proprioceptors and spinal reflexes

lower motor neuron

neuron that directly innervates skeletal muscle

upper motor nueron

neurons that modulates the activity of lower motor nuerons directly and via interneurons

alpha motor nuerons

lower motor neurons; ventral horn cells

• axons that leave the spinal cord via ventral root and innervate extrafusal muscles

• may innervate one or more muscle fiver

muscle fiber

single, multi-nucleated cell that receives input from one (alpha) motor neuron

motor unit

a single alpha motor nueron and all of the muscle fibers that it innervates

motor end plate

where alpha motor neurons synaptically connect with muscle fibers 

small motor unit

provide precise control of motor activity

large motor unit

provide for less precise motor activity; mediate activities that require sustained contractions or strength moves

motor pool

a group of motor neurons in the ventral horn of spinal cord whose axons project to a single muscle

lateral

proximal

somatotopic organization of ventral horn 

1. BLANK ventral horn control distal muscles

2. medial ventral horn control BLANK muscles 

firing frequency

muscle contraction at the level of motor unit:

• the force of contraction of individual muscle fibers is determined by BLANK BLANK of the motor neuron that innervates them

temporal summation

greater

fused

tetanus

muscle contraction (at the level of motor unit) demonstrate BLANK BLANK - as frequency of firing rate of alpha motor neuron increases, there is summation of muscle tension producing BLANK force of contraction

• at maximum rate, individual twitches are not evident and muscle is BLANK or complete BLANK

tetany

sustained muscular contraction caused by a series of stimuli repeated so rapidly the individual muscular responses are fused

recuirtment 

activation of additional motor units to accomplish an increase in contractile strength in a muscle 

number

contraction at the level of motor pool:

• total force of contraction of a muscle is determined by BLANK of alpha motor nuerons that are active

muscle spindles

detect muscle length and velocity of muscle length changes

• found in muscles itself; attached to extrafusal muscle fibers

golgi tendon organs

detect msucle tension/force information

• found in tendons

• activate when muscles contract - result in tension in tendon and nerve fiber stretching

group 1A axons

type 1 axons; innervate muscle spindles; cell body located in dorsal root ganglion

intrafusal muscle fibers

provide regulation of muscle spindle stiffness for variable sensitivity to strength

primary sensory axons

Type 1A axons; respond to stretching of muscle; detect length and rate of change in length

• cell body of origin = DRG

gamma motor axons

regulate intrafusal tension/stiffness; cell bodies of origin in ventral horn of spinal cord

• regulate sensitivity of muscle spindle to stretch stimulation

extrafusal muscle

regular contractile of muscle

annulospiral ending

formed by primary afferent (1A) neurons that loop around the nuclear region of muscle

central process of DRG neuron

enters the CNS via the dorsal root and courses to ventral horn to synapse on alpha motor neuron; the motor neuron projects back to the SAME MUSCLE and generates contraction of muscle fiber it innervates 

muscle contraction…

….causes the muscle spindle to relax, this leads to decrease afferent input and this leads to decrease motor output to extrafusal muscles

1A afferents

extrafusal

decreases

decreased

decreased

gamma

intrafusal

1A

generate

always

alpha-gamma coactivation:

alpha motor neurons are activated by BLANK BLANK; these neurons innervate the BLANK muscle fibers and intiate contraction. This BLANK tension on muscle spindles - this leads to BLANK activity of IA afferents and BLANK input to alpha motor neurons and muscle fibers innervated by that a. neuron.

IA afferents also innervate BLANK motor neurons - this neurons induce contraction of the BLANK muscle fibers and maintains tension on muscle spindle. This also maintains tension on BLANK afferents, allowing some ion channels to open and BLANK action potentials, which maintains excitatory output to alpha motor neurons and maintains contraction of the extrafusal muscle fiber.

There is BLANK some level of tension in muscles AT ALL TIMES

1B afferents

sensory neurons that detect muscle tension and convey this information from golgi tendon organs to CNS

1B

inhibitory

inhibit

decrease

excitatory

contract

circuitry of golgi tendon organs:

BLANK afferents activate when muscle contracts, they synapse on IB BLANK interneurons in the spinal cord. The interneuron BLANK the motor neuron that projects to the contracting muscle and BLANK tension to prevent tendon damage.

IB afferents also activate BLANK interneurons that project to the antagonist muscle and BLANK it

Brodmann’s area 3a

proprioceptors terminate in this region of the primary sensory cortex

• transmit information to BA #1, #2 and #5 (parietal association areas)

reflext circuit

produce automated responses adaptive for typical situations; sensory input synpases on the motor output which then elicits a muscle contraction

• may contain interneurons that excite or inhibit population of motor neurons

sensory input

motor output

2 components of reflex circuit

spinal reflexes

independent of supraspinal control; function even if the spinal cord is transected

• supraspinal input is usually inhibitory - absence of descending control causes these reflexs to be exaggerated

stretch reflex with reciprocal inhibtion

when a muscle quickly streches, 1A afferents excite the muscles alpha motor neurons which causes muscle contraction

• the 1A afferents synapse on a 1A inhibitory interneuron, which then inhibits the alpha motor nuerons of the antagonist muscle and causes it to relax 

antagonist muscle

the muscle on the opposite side of the joint

• tricep → bicep

• quadricep → hamstrings

flexor withdrawal with crossed extension

results from activation of nociceptor (alpha-delta)

inhibitory

excitatory

excitatory

inhibitory

flex withdrawal with cross extension:

1. BLANK interneurons inhibit alpha motor neuron that project to quadriceps and BLANK interneurons that activate flexor/hamstring on the side where th painful stimulus is activated

2. BLANK interneurons on the contralateral side of the spinal cord that activate the contralteral quadricept and BLANK interneurons that inhibit the contralateral flexor/hamstrings

flexor muscles contract

extensor muscles relax

withdrawl reflex - polsynaptic