KIN 4571 - Neurons and Receptors

what are neurons?

nerve cells transferring signals through action potentials

what is retrograde transport?

when the information is sent from the soma to the axon

what is anterograde transport?

when the information is sent from the axon to the soma

what does the synapse include?

axon terminal, dendrites of receiving cells, and neuroglia

what do glial cells (satellite cells) do?

provide nutrients and support to neurons, maintain the chemical environment, and myelinate and protect neurons (do NOT always outnumber neurons)

what are the types of neurons based on morphology?

bipolar, pseudounipolar, and multipolar

what do bipolar neurons do?

information pass through the soma

what do pseudounipolar neurons do?

information does NOT pass through the soma making it faster

what do multipolar neurons do?

most common type; information is going from up to down (soma to axon)

what are the types of neurons based on function?

motor neuron, interneuron, and sensory neuron

what do motor neurons do?

transmit signals from CNS to PNS; send message to muscles/muscle spindles to control muscle contraction

what do interneurons do?

signals within CNS; intergrate and process signals, form connections between neurons, and project “messages” from the spinal cord and higher brain centers

what do sensory neurons do?

transmit signals from PNS to CNS; receive sensory information

where are the sensory neurons located?

Outside spinal cord, in dorsal root ganglion; in ganglia adjacent to, or nuclei within the brain stem

what are the pathways of sensory neurons with or without interneurons?

motor neuron (reflex arc)

what are the pathways of sensory neurons with interneurons?

thalamus/cerebral cortex (making sense of stimuli)

where are the cell body locations of motor neurons?

spinal cord and brainstem nuclei

what are the types of motor neurons?

upper motor neurons (CNS) and lower motor neuron (CNS & PNS): alpha, beta, gamma motor neurons

how much do interneurons make up neurons?

99.9% of the 100 billion neurons

what is the relationship between upper and lower motor neurons?

upper motor neurons has its cell body in the cerebral cortex that descends through the brainstem and spinal cord to innervate lower motor neurons in the ventral horn; activating target muscles

what do alpha motor neurons do?

innervate extrafusal muscle fibers which leads to muscle contraction

what do gamma motor neurons do?

innervate intrafusal muscle fibers at muscle spindle which maintain muscle fiber

what are extrafusal muscle fibers?

cylindrically shaped cells that are innervated by alpha motor neurons

how do you find the motor unit?

by adding one alpha motor neuron will all extrafusal muscle fibers

what can one alpha motor neuron innervate?

several muscle fibers; # varies with the need for fine control;

which has better fine control: eye: 1:10-50 or hand 1:100

eye: 1:10-50

which has better fine control? hand: 1:100 or leg: 1:200-2000

hand: 1:100

what are the types of sensory receptors based on source?

Exteroceptors, Proprioceptors, and Enteroceptors

where can one find Exteroceptors?

in skin

where can one find Proprioceptors?

in muscles, tendons, joints

where can one find Enteroceptors?

in viscera

what are the types of senory receptors based on mode of detection?

Mechanoreceptors, Thermoreceptors, Photoreceptors, Chemoreceptors, and Nociceptors (free nerve endings)

what do mechanoreceptors do?

response to touch, pressure, vibration, and stretch

what do Thermoreceptors do?

sensitive to changes in temperature

what do Photoreceptors do?

response to light

what do chemoreceptors do?

response to chemicals (e.g., smell, taste)

what do Nociceptors do?

sensitive to pain (pain receptors)

where are joint receptors found?

Within and around joint capsule

what do joint receptors do?

Respond to start and end of movements as well as the joint position

what do muscle spindles do?

respond to strength/lengthening (absolute length and velocity) due to muscle contraction and muscle stretch

when does muscle contraction occur?

when there’s high threshold, but it only sometimes respond

when does muscle strecth occur?

when there’s low threshold and it always respond

here are muscle spindles located in?

muscle fibers, parallel to the extrafusal muscle fibers

what do muscle spindles detect?

movement and muscle stiffness

how do muscle spindles detect movment?

through proprioception (sense of limb position)

how do muscle spindles detect muscle stiffness?

through muscle tone (level of excitable motor neurons controlling a muscle; also determines the intrinsic stiffness of extrafusal muscle fibers and level of sensitivity of the reflex

______ muscle spindle density equal ______ movement precision

high; high

what do golgi tendon organs (GTO) do?

respond to force/tension and are located at the muscle tendon junction

what is the composition of golgi tendon organs (GTO)?

contains nuclear bag 1 (dynamic nuclear bag; IA afferent axon), nuclear bag 2 (static nuclear bag; IA and II afferent axons), nuclear chain (IA and II afferent axons), and gamma motor efferent axons

what are primary IA afferent axons?

sensory endings centrally located and project to spinal cord, cerebellum, brainstem, and motor cortex; have a large diameter (70-120 m/s), fire most at initial stretching, input from all 3 intrafusal muscle fibers, and are velocity and length sensitive

what are secondary II afferent axons?

sensory endings located and project to spinal cord, cerebellum, brainstem, and motor cortex; have a smaller diameter (30-70 m/s), input from nuclear bag 2 (static nuclear bag) and nuclear chain, and are muscle strength and length sensitive

what are gamma motor axons (gamma motor neurons/ fusimotor neurons)?

are efferent and found towards the ends, receive information from CNS, maintain spindle sensitivity, and contracts the intrafusal fibers with extrafusal for “readiness”

where can gamma dynamic motor axons be found in?

nuclear bag 1 fibers

where can gamma static motor axons be found in?

nuclear bag 2 and chain fibers

why is it important that we have muscle spindles?

because they are responsible for coordinating and smoothing out muscle contraction

what does golgi tendon organs (GTO) respond to?

force and tension during muscle contraction; to multiple extrafusal muscle fibers (while, not individual), leading to smooth muscle contraction

why and when do GTOs respond to muscle contraction?

when there’s low threshold (always response)

why and when to GTOs respond to muscle stretch?

when there’s high threshold (sometimes response)

where are GTOs located at?

global representation of the muscle (in series of extrafusal muscle fibers)

how many muscle fibers do GTOs contain?

10-15; each from a different motor unit

why are GTOs important?

for postural control; more GTOs in anti-gravity than distal muscles

what is the formation of GTOs discharge?

nonlinear with force

______ respond to passive stretch

muscle spindle

_____ respond to active contraction

GTO

what is the function of visceral receptors?

visceral reflexes at subconscious level

what are the types of visceral receptors?

Mechanoreceptor, Chemoreceptor, and Nociceptors

what do mechanoreceptors do?

changes in blood pressure

what do chemoreceptors do?

changes in pH or blood gases

what do nociceptors do?

distention of an organ (belly ache)