles) and endoneurium (connective tissue sheath around individual axons)

What structural difference exists between cell bodies in spinal ganglia and autonomic ganglia?

Spinal ganglia contain unipolar cell bodies, while autonomic ganglia contain multipolar cell bodies that innervate vesicles.

What are the six primary functional nerve fiber types in the PNS and their targets?

1. Somatic Efferent (SE): skeletal muscles. 2. Visceral Efferent (VE): smooth muscle, cardiac muscle, and glands. 3. General Somatic Afferent (GSA): skin, muscles, tendons, and joints. 4. General Visceral Afferent (GVA): viscera. 5. Special Somatic Afferent (SSA): vision, hearing, and vestibular sense (cranial nerves only). 6. Special Visceral Afferent (SVA): taste and smell (cranial nerves only).

What are the four primary branches into which a spinal nerve divides?

1. Meningeal branch (recurrent meningeal nerve). 2. Ramus communicans. 3. Ventral branch. 4. Dorsal branch.

What structures are innervated by the meningeal branch of a spinal nerve?

It provides sensory innervation to the meninges, denticulate ligament (DLL), annulus fibrosus of the intervertebral disc (IVD), and periosteum of the vertebrae.

What neurotransmitters are released by preganglionic and postganglionic autonomic neurons?

Preganglionic neurons always release acetylcholine (ACh). Postganglionic parasympathetic neurons release ACh, while most postganglionic sympathetic neurons release norepinephrine (NE).

What are the preganglionic origins of the sympathetic and parasympathetic divisions of the ANS?

The sympathetic division originates in the thoracolumbar spinal cord ($T1\text{--}L4$). The parasympathetic division originates in the brainstem (cranial nerves $III$, $VII$, $IX$, $X$) and sacral spinal cord ($S1\text{--}S3$).

What are the conduction velocities of A-\beta, A-\delta, and C nerve fibers?

A-\beta fibers conduct at $30\text{--}70\text{ m/s}$, A-\delta fibers conduct at $2\text{--}10\text{ m/s}$, and C fibers conduct at $1\text{ m/s}$.

What are the general fiber classifications from fastest to slowest?

The general fiber classifications from fastest to slowest are A-eta, A- ext{delta}, and C fibers.

What are the afferent fibres to muscles & joints?

Group I-IV & grop I has subgroups 1a (muscle spindle) & 1b (golgi tendon)

What are the afferents to viscera?

Primarily A & C fibers

What are the efferent fibres?

*Somatic motor neurons - A alpha (extrafusal muscle) & A gamma (muscle spindle)

*Autonomic - B & C

Somatosensation includes sensory input from where?

Skin (cutaneous), muscles & tendons (proprioception), joints, and visceral organs (interoceptiveS

Somatosensory modalities include?

Light touch, pressure, vibration, temperature, pain, kinaesthesia

Free nerve endings detect what?

Pain & temperature

What do nociceptors respond to?

Mechanic, thermal & chemical stimuli

-damaged tissues release inflammatory chemicals that activate & lower their activation threshold (peripheral sensitization?

What are the 3 types of nociceptors and what fibres do they use?

Mechanical & thermal (use A delta) & polymodal (use C fibres)

Which cutaneous mechanoreceptor is located at the epidermal-dermal junction and responds to sustained light touch, texture, and edges?

Merkel's discs.

What are the location, stimulus response, and adaptation characteristics of Pacinian corpuscles?

Located in the deep dermis, subcutaneous tissue, joint capsules, and ligaments; they respond to deep pressure, high-frequency vibration, and rapid tissue deformation; they feature an 'onion skin' capsule and are rapidly adapting mechanoreceptors.

What is located in dermis, joint capsule & ligaments and responds to skin stretch, sustained pressure, joint movements?

Ruffini endings. Slowly adapting mechanoreceptor.

What receptor located in paw pads, esp lips & fingertips responds to light touch, low frequency vibration & skin motion?

Meissner's corpuscles. Rapidly adapting mechanoreceptor.

Which sensory receptors are likely stimulated during HVLA (High-Velocity Low-Amplitude) manipulation?

Pacinian corpuscles, Ruffini endings, muscle spindles, and Golgi tendon organs.

What does light palpation activate?

Merkel & Meissnar

What does joint mobilization activate?

Ruffini, Pacinian, and muscle spindle

How do A-\delta fibers and C fibers differ in pain transmission?

A-\delta fibers mediate first pain (rapidly conducted, sharp pain), whereas C fibers mediate second pain (slowly conducted, dull/burning pain).

What are the functional differences between Joint Receptor Types I-III and Type IV?

Types I, II, and III monitor joint position and movement to contribute to proprioception. Type IV receptors act as nociceptors that respond to injury and inflammation.

What do intrafusal vs extrafusal fibres do?

Extrafusal:

-connect w tendons

-generate movement

Intrafusal:

-proprioceptors

-detect changes in length & velocity

• 2 types: nuclear bag & chain fibres

Explain how a muscle spindle works

When you stretch a muscle ie. reflex hammer - picked up by Type 1a fibres & activates them

Nuclear chain fibres have type II fibres which are sensory

At the end is gamma motor neurons which release ACh which causes the ends to contract which strengthens and lengthens the fiber

Stimulus is stretch - causes stretch of sensory fibres or 1a fibres and the gamma motor neuron causes the ends of the intrafusual fiber to contract so that the muscle spindle maintains sensitivity to further stretching.

What do Golgi tendons detect? Where are they located?

Degree of tension generated by muscle contraction & are located in the tendon

Describe how the GTO works

Muscle contracts - stimulates golgi tendon to prevent avulsion - stimulates the IB fiber to inhibit further contraction, protecting the muscle and tendon from damage. It inhibits the alpha motor neuron that supplies the muscle associated with that stimulated GTO. The GTO do not have a mechanism to modulate their sensitivity.

Describe the myotactic reflex

When the muscle contracts it pulls on that tendon & the GTO is activated which causes Ib fibres become activated → CNS → DR ganglion → bifurcates & the alpha motor neuron inhibits the contracted muscle to relax & take off tension & then the other stimulates the alpha motor neuron to contact the antagonistic muscle.

What key functional and structural differences distinguish Muscle Spindles from Golgi Tendon Organs (GTOs)?

Muscle spindles are in the muscle belly, detect muscle length/stretch via Group Ia & II afferents, excite \alpha motor neurons (stretch reflex), and receive \gamma motor innervation. GTOs are located in the tendon (MTJ), detect muscle tension via Group Ib afferents, inhibit \alpha motor neurons (autogenic inhibition reflex), and lack \gamma motor innervation.

What is the function of gamma (\gamma) motor neurons in muscle spindle regulation?

Gamma (\gamma) motor neurons innervate intrafusal muscle fibers, allowing the muscle spindle to contract and maintain sensitivity to muscle length during muscle contraction.