Basic Anatomy of the Somatic Nervous System

Question 1

what is the role of the NS?

Answer 1

important in integrating information coming from our external environment AND regulating internal processes

Question 2

describe the different divisions of the NS

Answer 2

Question 3

what is the NS split into?

Answer 3

central nervous system
peripheral nervous system

Question 4

what is an afferent neurone?

Answer 4

send sensory impulses from receptors in the PNS to targets in CNS (PNS –> CNS)

Question 5

what is an efferent neurone?

Answer 5

send impulses from the central nervous system to your limbs and organs (effectors) located in the PNS (CNS –> PNS)

way to remember: EFFErent goes to EFFEctors

Question 6

why does only efferent system divide further and not afferent?

Answer 6

afferent system is simply just a sensory system (sensory neurones).
so they don’t divide into somatic and autonomic systems.
only the efferent system does as it only makes sense that effects are voluntary and involuntary.

Question 7

what is the somatic and autonomic NS?

Answer 7

somatic- controls voluntary movement
autonomic- controls involuntary movement

Question 8

what does the autonomic NS divide into?

Answer 8

sympathetic- fight or flight
parasympathetic- rest and digest
enteric- NS involved in regulating your gastrointestinal tract

Question 9

what are the anatomical, directional and functional devisions of the NS?

Answer 9

anatomical- CNS and PNS
directional- afferent and efferent (gives you direction of impulse)
functional- somatic and autonomic (voluntary/ involuntay)

Question 10

what does the CNS consist of?

Answer 10

• the brain and spinal cord

Question 11

what layers of tissue cover the brain and spinal cord?

Answer 11

Meninges:
a layer of connective tissue which acts as a membrane around the brain and the spinal cord.
consists of:
- dura mater
- arachnoid matter
- pia matter.

Question 12

what fluid is the brain and spinal cord suspended in?

Answer 12

Cerebrospinal Fluid - This is present in the subarachnoid space (between arachnoid and pia matter)

Question 13

what is the superficial layer of the CNS?

Answer 13

Cranium (brain)
Vertebral Column (spinal cord)

Question 14

what does the PNS do?

Answer 14

• nervous system outside the CNS
• connects CNS to target organs
• connects sensory to CNS
• so that the CNS can process information

Question 15

what does the PNS consist of?

Answer 15

• 31 spinal nerve pairs
• 12 cranial nerve pairs
• Ganglia (autonomic/sensory) - bunches of neuronal cell bodies
• Nerve plexuses (Brachial/ lumbar/sacral) - networks of nerves that carry both afferent & efferent systems
• Enteric nervous system - regulating gastrointestinal tract

Question 16

what cells does the NS mainly consist of?

Answer 16

• neurones (nerve cells) ~10%
• glial cells (supporting cells) ~90%

Question 17

what is the structure of a neurone?

Answer 17

the nerve cell responsible for
conducting impulses
micro-anatomy of a neurone:
cell body (perikaryon) = 1
dendrites = many
axon = usually 1
terminal bouton/ synapses=
many

Question 18

what is an axon hillock?

Answer 18

region where an action potential is generated.
produces ‘kink’ in neurones – visible in micrographs

Question 19

what are the 3 types of neurones?

Answer 19

Multipolar = Motor (part of efferent NS)
Bipolar = Relay or special sense
Pseudo-unipolar = Sensory

Question 20

what is the structure of a multipolar neurone?

Answer 20

• multiple dendrites
• one axon that divides into multiple synapses

Question 21

what is the structure of a bipolar neurone?

Answer 21

• one dendritic process emerging to the cell body
• one axon that carries impulse from cell body to synapse
  (hence bipolar)
• mainly found in special senses ie. olfactory epithelium in nose, neuronal layers in retina

Question 22

what is the structure of a pseudo-unipolar neurone?

Answer 22

• one process coming off cell body and immediately branching into dendrites and axons
• adv: info from dendrites can pass directly to the terminal synapse w/o having to go to cell body

Question 23

what are glial cells?

Answer 23

comes form the greek term “glue”
most predominant cell type within the CNS
responsible for creating optimum microenvironment for neuronal activity

Question 24

what are the 4 main types of glial cells?

Answer 24

• Astrocytes
• Microglia
• Ependymal
• Oligodendrocytes

Question 25

what is the structure of astrocytes and what do they do?

Answer 25

• Star shaped
• Contribute to the blood-brain barrier- take nutrients from the blood into extracellular matrix of brain and neurones
• Most abundant cell type
• Provide structural and metabolic support
• have the ability to regenerate and can form gliomas (tumour)
• involved in removing synapses at the end of their life
• some synapses are tripartite (3 terminals rather than 1) the 3rd terminal comes from the astrocyte - involved in up taking neurotransmitters that are released into the synaptic cleft to prevent over/ under activation of synapse

Question 26

what is the structure of microglia and what do they do?

Answer 26

• immune function- phagocytosis
• when resting, they communicate with other neurones & cells to make sure that everything is controlled (Ramified)
• they pick up any signals from other neurones indicating that something isn’t right, hence activating them
• maintain brain homeostasis
• dysregulation can lead to neurological disease - through increased activation of microglia

Question 27

what is the structure of ependymal cells and what do they do?

Answer 27

• simple ciliated epithelial cells lining the ventricles
• cerebrospinal fluid synthesis: provides nutrients and creates perfect environment for neurone function
• protection of brain and buoyancy

Question 28

what happens if cilia doesn’t work properly?

Answer 28

if cilia is disregulated, it can lead to conditions relating to flow of CSF e.g hydrocephalus - build up of CSF in the brain

Question 29

what is the structure of oligodendrocytes and what do they do?

Answer 29

• cells with few processes
• create myelin sheaths around neurones in the CNS
• can myelinate several neurones at the same time
• provide metabolic support and electrical insulation

Question 30

what is a glial unit?

Answer 30

when one oligodendrocyte or Schwann cell myelinates one portion of the axon

Question 31

what do Schwann cells do?

Answer 31

• create myelin sheaths around neurones in PNS
• provide metabolic support and electrical insulation

Question 32

what is the difference between Schwann cells and oligodendrocytes?

Answer 32

one schwann cell can only myelinated one axon but oligodendrocytes can myelinate multiple in one go
Schwann cells are found in the PNS ONLY
oligodendrocytes are found in the CNS ONLY

Question 33

what is myelin?

Answer 33

Myelin is a lipid sheath 0.5-2.5μm
its an extension of the bilayer and wraps around the axon
thick
created by glial cells which surrounds the axon of a neurone
Increases the speed of conduction, provides protection and structural support
Created by:
Oligodendrocytes in CNS
Schwann Cells in PNS

Question 34

what are the gaps between the myelin sheath called?

Answer 34

nodes of Ranvier

Question 35

what is a nodal membrane?

Answer 35

Membrane at the node of ranvier

Question 36

what is an internodal membrane?

Answer 36

Membrane at the regions that are myelinated

Question 37

what are unmyelinated neurones?

Answer 37

axons are still surrounded by schwann cells - don’t completely wrap around the axon
the schwann cells have a more supportive role

Question 38

what is the somatic NS?

Answer 38

• specific collection of tissues of the nervous system
• the voluntary division of NS
• its sensations give rise to awareness of the sensory world around us– allows us to accurately localise sensations throughout the extent of the body
• gives us awareness of our body position in 3D space– known as proprioception

Question 39

what is the role of the afferent system in the somatic NS?

Answer 39

carry sensory information from the surface of the body to the NS
its sensations give awareness to the sensory experience
gain stimuli from the PNS are transmitted to the CNS

Question 40

where is the afferent neurone found in the somatic NS?

Answer 40

sensory neurone cell bodies are found in the dorsal root ganglion of the spinal nerve

Question 41

what is the role of the efferent system in the somatic NS?

Answer 41

carries motor info of the NS (output)
convey commands from the brain to skeletal muscles only (voluntary movements)
it manifests its actions as muscle contraction

Question 42

where is the efferent neurone found in the somatic NS?

Answer 42

motor neurone cell bodies are found in the ventral horn of the spinal cord or cranial nerve nuclei in the brain stem

Question 43

what is the afferent systems involvement in processing sensation?

Answer 43

sensation taken from areas such as the skin or special sense organs
processed by relevant areas of the CNS
elicits a reaction from the efferent division

Question 44

what is the region of the brain involved in processing sensory info?

Answer 44

primary somatosensory cortex

Question 45

what is the efferent systems role in movement of muscle?

Answer 45

only synapse with skeletal muscle
(smooth and cardiac are in autonomic NS)

Question 46

how do efferent neurones transmit impulses to the muscles?

Answer 46

following AP…
synaptic vesicles containing neurotransmitters fuse with PM release neurotransmitters to synaptic cleft
in muscle cell membrane there are receptors that detect neurotransmitters and cause contraction
neuromuscular junctions
(NMJs) or motor end
plates conduct stimuli to
skeletal muscle

Question 47

what is the difference between a neurone and a nerve?

Answer 47

MANY neurones make up ONE nerve

Question 48

what is the anatomy of a nerve?

Answer 48

multiple axons are grouped together to form a fascicle
many fascicles create a nerve numerous axon bundles (fascicles) surrounded by three protective layers

Question 49

what are the protective layers that surround fascicles?

Answer 49

from out to within:
epineurium - sheaths the entire nerve
interfascicular bands attach adjacent nerve fascicle

perineurium - axons with common origins and common destinations will travel together forming a nerve fascicle.
perineurium ensheaths a nerve fascicle

endoneurium - ensheaths a single cell’s axon
gives it structure and isolates it

Question 50

what are some typifying characteristics of the somatic NS? (hamzah’s notes)

Answer 50

It can be said to exist in a bi-stable state
Thus, it is either
Active (awake)
or
Inactive (sleep)
It is largely paralysed during [rapid eye movement sleep]
Its effector organ is the skeletal muscle only
It is responsible for movement (via muscles) and motor tone of the body

Question 51

what are characteristic anatomical properties of neurones of the somatic NS? (hamzah’s notes)

Answer 51

Dendrites – very important as they increase the surface area of the cell’s membrane.
synaptic impulse are directed through these membranes.
Dendrites with direct connection to the cell body are known as proximal dendrites
Dendrites with no connection to the cell body are called distal dendrites
The diameter of the dendrites the lower the cross-sectional resistance to electrical impulses, so the faster the impulse travel
The Cell body has a multipolar cellular morphology – the cell body has a lot of processes radiating in multiple directions
Cell body is located in the central nervous system (Ventral horn of the spinal cord or Cranial nerve motor nuclei of the brain)
They are largely myelinated:
Alpha motor neurons (fastest) (main) innervate the highly contracting fibres that supply the muscle with its power.
Gamma motor neurons (slow) innervate slightly contract fibres – allows muscle to be sensitive to stretch, over a wide range of muscle lengths.

Question 52

describe the structure of somatic afferents/ sensory neurone? (hamzah’s notes)

Answer 52

The axon splits into two sub axons.
This is a sensory neurone so its electrical impulses are generated at the periphery end which converts electromechanical energy into electrical impulses
The impulses are then propagated along the periphery axon towards the cell body
The cell body receives a copy of the electrical impulse alerting the cell allowing it to respond metabolically to support the axon – e.g. makes sure nutrition is made available to the axon.
The impulse then moves along the axon towards the brain.
Bold cell body – Axonal terminals on the cell body would interfere with signal generated in the peripheral corrupting the signal.
This morphology is called pseudo unipolar morphology (characteristic primary sensory neurones) - one process radiating in multiple directions
Primary sensory neurones: neurones responsible for converting electromechanical energy into electrical impulses.
Cell body is located outside of the CNS they are located in ganglia.
Levels of myelination vary between functional sub-types
Some are myelinated:
Heavily myelinated (alpha) and (Beta)
Lightly myelinated (gamma)
The rest are unmyelinated:
Knows are C-fibres Or free nerve endings (largely responsible for pain sensation)

Question 53

what does the neuroglia in the PNS consist of? (hamzah’s notes)

Answer 53

Neuroglia of the PNS are:
The Schwann Cell
Main Function is myelination of Peripheral Nerve Axons
1 Schwann Cell myelinates only 1 axon segment
The Satellite Cells
Physical Support of Neurones in PNS
Microglia
(they are macrophages) Immune and Inflammatory functions

Question 54

describe the myelination of axons of the PNS in the somatic NS (hamzah’s notes)

Answer 54

Are variable in thickness & length
Some are not myelinated - Thin axons
Others are heavily myelinated -Thick axons
The level of myelination varies between neuronal types
Unmyelinated axons are only found on C fibres (responsible for pain sensation) - somatic sensory neurones
All motor neurones are myelinated
Conduction speed of electrical impulses is correlated to the level of axonal myelination
Expressed mathematically as Conduction velocity (CV) =5.8 x fibre diameter (FD)

Question 55

describe the myelinated axon in X-Section of the somatic NS
what diseases can demylinate axons? (hamzah’s notes)

Answer 55

Thickness of myelin varies between neuronal types
Myelin of thin axons can be 0.5 micrometres thick
Myelin of thick axons can be 2.5 micrometres thick
On average, a Schwann Cell wraps itself 100x around an axon
Certain diseases demyelinate axons
Multiple Sclerosis
Diabetes
Guillain Barre
Polyneuropathies
They provide insulation - increases the speed of conduction of nerve impulses
Reduces the capacitance of neurones – makes it easier for the neurone to be activated
If a nerve axon is cut and it dies away, the myelin sheath left behind maintains the tunnel where the old axon ran. This creates a path taken by a new/regenerated axon.

Question 56

what is a motor neurone? (hamzah’s notes)

Answer 56

It is an efferent neurone that supplies
Skeletal muscle - effectors of the somatic nervous system
Smooth muscle - effectors of the autonomic nervous system
Cardiac muscle are myogenic
Role of skeletal muscles:
Bring about displacement of limb (movement)
Set muscle Tone
Taken together motoneurons make up the motor system of the body

Question 57

describe the anatomy of the NMJ (hamzah’s notes)

Answer 57

It is confined to a specific area of the muscle known as the ‘nerve entry point’ or neurovascular hilum
The nerve entry point is the:
Geographical centre of any given muscle (allows contraction to be centralised)
Site of entry of a motoneuron into the substance of the muscle
Site of entry of arterial supply to a muscle
Site of exit of venous drainage of the muscle
Site of collection of nicotinic receptors of healthy muscles – that allow for electrical impulses of the nerve to produce contraction of the muscle done with production of neurotransmitter – Acetyl choline.