KIN 4571: Neurons and Receptors

Neurons

nerve cells transferring signals through action potentials

Structural components of neurons

- Soma (Perikaryon): Cell Body

- Dendrite: Message receiver

- Axon: Message-sending extension

Other components of neurons

Synapse and Glial cell

Microbes are transported through?

microtubules (antegrade transport and retrograde transport)

Antegrade transport

soma --> synapse

Retrograde transport

synapse --> soma

Neurons can be based on?

morphology or function

Based on morphology includes:

bipolar, pseudounipolar, and multipolar

What is the most common morphology?

multipolar

Based on function includes:

sensory neuron, motor neuron, and interneuron

Sensory neuron

cell body in dorsal root ganglion outside of spinal cord, receives sensory information (PNS --> CNS)

Motor neuron

cell body in spinal cord, sends messages to muscles to control muscle contractions (CNS --> PNS)

Interneuron

Form connections between motor and sensory neurons

Sensory receptors

detect stimuli from the environment and convert them into receptor potentials

What happens when the receptor reacher a threshold?

action potentials are generated

Synapse

- axon terminal

- dendrites receiving cells

- Glial cell process

Satellite cells (neuroglia): non neural cells, provide?

nutrients and support to neurons

Satellite cells generate?

neurons

Satellite cells __________ and ________ neurons

myelinate and protect

Do satellite cells always outnumber neurons?

NO

What is the most abundant type of neuron?

multipolar and interneuron

What is the function of a sensory neuron?

to receive sensory information

What is the function of a motor neuron?

send message to muscles/muscle spindles to control muscle contractions

What is the function of a interneuron?

to integrate and process signals, form connections between neurons, and project "messages" from the spinal cord and higher brain centers

What percentage of the 100 billion neurons are interneurons?

99.9%

Real life example

when you touch something hot and withdraw your hand you receive the information from the sensory receptors on your fingers and that information will send from the dendrites up the sensory neurons. The axons of the sensory neurons will enter the spinal cord through the dorsal root of the spinal cord. The synapse is in the interneuron in this case. Synapse is the motor neuron. The axons of the motor neuron exits the spinal cord through the ventral root. Then the synapse is muscle (effector). The muscle will contract and you withdraw your hand.

Receptor adaptation includes:

slowly adapting receptors and rapidly adapting receptors

Slowly adapting receptors

adapt very little over time and remain responsive

Rapidly adapting receptors

Adapt very quickly, only detecting onset of stimuli

Sensory receptors are based on?

mode of detection and source of the stimulus

Mode of detection includes:

mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors

Mechanoreceptors

response to touch, pressure, vibration, and stretch

Thermoreceptors

sensitive to changes in temperature

Photoreceptors

response to light

Chemoreceptors

response to chemicals

Nociceptors

sensitive to pain

Source of the stimulus includes:

exteroceptors, proprioceptors, and enteroceptors

Exteroceptors

skin (hair follicle receptor, Merkel DIsk, Meissner Corpuscle, Pacinian Corpuscle, Ruffini Endings, Nocioceptors)

Proprioceptors

muscle spindle (detect muscle length, Gorgi tendon unit (detect muscle tension), joint receptors (detect movements and joint positions)

Enteroceptors

visceral receptores (eg. mechanoreceptors, chemoreceptors, nociceptor)

Motor unit consists of?

an alpha motor neuron and all muscle fibers it innervates

Neuromuscular Junction

the chemical synapse between motor nerves and muscle fibers

Motor Unit =

1 alpha motor neuron + all extrafusal muscle fibers

Joint Receptors are within and around what?

joint capsule

Joint Receptors respond to?

start and end of movements as well as the joint position

Muscle Spindles respond to?

stretch/lengthening (absolute length and velocity)

Where are Muscle Spindles located?

in the muscle fibers, parallel to the extrafusal muscle fibers

What do Golgi Tendon Organs (GTO) respond to?

force/tension

Where are GTO's located at?

the muscle tendon junction

Muscle Spindles response to muscle stretch and lengthening due to?

muscle contraction - high threshold (sometimes response) and muscle stretch - low threshold (always response)

What is the function of Muscle Spindles?

detect movements (proprioception - sense of limb posittion) and detect/maintain muscle stiffness (muscle tone - level of excitable m.n. controlling a muscle)

What do muscle spindles determine?

intrinsic stiffness of extrafusal muscle fibers and level sensitivity of the reflex

What is the muscle spindle composition?

- Intrafusal fibers: connected in parallel with extrafusal fibers

- Ia and II sensory nerve endings (afferent): centrally located

- gamma (y) motor axons (efferent): located towards the ends

Intrafusal Muscle Fibers are the main part of the?

muscle spindle (parallel with extrafusal)

Nuclear bag 1 (dynamic nuclear bag)

Ia afferents

Nuclear bag (static nuclear bag)

Ia and II afferents

Nuclear chain

ia and II afferents

Nuclear bag 1, nuclear bag 2, and nuclear chain all belong to?

Intrafusal Muscle Fibers

Motor endings are?

efferent, towards the ends

Motor endings have?

gamma motor neurons (fusimotor neurons)

Motor endings receive information from?

CNS

What is the function of motor endings?

spindle sensitivity, contracts the intrafusal fibers with extrafusal for "readiness"

- dynamic

- static

Dynamic

Nuclear bag1 fibers, 1a afferents used for dynamic stretch-velocity

Static

Nuclear bag2 and chain fibers, Ia and II, used for muscle tone

Sensory endings are?

afferent, centrally located

Sensory endings project to?

spinal cord, cerebellum, brainstem, motor cortex

Primary sensory endings

primary - Ia afferent axon

- large diameter -- 70-120m/s

- velocity and length sensitive

- fire most at initial stretching

- input from all 3 intrafusal muscle fibers

Secondary sensory endings

secondary - II afferent axon

- smaller diameter -- 30-70m/s

- muscle length sensitive

- input from nuclear bag2 (static nuclear bag) and nuclear chain

What are muscle spindles important for?

coordinating and smoothing out muscle contraction

In monosynaptic reflexes how many synapses are involved?

only one synapse is involved, no interneuron

What is the purpose of stretch reflexes?

they prevent over-stretching a muscle

Monosynaptic reflexes flow

muscle spingle --> Ia and II afferent --> alpha motor neuron --? facilitate agonist muscle (and synergist muscles)

Effectors Skeletal Muscle: Extrafusal Muscle Fibers

- cyndrically shaped cells

- innervated by alpha motor neurons (a.m.n.)

- 1 alpha motor neuron can innervate several muscle fibers

Which has better fine control?

Eye: 1:10-50

Hand: 1:100

Leg: 1:200-2000

Eye

# varies with the need for fine control

What is a global representation of the muscle (in series of extrafusal muscle fibers) for GTO's?

contains many muscle fibers (10-15) and each fiber is from a different motor unit

GTO's are supplied by?

type 1b sensory afferent --> 1b interneurons

- mediate nonreciprocal (autogenic) autogenic inhibition:

-> "-" input to agonist muscle

-> "+" input to antagonistic muscle

What is the purpose of nonreciprocal (autogenic) inhibition?

They prevent over contraction of a muscle

Nonreciprocal (autogenic) inhibition flow

GTO --> 1b afferent --> 1b interneuron

- inhibit agonist muscle

- facilitate antagonist muscle

Are there more GTO's in anti-gravity (postural) than distal muscle?

Yes

GTO's receive information from CNS to control?

force adjustments

GTO's respond to the force/tension during?

muscle contraction

GTO's respond to multiple extrafusal muscle fibers (whole, not individual), leading to?

smooth muscle contraction

Do GTO's discharge linearly with force?

No

What happens when you are holding a cup and someone pours water in it?

It will create a passive stretch on your bicep brachii so the muscle spindle will send information to the spinal cord. The purpose of the reflex is to prevent over stretching of the bicep brachii.

What are sensory receptors that transmit pain information?

Nociceptors

Assume that you are slowly flexing your arm by only contracting your biceps brachii. This would stimulate the _________ ___________ ________ in the ______ ______ and the _______ _______ in the ________ ________ ________.

This would stimulate the Golgi Tendon Organs in the Biceps brachii and the muscle spindles in the Triceps brachii muscle(s).

_____________________ intrafusal muscle fibers would be most likely to respond to respond different to your performing the same movement twice, but at different speeds?

Nuclear bag1

Sensory neurons - in ______ --> to ______

in PNS to CNS

motor neurons - in ______ --> to _______

in CNS to PNS

Interneurons - within ______

CNS

Extrafusal muscle fibers are innervated by ____ motor neuron. Intrafusal muscle fibers are innervated by ____ motor neuron.

alpha - extrafusal

gamme - intrafusal

REVIEW: muscle spindles

- maintain muscle tone

- are located in the muscle, parallel to extrafusal fibers

- innervated by Ia sensory ending

- innervated by II sensory ending

REVIEW: GTOs

- responds to change in muscle tension

- used for standing posture control, so are found in abundance in your lower back and leg muscles

- makes you aware of your arm location when you reach for a door handle in the dark

- innervated by Ib sensory ending

Monosynaptic/stretch reflex: muscle spindle or GTO

muscle spindle

Nonreciprocal/autogenic inhibition: muscle spindle or GTO

GTO