AHN 4: Nervous system

Define the categories of the nervous system: central, peripheral, autonomic, sympathetic and parasympathetic. 1.9 Describe and compare the basic structure and organisation of the somatic and autonomic nervous systems 1.10 Describe the main features of spinal nerves and cranial nerves and distinguish their main structural and functional differences 1.11 Define the characteristics of a reflex and describe the anatomy of a basic reflex circuit 1.12 Describe the basic plan of connections of the parasympathetic autonomic ganglia of the head

What are the three main functions of the nervous system? 

• (1) Sensory input: Receives information (stimuli) from inside and outside the body. 

• (2) Integration: Processes and interprets sensory information, deciding how the body should respond. 

• (3) Motor output: Initiates an action or response. 

• AND maintains homeostasis, meaning the regulation of the body's internal environment to keep it relatively stable 

Compare/contrast CNS and PNS  

Division	Structures	Function
CNS (central nervous system)
PNS (peripheral nervous system)

Division	Structures	Function
CNS (central nervous system)	Brain and spinal cord	Processes and integrates information. Control centre.
PNS (peripheral nervous system)	Cranial and spinal nerves	Connects the CNS to the rest of the body

Define afferent vs efferent 

• Afferent = arriving at the CNS (sensory). 

• Efferent = exiting the CNS (motor). 

Within the PNS what branch controls the skeletal muscle and which controls the autonomic motor output? 

• Somatic motor = Controls skeletal muscle (voluntary movement). 

• Autonomic motor = Controls smooth muscle, cardiac muscle and glands (involuntary). 

Compare/contrast somatic vs autonomic  

Feature	Somatic	Autonomic
Control	Voluntary/involuntary	Voluntary/involuntary
Target
Main role
Motor pathway

Feature	Somatic	Autonomic
Control	Generally voluntary	Involuntary
Target	Skeletal muscle	Smooth muscle, cardiac muscle and glands
Main role	Movement	Regulation of internal organs
Motor pathway	One motor neurone from CNS to muscle	Two-neurone pathway involving a ganglion

What are the 4x different types of afferent fibres and what are the 3x efferent fibres?  

• Afferent (sensory): (1) General somatic, (2) general visceral, (3) special somatic, (4) special visceral  

• Efferent (motor): (1) general somatic, (2) general visceral, (3) special visceral  

Sensory afferent fibres: 

Fibre type	What it carries
General somatic afferent (GSA)
General visceral afferent (GVA)
Special somatic afferent (SSA)
Special visceral afferent (SVA)

Why are SSA and SVA divided?

Fibre type	What it carries
General somatic afferent (GSA)	Touch, pain, temperature and proprioception from skin, muscles and joints
General visceral afferent (GVA)	Visceral - ORGANS…Sensory information from internal organs
Special somatic afferent (SSA)	Vision, hearing and balance
Special visceral afferent (SVA)	Taste and smell

SSA and SVA divided because they are derived from different embryological origins. Special somatic afferent fibres derive from ectoderm, Special visceral afferent fibres derive from chemoreceptors from endoderm.  

Motor efferent fibres:  

Fibre type	What it supplies
General somatic efferent (GSE)
General visceral efferent (GVE)
Special visceral efferent (SVE)

Fibre type	What it supplies
General somatic efferent (GSE)	Skeletal muscles
General visceral efferent (GVE)	Visceral - ORGANS…So Smooth muscle, cardiac muscle and glands
Special visceral efferent (SVE)	Skeletal muscles derived from the pharyngeal arches, e.g. muscles of mastication and facial expression

All spinal nerves are mixed nerves – containing sensory and motor fibres. There are 31 pairs of spinal nerves how are they divided across regions? 

Spinal nerves are mixed nerves - motor and sensory function.

• Cervical, 8 

• Thoracic, 12 

• Lumbar, 5 

• Sacral, 5 

• Coccygeal, 1 

• Total, 31 

There are 8 cervical spinal nerves but only 7 cervical vertebrae how do they relate to each other?  

• C1 comes out above the first cervical vertebra.  

• Then the next 7x Cervical vertebra: C2-C8 come out ABOVE its corresponding vertebra (under the vertebra above E.g. C2 comes out ABOVE C2 vertebra by under C1 vetebra).  

• C8 comes out under vertebra 7. Then thereafter the thoracic nerves come out under their corresponding vertebra e.g.   T1 comes out UNDER CORRESPONDING thoracic vertebra 1.

What are the following structures  

(1) Dorsal Horn: receives and processes sensory information.

(2) Ventral Horn: contains the cell bodies of somatic motor neurones. 

(3) Dorsal root: carries sensory (afferent) information towards the spinal cord. 

(4) Ventral root: carries motor (efferent) information away from the spinal cord.

(5) Spinal nerve: Dorsal and ventral roots join to make mixed spinal nerve.  

What is the function of spinal horn, root and rami?

• Horn: (internal grey matter of the spinal cord is in a H shape so different sections have different functions). - anterior/ventral horn, posterior/dorsal horn and lateral horn.  

• Root: connect the spinal cord to the emerging spinal nerves. The dorsal and ventral roots unite just outside the spinal cord within the intervertebral foramen to form a single mixed spinal nerve.  

• Rami: after exiting the intervertebral foramen, each mixed spinal nerve splits into distinct branches called rami – anterior/ventral, posterior/dorsal 

What is the function of a dorsal, ventral and lateral horn? 

Spinal horn = internal grey matter of the spinal cord is in a H shape so different sections have different functions) 

• Dorsal horn: receives and processes sensory information. 

• Ventral horn: contains the cell bodies of somatic motor neurones. 

• Lateral horn: contains autonomic preganglionic neurone cell bodies, particularly in the thoracolumbar region. 

(Sympathetic origin thoracolumbar region, their preganglion neurones are SHORT and post ganglionic neurones are long) vs (Parasympathetic origin is the craniosacral and their ganglia are near target organ so their preganglionic neurones are long and post are short).

Where does the cell body of sensory neurone feeding information into the dorsal horn sit?

Cell body of sensory neurone sits in the dorsal root ganglion. Information travels from the body to the ganglion then via the root into the dorsal horn.

What is the spinal root and which is sensory and which is motor? 

• Dorsal root: carries sensory (afferent) information towards the spinal cord. 

• Dorsal root ganglion: contains the cell bodies of sensory neurones. 

• Ventral root: carries motor (efferent) information away from the spinal cord. 

• Dorsal and ventral roots join to make mixed spinal nerve.  

What is a dorsal root and ventral root ganglion? 

• Ventral root ganglion does not exist.  

• BUT dorsal root ganglion is an expansion of dorsal root. A peripheral process carrying sensory information from sensory receptors towards the dorsal ganglion. A central process carrying sensory information from the ganglion into the spinal cord (dorsal horn). 

What are the spinal rami? And what does the dorsal ramus vs ventral ramus supply? 

• Dorsal ramus: supplies the posterior (back) of the body, including deep back muscles and overlying skin. 

• Ventral ramus: supplies the anterior and lateral body wall and limbs, including through the nerve plexuses. 

Are roots or rami mixed sensory/motor? 

• Roots: dorsal = sensory; ventral = motor. 

• Rami: of spinal nerves are both dorsal and ventral are mixed, containing sensory and motor fibres. 

Describe the stepwise process of reflex arc in response to heat stimulus.  Include the components of spinal cord.  

• Withdrawing from the hot stimuli is a response coordinated at spinal cord level, although information also travels to the brain so you become consciously aware of the pain. 

• The basic sequence is: (1) Receptor: Detects heat stimulus. (2) Sensory neurone: Carries the information towards the spinal cord through the dorsal root. (3) Integration centre: The sensory neurone communicates with a motor neurone, often through an interneurone. (3) Motor neurone: Carries the response away from the spinal cord through the ventral root.(4) Effector: A muscle or gland produces the response. 

What are all 12 cranial nerves? 

Dont forget to specify pupil constriction and taste

No.	Cranial nerve	Main function
I		1
II		1
III		2
IV		1
V		2
VI		1
VII		3
VIII		2
IX		3
X		3
XI		1
XII		1

No.	Cranial nerve	Main function
I	Olfactory	Smell
II	Optic	Vision
III	Oculomotor	Most eye movements; pupil constriction
IV	Trochlear	Superior oblique eye muscle
V	Trigeminal	Facial sensation; muscles of mastication  V1 – Ophthalmic: upper face and forehead.  V2 – Maxillary: upper teeth, maxilla, midface.  V3 – Mandibular: lower teeth, mandible, lower face. V3 also provides motor innervation to the muscles of mastication.
VI	Abducens	Lateral rectus muscle; abducts eye
VII	Facial	Facial expression, crying, taste anterior ⅔ of tongue; salivary and lacrimal glands
VIII	Vestibulocochlear	Hearing and balance
IX	Glossopharyngeal	Taste posterior ⅓ of tongue; parotid gland, carotid body,  pharyngeal sensation
X	Vagus	Parasympathetic supply to thoracic and abdominal organs; swallowing and voice
XI	Accessory	Sternocleidomastoid and trapezius - turning head and shrugging shoulde
XII	Hypoglossal	Tongue movements

Classify each of the cranial nerves into motor, sensory or mixed.

No.	Cranial nerve	Classification
I	Olfactory	Sensory = Smell
II	Optic	Sensory = Vision
III	Oculomotor	Motor = Most eye movements; pupil constriction
IV	Trochlear	Motor = Superior oblique eye muscle
V	Trigeminal	MIXED sensory/motor   Facial sensation; muscles of mastication  V1 – Ophthalmic: upper face and forehead.  V2 – Maxillary: upper teeth, maxilla, midface.  V3 – Mandibular: lower teeth, mandible, lower face. V3 also provides motor innervation to the muscles of mastication.
VI	Abducens	Motor: Lateral rectus muscle; abducts eye
VII	Facial	Mixed sensory/motor   Facial expression, crying, taste anterior ⅔ of tongue; salivary and lacrimal glands
VIII	Vestibulocochlear	Sensory: Hearing and balance
IX	Glossopharyngeal	Mixed sensory/motor  Taste posterior ⅓ of tongue; parotid gland, carotid body, pharyngeal sensation
X	Vagus	Mixed: Parasympathetic supply to thoracic and abdominal organs (sensory and motor); swallowing and voice
XI	Accessory	Motor: Sternocleidomastoid and trapezius: turning head and shrugging shoulder
XII	Hypoglossal	Motor: Tongue movements

What is the dental function/ relevance of CN5? 

• Trigeminal nerve. 

• V1 – Ophthalmic: upper face and forehead. 

• V2 – Maxillary: upper teeth, maxilla, midface. 

• V3 – Mandibular: lower teeth, mandible, lower face. V3 also provides motor innervation to the muscles of mastication. 

What is the dental function/ relevance of CN7? 

• Controls muscles of facial expression. 

• (Crying).

• Carries taste from the anterior two-thirds of the tongue. 

• Provides parasympathetic supply to the submandibular, sublingual and lacrimal glands, amongst others. 

What is the dental function/ relevance of CN9? 

• Carries taste and sensation from the posterior third of the tongue. 

• Provides parasympathetic supply to the parotid gland through the otic ganglion. 

How many motor neurones are used to reach muscles in somatic vs autonomic nervous system? 

• Somatic NS to reach skeletal muscle: 1x motor neurone.  

• Autonomic NS to reach muscle: 2x neurones.  Where the two nerves meet is in the parasympathetic/sympathetic ganglion.

What are the two motor nerves involved in autonomic nervous system to innervate muscle? 

• Preganglionic neurone: Its cell body is in the CNS and its axon travels to an autonomic ganglion. 

• Postganglionic neurone: Its cell body is in the ganglion and its axon travels to the target organ. 

What is craniosacral outflow vs thoracolumbar outflow?  

• Craniosacral vs thoracolumbar outflow represents = the anatomical origins of the two main divisions of the autonomic nervous system (ANS): the sympathetic and parasympathetic systems.  

• Thoracolumbar = sympathetic origin (nerve fibres originate from the thoracic and lumbar regions of the spinal cord).  

• Craniosacral = parasympathetic origin (nerve fibres originate from brain stem via cranial nerves and the sacral region of the spinal cord).  

What is the origin of the sympathetic nervous system? Which are longer pre or postganglionic neurons? And why? 

• Thoracolumbar = nerves arise from thoracic and lumbar regions of the spinal cord.  

• This means its preganglionic neurones originate in the spinal cord between T1 and L2. 

• The sympathetic neurones leave the spinal cord in the thoracolumbar region and form a sympathetic chain alongside the vertebral column (of sympathetic ganglia). The post ganglionic neurones then travel from here to their target. As such the preganglionic neurones are SHORT and the post ganglionic neurones are LONG. 

Sympathetic trunk with ganglions.

Splachnic nerve (accessory ganglion).

(Dorsal and ventral roots)

What is the origin of the parasympathetic nervous system? Which are longer pre or postganglionic neurons? And why? 

• Craniosacral outflow = nerve fibres originate from brainstem via cranial nerves and the sacral region of the spinal cord.  

• Brainstem/ cranial nerve origin: CN III – Oculomotor, CN VII – Facial, CN IX – Glossopharyngeal and CN X – Vagus.  

• Preganglionic neurones also originate from spinal cord segments S2–S4. These travel in the pelvic splanchnic nerves.  

• The parasympathetic ganglia are located close to, or within, the target organ. Therefore, the preganglionic neurone has to travel a relatively long distance from the brainstem or sacral spinal cord to reach the ganglion. 

What are there are four major parasympathetic ganglia in the head? 

Cranial nerve	Ganglion	Target
CN III – Oculomotor	Ciliary	Eye – pupil constriction
CN VII – Facial	Pterygopalatine	Lacrimal, nasal and palatal glands
CN VII – Facial	Submandibular	Submandibular and sublingual glands
CN IX – Glossopharyngeal	Otic	Parotid gland

What is the effect of the sympathetic nervous system activation on HR, vessel diameter, gut motility, adrenaline concentrations and pupil dilation? 

Effect	Explanation
Increased heart rate	Increases blood supply to tissues
Vasoconstriction	Constricts many blood vessels
Reduced gut motility	Digestion becomes less of a priority
Adrenaline release	Stimulates the fight-or-flight response
Pupil dilation	Allows more light into the eye

How do sympathetic neurones travel from the spinal cord to their target organs? 

• Step 1 – Preganglionic neurone leaves the spinal cord. The axon exits through the ventral root and enters the spinal nerve. 

• Step 2 – Enters the sympathetic chain. The preganglionic fibre enters the sympathetic chain through the white ramus communicans. The sympathetic chain is a series of ganglia running alongside the vertebral column. 

• Step 3 – Synapses in a ganglion. The preganglionic neurone synapses with a postganglionic neurone.  

  • For the head and thorax, this synapse occurs in the sympathetic chain.  

  • For some abdominal organs, preganglionic fibres pass through the chain without synapsing and travel to prevertebral ganglia, associated with the abdominal aortic plexuses. 

• Step 4 – Postganglionic neurone travels to its target.  

 What are white vs grey rami communicants?  

• The preganglionic fibre enters the sympathetic chain through the white ramus communicans. 

• For fibres returning to spinal nerves, the postganglionic axon travels back through the grey ramus communicans. It can then travel through the spinal nerve branches to reach its target. 

White ramus	Grey ramus
Preganglionic fibres	Postganglionic fibres
Myelinated	Unmyelinated
Enters sympathetic chain	Returns to spinal nerve

What is the origin of the parasympathetic nervous system? 

• Craniosacral outflow = nerve fibres originate from brain stem via cranial nerves and the sacral region of the spinal cord 

• preganglionic neurones originate from: Brainstem – cranial nerves III, VII, IX and X. OR Sacral spinal cord – S2–S4 

What is the effect of the parasympathetic nervous system activation on HR, vessel diameter, gut motility, adrenaline concentrations and pupil dilation? 

Effect	Explanation
Decreased heart rate	Reduces cardiac activity
Increased gut motility	Promotes digestion
Increased saliva	Supports digestion
Pupil constriction	Reduces pupil size

The parasympathetic NS also uses two neurone pathway like the sympathetic NS – however how is it different?  

• Long preganglionic neurone. 

• Short postganglionic neurone. 

• Ganglia located close to, or within, the target organ. 

How is saliva production controlled by the autonomic nervous system? 

• CN VII stimulates the submandibular and sublingual glands via the submandibular ganglion. 

• CN IX stimulates the parotid gland via the otic ganglion. 

• More parasympathetic stimulation increases the watery saliva secretion. 

Compare contrast the sympathetic vs parasympathetic autonomic nervous system 

Feature	Sympathetic	Parasympathetic
Function
Origin
Spinal origin
Cranial nerves
Preganglionic neurone
Postganglionic neurone
Ganglia
Heart rate
Gut motility
Pupil

Feature	Sympathetic	Parasympathetic
Function	Fight or flight	Rest and digest
Origin	Thoracolumbar	Craniosacral
Spinal origin	T1–L2	S2–S4
Cranial nerves	None	III, VII, IX, X
Preganglionic neurone	Short	Long
Postganglionic neurone	Long	Short
Ganglia	Near spinal cord	Near or within target organ
Heart rate	Increases	Decreases
Gut motility	Decreases	Increases
Pupil	Dilates	Constricts