Spinal cord (+CNS) (wk3)

Question 1

How info travels up and down spinal cord
-Describe the spinal cord

Answer 1

-It creates pathways to and from the brain
-There are 4 parts; cervical, thoracic, lumbar and sacral

Question 2

How info travels up and down spinal cord
-White and grey matter

Answer 2

-The middle of the cord is made up of neurones and other cells (grey matter)
-The outside of the cord is made up of fibres (white matter) that carry info up and down the cord

Question 3

How info travels up and down spinal cord
-Sensory info

Answer 3

-Sensory info comes into the spinal cord at the dorsal horn.
-Info about the body such as; fine touch, proprioception vibration, pain and temperature
-It arrives in the dorsal grey matter

Question 4

How info travels up and down spinal cord
-Motor neurons/motor unit

Answer 4

-Motor neurones -> Located in the ventral spinal cord. These neurones make direct contact onto the muscles. Stimulation of these nerves causes movement. Each cell is a motor unit
-Motor unit -> Make up motor pools. A lower motor neurone in the spinal cord innervates a muscle. Each motor neuron synapses with multiple fibres within the muscle. The motor neurone and all the muscle fibres it contracts, defines a motor unit

Question 5

How info travels up and down spinal cord
-Muscle pools

Answer 5

-All the muscle neurones that innervate a single muscle are called a motor pool
-The size of the innervation is important as muscles that are capable of fine movements are innervated by more neurones

Question 6

How info travels up and down spinal cord
-Describe somatotopy and spinal white matter

Answer 6

-Somatotopy -> Maps are referred to as somatotopic when ‘space’ is related to locations on the body and the adjacent neurons in the neural tissue respond selecetively to stimuli presented to specific locations on the body
-Spinal white matter -> There are fibre tracts that carry info to and from the brain, with 2 major descending and 2 major ascending systems

Question 7

How info travels up and down spinal cord
-Lateral (red) descending systems

Answer 7

-Include the corticospinal and rubrospinal tracts. Fibers of the system are in the dorso-lateral part of the spinal cord. They connect to motor neurones in the lateral part of the ventral horn. This system influences lateral musculature.
1. Corticospinal fibers strongly influence movement of every part of the body
2. The rubrospinal tract can compensate for the loss of corticospinal input - but loss of individual fingers can go
3. The descending corticospinal tract has the ability to use fingers individually and are the sole province of the system

Question 8

How info travels up and down spinal cord
-Medial (blue) descending systems

Answer 8

-Include the vestibulospinal and reticulospinal tracts. Fibers of the medial system are in the ventro-medial part white matter. They connect to motor neurones in the medial part of the ventral horn. It influences medial musculature.
1. The systems are involved in the control of balance and posture - mainly for the medial sections of the body
2. The functions happen with little conscious control
3. The vestibulospinal tract retains balance when the body is moved (external disturbance)
4. The reticulospinal tract helps us retain posture and balance during our own volitional movements (internal disturbance)

Question 9

How info travels up and down spinal cord
-Describe the dorsal column pathway and spinothalamic pathway (ascending)

Answer 9

-Dorsal column pathway -> Carries sensory info from the joint and skin about; fine touch, vibration, two-point discrimination and proprioception (position) from the skin and joints
-Spinothalamic pathway -> The lateral spinothalamic tract conveys; crude touch (a sense of being touched without knowledge of where), pain and temperature.

Question 10

How info gets in and out of the spinal cord
-Dorsal Root Ganglion

Answer 10

-The cell bodies of incoming sensory neurones lie outside the spine in a series of ganglion (the DRG)
-They are unlike motor neurones which have their cell bodies in the ventral horn

Question 11

How info gets in and out of the spinal cord
-Spinal nerves

Answer 11

-Both the sensory and motor axons run in the same nerves, regardless of where the cell bodies are
-There are 31 pairs of spinal nerves
-The positions in the spine determine what part of the body each spinal nerve serves
-The size of the nerve and amount of info carried by the nerve will be different as some parts of the body have more muscles and more sensory receptors

Question 12

How info gets in and out of the spinal cord
-Dermatomes

Answer 12

-An area of the skin supplied by nerves from a single spinal root.
-Example -> After recovery from chickenpox, the virus remains in your DRG. This can reactivate later in life causing an itchy rash that is isolated to a single dermatome

Question 13

How info gets in and out of the spinal cord
-2-point discrimination

Answer 13

-Is the ability to discern that 2 objects are 2 distinct points, not 1
-This ability reflects how finely innervated an area of skin is

Question 14

How info gets in and out of the spinal cord
-Spinal enlargements

Answer 14

-The arms and legs (and hands/feet) have many highly innervated muscles and have a high density of sensory receptors.
-This means that the spinal cord that provides the spinal nerves to the arms and legs are enlarged (due to the high number of innervated muscles)

Question 15

How info gets in and out of the spinal cord
-Info flow in a spinal segment (refer to diagram 10/10)

Answer 15

1. Dorsal horn -> Contains sensory neurones and these receive sensory info and send this up to the brain
2. Ventral horn -> Contain neurones that send messages directly to the muscles
3. Intermediate zone -> Contain interneurons and these integrate info e.g. inhibition

Question 16

Coding in the nervous system
-Sensory system coding

Answer 16

-Stimulus and intensity determines the size of receptor potential and the frequency of action potentials
-Firing frequency for intensity (rate code) -> very non-linear and usually codes for intensity of contrast between 2 levels

Question 17

Coding in the nervous system
-Motor system coding

Answer 17

-> Motor neurones use a rate code to signal the force exerted by a muscle
-An increase in the rate of action potentials fired by the motor neurone causes an increase in the amount of force that the motor unit generates
-Maximal contractions (when the muscle is unable to contract further) is called a tetanic contraction and occurs when the input is at a frequency that is so fast that the muscle cannot relax between action potentials.

Question 18

Coding in the nervous system
-Rate coding in the motor system

Answer 18

-For small forces motor units are recruited first, as the required force increases, larger motor units are recruited
-Size principle states that, with increasing strength of input onto motor neurons, smaller motor neurons are recruited and fire action potentials before larger motor neurones are recruited
-When multiple potentials occur in quick succession, the force in the muscle builds up.

Question 19

Coding in the nervous system
-The effect of stimulation rate on muscle tension (refer to diagram 10/10)

Answer 19

A – At lower frequencies of stimulation, the action potential in the motor neuron results in a single twitch of the muscle fibres
B – (20 Hz) At higher frequencies, the twitches sum produces a force greater than that of single twitches
C – (80 Hz) At higher frequencies, the force produced is greater, but single twitches can still be seen. This response is called unfused tetanus.
D – (100 Hz) At higher rates of motor neuron activation, individual twitches cannot be seen – this is called fused tetanus

Question 20

Coding in the nervous system
-Early increases in strength produced by training are neuronal in origin (refer to lecture in week 3)

Answer 20

-The increased rate of tension achieved after training is accompanied by an increase in accompanied rectified surface EMG activity in the early phase of contraction