Sensorimotor systems of the spinal cord

• communication between incoming sensory signals and outgoing motor signals

• afferent information entering the cord and how it is integrated by neurons in the gray matter of the spinal cord to activate muscles

• Just a few spinal cord neurons can mediate and sustain intricate, complex movements without input from the brain

Sensorimotor Systems

__ entering the cord and how it is integrated by neurons in the gray matter of the spinal cord to activate muscles

afferent information

Just a few spinal cord neurons can mediate and sustain intricate, complex movements without input from the __

brain

• cervical 

• thoracic 

• lumbar 

• sacral

spinal cord

• Has largest diameter – easy to injure

  • Signals between the brain and all 4 limbs pass through the White Matter of Cervical spinal cord

  • The amount of white matter increases the more rostral you go

• Spinal Nerves to:

  • Neck, Arm & Hand

cervical

which part of the spinal cord has the largest diameter and easiest to injure?

cervical 

• Lateral horns - contain sympathetic neuron cell bodies

• Spinal Nerves to:

  • Trunk

thoracic

contain sympathetic neuron cell bodies

lateral horns

• No Cuneate Fasciculus!

• Only Gracile Fasciculus at this level

• Spinal Nerves to:

  • Legs and Feet

lumbar

T/F: the lumbar spinal cord has a cuneate fasciculus

false 

• Defecation, urination, sexual function

• Spinal Nerves to:

  • Posterior legs

  • Sensation from perineum and anus

  • Motor control of External Anal Sphincter

sacral

Both parts are anatomically separated from autonomic nervous system:

• sympathetic 

• parasympathetic 

where does sympathetic originate?

lateral horn

• The unconscious nervous system

• Receives information from visceral receptors

• Has motor control over viscera

Autonomic Nervous System

the Autonomic Nervous System has __ over viscera

motor control

exit the Brainstem via Cranial nerves (esp. Vagus (CNX)) and exit the Spinal Cord through spinal nerves from segments S2 – S4

Parasympathetic signals

exit the Spinal Cord through spinal nerves from segments T1 – L2

All Sympathetic signals

• On their way into and out of the CNS, __ pass through Ganglia

• Such as the Sympathetic Trunk Ganglia (AKA Sym. Chain)

autonomic signals

Autonomic signals entering and exiting the spinal cord travel through __ between the spinal cord and ganglia

Communicating Rami (instead of the Ventral Ramus)

Most motor signals to viscera pass through __

Splanchnic Nerves

• Contain cells bodies of Alpha Motor Neurons for each of the muscles of the body

• Medial Group

  • Supply axial muscles

• Lateral Group

  • Supply distal muscles

• Intermediate Zone – between dorsal and ventral horns

  • Spinal Interneurons

  • For reflexes and organization of muscle synergies

  • Connect L&R sides of the spinal cord for alternating gait movements

Lower (α)Motor Neurons of the Ventral Horn

Lower (α)Motor Neurons of the Ventral Horn contain cell bodies of __ for each of the muscles of the body

Alpha Motor Neurons

Supply axial muscles

Medial group of ventral horn

Supply distal muscles

Lateral Group of ventral horn

• between dorsal and ventral horns

• Spinal Interneurons

• For reflexes and organization of muscle synergies

• Connect L&R sides of the spinal cord for alternating gait movements

intermediate zone

what is the intermediate zone between?

dorsal and ventral horns

what is the intermediate zone for?

For reflexes and organization of muscle synergies

what does the intermediate zone connect?

Connect L&R sides of the spinal cord for alternating gait movements

For movement, motor units, not individual muscle cells, are activated

Motor Units – αMN + muscle cells

• an alpha motor neuron and the muscle cells it innervates

  • An Action Potential in an alpha motor neuron generates a
    twitch in each muscle cell it connects with

motor unit

motor units differ by

• Number of muscle fibers

  • Less muscle fibers per motor unit = more precise control

• Type of muscle fibers

Less muscle fibers per motor unit = more precise control

Number of muscle fibers

muscle cell types

• slow twitch 

• fast twitch 

• intermediate fibers

small, low force, endurance, aerobic

slow twitch

large, high force, fatigue, anaerobic

fast twitch

possesses properties of both

intermediate fibers

• Motor units with low numbers of muscle cells will do light lifting – generate small forces

  • Also allow for precision movements

  • motor units in eye muscles contain ~10 muscle fibers

• Motor units with high numbers of muscle cells will do heavy lifting – generate large forces

  • Allows for less precise movements

  • Motor units in the gastrocnemius contain ~2000 muscle fibers

• Motor Units of increasing size are recruited to increase tension

number of muscle fibers

• Also allow for precision movements

• motor units in eye muscles contain ~10 muscle fibers

Motor units with low numbers of muscle cells will do light lifting – generate small forces

• Allows for less precise movements

• Motor units in the gastrocnemius contain ~2000 muscle fibers

Motor units with high numbers of muscle cells will do heavy lifting – generate large forces

Motor Units of increasing size are recruited to __

increase tension

• Motor units with slow-twitch fibers will do light, repetitive
  lifting – generate small forces with endurance

•  Motor units with intermediate fibers will do medium lifting –
  generate medium forces with medium speed

• Motor units with fast-twitch fibers will do quick heavy lifting –
  generate large forces with fatigue

• Motor Units of increasing strength are recruited to increase tension

type of muscle fibers

what will motor units with slow twitch fibers do?

Motor units with slow-twitch fibers will do light, repetitive
lifting – generate small forces with endurance

what will motor units with intermediate fibers do?

Motor units with intermediate fibers will do medium lifting –
generate medium forces with medium speed

what will motor units with fast twitch fibers do?

Motor units with fast-twitch fibers will do quick heavy lifting –
generate large forces with fatigue

How do αMNs Code for Contraction Strength? Its all about the Frequency!

• Summation

• Unfused Tetanus

  • Maximal tension with a “shake”

    • Partial relaxation occurs, allowing slight recovery

• Fused Tetanus

  • Fatigue occurs quickly

The force, speed, and duration of muscle contraction are determined in the spinal cord

• Maximal tension with a “shake”

  • Partial relaxation occurs, allowing slight recovery

unfused tetanus

• smooth, sustained contraction

  • Fatigue occurs quickly

fused tetanus

• response to change in stimulus frequency

  • increased stimulus frequency (muscle does not completely relax between stimuli) → second contraction of greater force

summation: wave (temporal)

If Alpha (α) Motor Neurons Control Muscle Contraction, what controls the α Motor Neurons ?

1. Descending signals (including UMNs) from the brain

2. Somatosensory (afferent) signals from receptors (Cut. + Proprio.)
   • Afferent Drive

3. Spinal Interneurons in the intermediate zone of the spinal cord grey matter

• Stretch reflex features a monosynaptic circuit

  • allows for a rapid reaction to proprioceptive stimulus

Afferent Drive (incoming somatosensory information)

• Helps coordinate synergies and gait patterns

• 1 sensory neuron synapses with Many cells in the CNS

• αMN and Interneurons

• Axon collaterals split from each other in the grey matter

divergence of the signal

• __ synapses with Many cells in the CNS

  • αMN and Interneurons

1 sensory neuron

• Receive signals from the brain and sensory receptors

• “Decide” if an activation signal is strong enough to be “worthy” of generating an Alpha NM action potential signal to its motor unit of muscle cells

  • Last-order Interneuron: interneurons that synapse with α motor neurons

• Types of ‘Last-order’ Interneurons:

  • 1a Inhibitory

  • 1b Inhibitory

  • Renshaw Cells

Spinal Interneurons

where does spinal interneurons receive signals from?

the brain and sensory receptors

what do spinal interneuron “decide”?

• if an activation signal is strong enough to be “worthy” of generating an Alpha NM action potential signal to its motor unit of muscle cells

  • • Last-order Interneuron: interneurons that synapse with α motor neurons

interneurons that synapse with α motor neurons

last order interneuron

• Involved in the stretch reflex

• Sums together all Excitatory and Inhibitory Signals

• Inhibits α motor neurons to antagonists muscles

  • The stretched muscle can contract unopposed

1a inhibitory

which reflex is 1a inhibtory involved in?

Involved in the stretch reflex

what does 1a inhibitory sum up together?

Sums together all Excitatory and Inhibitory Signals

1a inhibitory inhibits alpha-motor neurons to what muscles?

antagonists muscles

• Involved in the Golgi Tendon reflex

• Sums together all Excitatory and Inhibitory Signals

• Inhibits α motor neurons to tensed muscles

  • The inhibited muscle can relax so the antagonist muscle can contract unopposed

1b inhibtory 

what reflex is 1b inhibitory involved in?

Involved in the Golgi Tendon reflex

what does 1b inhibitory sum up together?

Sums together all Excitatory and Inhibitory Signals

1b inhibitory inhibits alpha-motor neurons to what muscles?

• Inhibits α motor neurons to tensed muscles

• The inhibited muscle can relax so the antagonist
  muscle can contract unopposed

• Provides negative feedback to firing αMNs

  • Receives signals from αMNs and descending signals from the brainstem

• Inhibit 1a Inhibitory Interneurons

• Rhythmically active during locomotion

• Balances contractions of agonist, antagonists and synergists at a joint during locomotion

• help stabilize the motor system and prevent
  overstimulation of muscles

renshaw cells (inhibitory)

what does Renshaw cells provide?

• Provides negative feedback to firing αMNs

  • Receives signals from αMNs and descending signals from the brainstem

when is Renshaw rhythmically active?

• Rhythmically active during locomotion

• Balances contractions of agonist, antagonists and synergists at a joint during locomotion

what does the Renshaw cells stabilize?

help stabilize the motor system and prevent overstimulation of muscles

• An alpha motor neuron is excited and sends an action potential
towards its motor unit.
• A collateral (a small branch) of that motor neuron's axon loops
back and forms an excitatory synapse with a Renshaw cell.
• The Renshaw cell releases the inhibitory neurotransmitters
glycine and GABA onto the original motor neuron
• This inhibitory feedback reduces the firing rate of the motor
neurons, creating a self-regulating system that prevents
excessive muscle contraction

how renshaw cells work

An alpha motor neuron is excited and sends an __ towards its motor unit

action potential

A collateral (a small branch) of that motor neuron's axon loops back and forms an __ with a Renshaw cell.

excitatory synapse

The Renshaw cell releases the

inhibitory neurotransmitters glycine and GABA onto the original motor neuron

This inhibitory feedback reduces the firing rate of the motor neurons, creating a __ that prevents excessive muscle contraction

self-regulating system

attacks Renshaw cells, resulting in uncontrolled muscle spasm

tetanus toxin

If you injure your right foot, your left foot has to carry more body weight, therefore contralateral muscles will be activated to stabilize the contralateral limb

Also crucial for communication to the contralateral lower motor neurons

Spinal Interneurons - Crossed Extensor Reflex

• A muscle stretch/length receptor

• Takes part in:

  • regulation of movement

  • maintenance posture

• Produces a reflex contraction in response to a rapid increase in muscle length

• Thigh muscles play a huge role in standing posture

• If one is standing upright and starts to sway to the left, muscles in the legs and torso are stretched, activating the myotatic reflex to counteract the sway

muscle spindle reflex

what does muscle spindle reflex take part in?

• regulation of movement

• maintenance posture

muscle spindle reflex produces what?

Produces a reflex contraction in response to a rapid increase in muscle length

Thigh muscles play a huge role in

standing posture

If one is standing upright and starts to sway to the left, muscles in the legs and torso are stretched, activating the __ to counteract the sway.

myotatic reflex

Stretch Reflex Tests are called __ because they are tested by tapping the muscle tendon in order to stretch the muscle suddenly

Deep Tendon Reflex Tests

• Assess health of all reflex arc components, spinal nerves, etc.
• Assess Myotome function
• Help determine if a motor issue is the result of a UMN injury (hyperreflexia) or LMN injury (areflexia/hyporeflexia)

purpose of deep tendon reflex tests

Help determine if a motor issue is the result of a __ or __

UMN injury (hyperreflexia) or LMN injury (areflexia/hyporeflexia)

CPGs produce rhythmic motor patterns without input from the brain

Central Pattern Generators

from the Lower extremity can provide the “go signal” that produces alternating excitation and inhibition of antagonistic muscle groups in a gait pattern

Somatosensory signals

CPGs are also responsible for

stimulating the right and left limbs in an alternating sequence of Swing and Stance

Once CPG’s receive a __ they become self- stimulating and produce repetitive synchronized activity that continues until a __occurs

“go signal” / “stop signal”

• CPGs feature lots of __

  • E.g., in order to flex a joint, simultaneous signals are sent to the flexor MNs and the Extensor MNs

    • Flexor MNs receive an excitatory signal

    • Extensors MNs receive an inhibitory signal

reverberating circuits

receive an excitatory signal

Flexor MNs

receive an inhibitory signal

extensor MNs