spinal cord : sensory and motor systems

Question 1

what are the two major parts of the nervous system ?

Answer 1

CNS - brain and spinal cord

PNS - nerves that are connected to tissues

Question 2

what are the motor components of the nervous system ?

Answer 2

. voluntary ( somatic ) - innervates skeletal muscle

. involuntary ( autonomic ) - innervates smooth and cardiac muscle and glands

Question 3

what is the function of baroreceptor?

Answer 3

located in the walls of large arteries ( aorta and carotid artery ) and they monitor blood pressure by monitoring the strength of the pulse
e.g baroreceptors will signal to the central nervous system that blood pressure is high , so motor nerves that come from particular motor neurons that come from the CNS that innervate heart muscle to slow heart muscle down .

Question 4

what do the 12 cranial nerves do?

Answer 4

12 pairs of cranial nerves connect face, head and neck to the brain

Question 5

what do the 31 pairs of spinal nerves do ?

Answer 5

31 pairs of spinal nerves connecting rest of the body to the spinal cord

Question 6

what is the peripheral and central nervous system relations and functions ?

Answer 6

1. stimulus
2. receptors which are responsive to certain stimuli , transduction occurs which is converting stimuli into electrical activity
3. send electrical activity via sensory/afferent nerves to the CNS
4. interneurons in the CNS are able to instruct and command via the motor/efferent nerve that innervate effector tissues in body
5. effector tissue is a general term for muscle and for glands that produce a response

Question 7

what is the basic structure and function of the spinal cord ?

Answer 7

. part of CNS
. enclosed by bones of the vertebral column
. long ( 0.5 - 0.75 m ) extends from the base of the brain to the level of 1st lumbar vertebrae
. bilaterally symmetrical , has a left and right side ( anatomically and functionally )
. connected to body tissues ( e.g. skin , muscle ) by spinal nerves passing between vertebral bones
. involved in sensory and motor functions of the body below the neck
. comprises 29 adjoining segments : similar structure
. top-to-bottom functional map of adjoining body parts

Question 8

what brainstem ?

Answer 8

. lowest part of the brain and is connected to the spinal cord

Question 9

how do spinal nerves exit/enter spinal cord ?

Answer 9

spinal nerves enter/exit between vertebral bones held apart by cartilage discs

Question 10

what happens if cartilage disc collapses ?

Answer 10

spinal nerves passing through get crushed and causes loss of function and pain
e.g. sciatica

Question 11

how many spinal segments do we have in the spinal cord ?

Answer 11

29 spinal segments

Question 12

what is the function of the 29 spinal segments ?

Answer 12

. mediate sensory and motor functions related to different body parts , top-to-bottom

Question 13

what is the basic structure of 29 spinal segments ?

Answer 13

. 8 cervical - concerned with sensory/motor functions coming from ( neck , upper limbs )
. 12 thoracic - concerned with sensory/motor functions coming from ( trunk )
. 5 lumbar - concerned with sensory/motor functions coming from front pelvis and lower limb
. 4 sacral - concerned with sensory/motor functions coming from ( back pelvis and lower limb )

. all segments have a common structural plan
. but with subtle regional differences in anatomy because of their relations to different body parts

Question 14

how do spinal nerves get in and out form peripheral tissues into the spinal cord through the gaps in the discs ?

Answer 14

each spinal segment has a pair of spinal nerves one on the left and one on the right , when spinal nerves get through gaps in the cartilage they divide in 2 inside the vertebral column to form dorsal ( back ) and a ventral (front )nerve root

Question 15

what is the function of dorsal root ?

Answer 15

send input into spinal cord : sensory input from receptors via axons of dorsal root ganglion ( DRG ) cells

Question 16

what is the function of ventral root ?

Answer 16

sends outputs from spinal cord : voluntary and involuntary motor outputs mainly to skeletal and smooth muscle

Question 17

what are examples of somatic sensory inputs that travel into dorsal part of spinal cord ?

Answer 17

. merkels disc - signal texture
. free nerve ending - signal pain or thermal
. meissner’s corpuscle - signal light pressure
. pacinian corpuscle - signal vibration and deep pressure
. ruffini endings - signal skin stretch

Question 18

what is the common structure in each spinal cord segment ?

Answer 18

inner = grey matter 
outer = white matter

Question 19

what is the structure and function of inner = grey matter in spinal cord segment ?

Answer 19

. contains spinal neuron cell bodies and dendrites
. H-shaped with 3 pairs of horns
. dorsal horn : touch , pain and kinesthetic sensory inputs
. ventral horn : send voluntary motor output to skeletal muscle
. lateral horn : send autonomic output to smooth muscle and glands

Question 20

what is the function and structure of outer white matter in spinal cord segment ?

Answer 20

. axon pathways ( tract )
. sends somatic sensory information up to the brain
. sends motor instructions from the brain to ventral or lateral horn motor neurons

Question 21

why is white matter in spinal segment white ?

Answer 21

white because axons in spinal cord have myelin around them

Question 22

what is myelin and its function ?

Answer 22

. membrane of particular type of glial cell that wrap repeatedly around axons to insulate them
. function is to speed velocity of impulse conduction in axon

Question 23

how does a typical cross section of cord segment look like ?

Answer 23

. dorsal = posterior
. ventral = anterior
. h- shaped grey matter has 9 layers of separate cell bodies of spinal cord which are numbered form 1-9 in roman numerals
- layer 1-6 : have dorsal horn , all nuerons are concerned with somatic sensations ( touch , pain )
- layer 8-9 : contain ventral horn of spinal cord , contain motor neuron that innervate skeletal muscle ( voluntary motor )
layer 7 : contains ( autonomic motor )

Question 24

where does each segment receive sensory input and send motor output from ?

Answer 24

. each segment receives sensory input and sends motor output to the same regions of the body via its pair of spinal nerves organised in the strips

Question 25

what is segmental topography : registration of sensory and motor maps ?

Answer 25

. sequential cervical - to sacral cord segments map sequential body parts from neck-to-foot with some minor distortion
. adjoining segments map adjoining sensory ( dermatomes ) and motor ( myotomes ) strips
. forming detailed sensory and motor maps

Question 26

what do the enlarged cervical and lumbar segments reflect ?

Answer 26

. they reflect the extra innervation ( sensory and motor ) of the upper and lower limbs
. these regions have more more sensory and motor receptors and thus more neurons in feet and hands

Question 27

what are the autonomic divisions ?

Answer 27

. sympathetic or thoraco-lumbar division

• originates from lateral horns motor neurons
• spinal levels T1 to L2

. parasympathetic or cranio-sacral divisions

• originates mainly from the brain ( cranium )
• plus some lateral horn motor neurons @ sacral levels S2-4

Question 28

what happens if there is no autonomic neurons ?

Answer 28

. there will be no lateral horns at C1-8 or L3-S1 of spinal cord

Question 29

what is the difference in function between sympathetic and parasympathetic nervous system ?

Answer 29

. sympathetic division - dilates pupil , accelerates heart beat , relaxes airways ,
. parasympathetic division - constrict pupil , slows heart beat , constrict airways

Question 30

what do the axons in the white matter do ?

Answer 30

some axons are sensory so they send signals to the brain and some are motor which come from the brain and go down to innervate neurons of ventral or lateral horns
these axons run in bundle of axons found in the right matter these regions are known as funiculus

Question 31

what do lateral , ventral and dorsal funiculus have ?

Answer 31

they contain different axon pathways

Question 32

what is the somatic sensory pathways ?

Answer 32

1. from sensory receptors and axons of the dorsal root ganglion cells to spinal cord and then via different ascending axon pathways running in parallel up to the brain
2. in the dorsal or ventral-lateral funiculi

Question 33

what are different somatic sensory pathways , different specialised functions ?

Answer 33

. conscious somatic sensation/perception
. control of voluntary movements
. brain arousal mainly from pain

Question 34

what is the voluntary motor pathways ?

Answer 34

1. from upper motor neurons in the brain
   via different descending axon pathways running in parallel in the lateral or ventral funiculi
2. to lower ventral horn motor neurons which send axons via the ventral root of spinal nerves directly to skeletal muscle

Question 35

what are different voluntary motor pathways , different specialised functions ?

Answer 35

. fine control complex movements e.g. hand functions
. rhythmic coordinated movements
. balance and posture

Question 36

what are the two major classes of motor neuron ?

Answer 36

there are two major classes of motor neurons associated with voluntary skeletal muscle

1. large ( alpha ) - innervate main (extra-fusal ) muscle fibres- when you contract makes muscle shorter
2. small ( gamma ) - innervate ( intra-fusal ) muscle spindle

Question 37

what are the major classes of motor neurons grouped int ?

Answer 37

they are grouped in pools : 1 pool supplies 1 muscle topographically orgsnized

. medial pools : midline( trunk )muscle ( balance/posture)
. lateral pools : lateral (limb) muscles ( complex actions )