Anatomy 3: Overview of the Nervous System

Question 1

What is the function of the nervous system?

Answer 1

The nervous system is the master controlling and communicating system of the body. It gathers information about what is occurring inside and outside the body. It then processes this information and makes decisions about what should be done moment by moment.

Question 2

What is nervous tissue composed of?

Answer 2

Nervous tissue consists of neurons and supporting cells. Neurons transmit, receive and process information.

Question 3

What is a neuron?

Answer 3

The neuron is the functional unit of the nervous system. They are sensitive to stimuli (mechanical, thermal or chemical) and are able to conduct messages.

Question 4

What is the structure of a neuron?

Answer 4

Extending from the cell body (soma, perikaryon) are:

• Numerous branching processes called dendrites, which receive electrochemical stimuli from other cells at synapses
• The axon, a long singular process that does not branch close to the body and transmits electrical signals away from the cell body, towards another neuron.
  
  • The axon is normally covered by a myelin sheath (lipid made by Schwann cells), which insulates the axons and speeds up electrical conduction along it.
  • Near the end of the axon, it splits into small branches called telodendria. These end as small swellings (axon terminals or terminal boutons)

Question 5

What are the two parts of the nervous system?

Answer 5

1. Central Nervous System:

The brain and spinal cord.

2. Peripheral Nervous System:

Everything else! The nerves and ganglia outside the brain and spinal cord. The PNS has voluntary (somatic) and involuntary (autonomic)
components. This includes both afferent (sensory) and efferent (coming) afferent means carrying sensations TO the spinal cord, whereas efferent means carrying sensations away from the spinal cord!

Question 6

What are the components of the peripheral nervous system?

Answer 6

1. Somatic (voluntary) component
2. Autonomic (involuntary) component

Question 7

What is the purpose of the somatic (voluntary) component of the nervous system?

Answer 7

Somatic (voluntary) component:

responsible for innervating (supplying, projecting) the skin, skeletal (striated) muscle, bones and joints – essentially dealing with the body wall and the limbs, being a volunatry process.

• The skin, bones and joints are all involved in sensory information to the spinal cord or brain
• Muscle involves commands going out to the muscle from the spinal cord (other way!).

Question 8

What is the purpose of the Autonomic (involunary) component?

Answer 8

Autonomic (involuntary) component:

responsible for innervating the involuntary parts of the body, including internal body organs (viscera) including blood vessels and glands.

• Can be further divided into the sympathetic and parasympathetic parts.

Question 9

In the PNS, there is a sensory (afferent) division and a motor (efferent) division. What are the subdivivisions of each?

Answer 9

Sensory (Afferent) Division

Somatic sensory

Visceral sensory

Motor (Efferent) Region

Somatic Motor

Visceral Motor

Question 10

What is the function of the Somatic Sensory division?

Answer 10

Somatic sensory: Sensory that comes from the body or the limbs, including a wide range of senses:

ƒ Touch ƒ Pain ƒ Pressure ƒ Vibration
ƒ Temperature ƒ Proprioception (knowing position of limbs in space, important for use of muscles)

Question 11

What is the function of the Visceral Sensory Division?

Answer 11

Visceral sensory: The reason most patients will see a doctor, for abnormality. Cardiac pain, carried back to spinal cord and then brought to conscious awareness by visceral sensory fibres – which come in lots of different shapes and sizes. Visceral sensory fibres include:

ƒ Stretch ƒ Pain ƒ Temperature ƒ Chemical changes (to be received as pain) ƒ Nausea and hunger

Question 12

What is the function of the Somatic Motor division?

Answer 12

Somatic motor: The reason we are able to move our limbs is because of somatic motor function – we can voluntarily contract skeletal muscle, due to the innervation of all skeletal muscles, due to innervation from the spinal cord and cranial nerves.

Question 13

What is the function of the Visceral Motor division?

Answer 13

Visceral motor: There is also smooth muscle (involuntary) control, and this includes cardiac muscles and glands, which are subconcious – all covered by visceral motor.

Question 14

The CNS is largely enclosed in the bony framework. Inside the skull, there are lots of membranes that surround and protect the brain. What are they called?

Answer 14

Meninges (They support and protect the brain)

Question 15

What are the layers of the meninges?

Answer 15

ƒ Dura mater – outside membrane

ƒ Arachnoid – delicate membrane (like a spider’s web!) Below the arachnoid there is a subarachnoid space – which has a fluid in it – CSF!

ƒ Pia mater – on the surface of the brain surface.

Question 16

What is the difference between grey and white matter?

Answer 16

Grey matter is where there are nerve cell bodies (as well as supporting cells and dendrites). grey matter sits on the outside of the brain (but inside the spinal cord). grey matter has unmeyelinated axons

White matter is positioned on the inside of the brain (but outside the spinal cord). White mater has myelenitated axons. White matter is where there are axons.

Question 17

What is the structure of the spinal cord?

Answer 17

The spinal cord is around 45 cm long.
It is a thin structure that attaches to the brain stem and runs down the neural canal of the vertebral column, finishing at around L1 (lumbar vertebra 1).
There are lots of nerves attached to the spinal cord, called spinal nerves .
It is divided into discrete segments according to the nerve regions.

• Cervical nerves 1-8 (7 cervical vertabrae)
• Thoracic nerves 1-12 (12 vertabrae)
• Lumbar 1-5 (5 vertabrae)
• Sacral 1-5 (5 vertabrae)
• ]Coccygeal – 1 (at coxis)

Question 18

Why is there a discrepancy in the spinal segement and nerve exit locations?

Answer 18

Because the spinal nerve is shorter then the vertebral column, it means that some of the spinal nerves actually have to run some distance before they can leave the vertebral column!

• At the top, the 1st cervical nerve comes out and the 1st cervical nerve.
• As you move down, the nerve must travel further and further before it leaves – but the nerve’s name is based on which vertebral column it leaves below!

Question 19

What is the spinal segment, and what are its components?

Answer 19

The spinal segment is the area of the spinal cord that gives rise to the rootlets of one spinal nerve.

There are 2 sets of rootlets – and one at the back, called dorsal rootlets , and one at the front, called ventral rootlets, which all join together to form a spinal nerve.

Question 20

What is the basic structure of a spinal nerve, and the surrounding structures?

Answer 20

Question 21

What are dorsal roots?

Answer 21

Where incoming s ensory fibres (eg. the skin) enter the spinal cord.

Question 22

What are ventrala roots?

Answer 22

Where outgoing motor fibres (eg. skeletal muscles, glands) leave the spinal cord.

Question 23

What are ganglion?

Answer 23

Positioned on the dorsal root . It is a collection of nerve cell bodies, outside of the CNS.

The cell body of a ganglion has two sensory nerves, a central process of which goes into the spinal cord and a peripheral process which goes out to the skin.

Question 24

What is the dura?

Answer 24

A membrane thaat covers the spinal cord. Has root sleeves to cover the rootlets as well as the dorsal root ganglic.

Question 25

What happens at the caudal end of the spine?

Answer 25

As we move down the spinal cord, the spinal nerves must travel a further distance before they move out of the vertebral column. At the bottom of the spinal nerve, after the **conus medullaris (end of spinal cord) there are still nerve fibres running down – which are technically dorsal and ventral roots. They form a bundle of nerve fibres, that looks like a horse’s tail**

Question 26

What is the purpose of the ANS, and the Hypothalmus?

Answer 26

* The ANS refers to matters that are out of voluntary control. * **The hypothalamus is sometimes referred to as the ‘visceral brain’ as it is involved in the overall control of the ANS and a lot of automatic functions**. Essentially, what needs to be known is there is a cascade of control , which eventually leads to the spinal cord.

Question 27

What are the classical divisions of ANS?

Answer 27

1. Symphatic path 2. Parasympathetic path

Question 28

What is the function of the sympathic path?

Answer 28

The sympathetic division is regularly thought be involved in dealing with EMERGENCY SITUATIONS. These include the **4 F’s: Fight, Flight, Fear and …Sexual Activity!** It is usually about expending a lot of energy to get out of that situation. This includes supply to the lungs (bronchodilation, decreased secretion) for increased intake of air. **The sympathetic path is also referred to as the thoracolumbar outflow.** The outflow from the spinal cord is **through very specific thoracic and upper lumbar segments – the middle part of the spinal cord.**

Question 29

What is the function of the Parasympathetic path?

Answer 29

The parasympathetic division is often referred to things related to being in the **VEGETATIVE STATE - tends to be about conserving energy or taking energy stores on board to be used late**. It is **responsible for storage, conserving energy, taking on board nutrients and bringing the body back to a state of rest.** The parasympathetic path is also referred to as the **craniosacral outflow**. There is a **flow of control through cranial nerves (nerves attached to the brain stem) and sacral nerves (nerves attached to sacral part of spinal nerve).**

Question 30

What is the difference between the location of the ganglia in the Sympathetic pathway and the Parasympathetic pathway?

Answer 30

In the sympathetic (thoracolumbar) pathway, the **ganglia is closer to the spinal cord (found in sympathetic trunk),** and so it has a longer post - ganglionic fibre than pre ganglionic fibre – and so are often referred to as mainly **POST-GANGLIONIC.** In the parasympathetic (craniosacral) pathway, t**he ganglia are in or closer to the visceral organ being served,** and so they have much longer pre - ganglionic fibre than post - ganglionic fibre , and so are thought to be **PRE-GANGLIONIC.**

Question 31

What is the difference betweenj the autonomic and somatic motor systems?

Answer 31

**Somatic (voluntary):** A somatic motor neurone is a voluntary neurone that has a cell body within the spinal cord (ventral horn) which sends its axon out through the ventral root to skeletal muscle. **The somatic motor system only has 1 nerve cell that goes straight to the target tissue.** **Autonomic (involuntary):** The autonomic outflow comes from a different flow. The autonomic (in this case sympathetic) neurone is connected to the lateral horn , with a fibre that heads out to a GANGLION, instead of going straight to the target structure (smooth muscle or gland etc.). There is usually 1 pre-ganglionic neurone in the spinal cord and then a postganglionic neurone inside a cluster of nerve cell bodies, which then sends an axon out to the body being supplied! The ANS usually has at least 2 nerve cells in the chain of th e outflow from the spinal cord to the target organ.

Question 32

What is the sympathetic trunk?

Answer 32

* A collection of nerve cell bodies (the ganglia) lie alongside the entire length of the vertebral column. The sympathetic trunk has very fine process that connect them to the spinal nerves. * Ramus communicans , connects the sympathetic trunk to the spinal nerve.

Question 33

What is happening in this picture?

Answer 33

As can be seen, there is a pre-ganglionic nerve fibre that comes out of the spinal cord, crosses the white ramus communicans to go the ganglia in the sympathetic trunk, where it then synapes to a post ganglionic nerve fibre, passes back along the gray ramu s communicans , and then heads out to the target tissue.