NERVOUS SYSTEM: NERVOUS TISSUECHAPTER OBJECTIVESWhen you have completed this chapter you should be able to:

Describe the anatomy of a neuron.

Classify the different types of neuron according to their structure and direction of the action potential.

Describe the types and functions of neuroglia.

Understand and describe how a nerve impulse is generated and propagated.

The nervous system along with the endocrine system work together to coordinate all of the body systems. It does this by detecting, storing, transmitting and responding to information or stimui.

The nervous system can be anatomically subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord and the peripheral nervous system consists of the spinal nerves and ganglia.

Nervous tissueNeurons or nerve cells are the basic components of the nervous system and our bodies contain billions of them. Supporting and protecting the neurons are neuroglia cells which form a type of connective tissue around the nerve cells.

Neurons

Neurons come in various shapes and sizes but they all contain a cell body and usually two processes; a dendrite and an axon. Dendrites are short, thin branched projections (the word dendrite is derived from the Greek word "dendron", which means tree) that receive signals and transmit them towards the cell body.  They form synapses with other neurons and respond to neurotransmitters. Axons are long straight projections which transmit signals (action potentials) away from the cell body. Their ends branch to form presynaptic terminals which contain neurotransmitters to send signals away from the cell.

Components of a Neuron Description

Cell body Contains a large nucleus and granular protoplasm. Axon Axons are long straight processes which transmit signals (action potentials) away

from the cell body. Their ends branch to form presynaptic terminals which contain neurotransmitters.

Dendrite Dendrites are short, thin branched processes which receive signals and transmit them towards the cell body.

Neurons can be classified according to their structure or by the direction in which the action potentials travel.

Structure

Neuron Types Neuron Structure Unipolar Has an axon but no dendrites. Mostly sensory fibres. Bipolar Has an axon and a dendrite. Part of specialised sensory organs. Multipolar Has one axon and numerous dendrites. Motor and interneurons.

Direction of Action Potential

Neuron Types Direction of the Action Potential Sensory (Afferent) Neurons Conduct signals to the CNS. Motor (Efferent) Neurons Conduct signals from the CNS to the muscles. Interneurons (Association Neurons) Conduct signals from one neuron to another.

Neuroglia

Neuroglia are essential for the normal functioning of the nervous system. They have a number of supporting roles throughout the nervous system and there are 5 different types of neuroglia cells which carry out these functions.

Neuroglia Location Function

Astrocytes CNS Star shaped cells that help keep the neurons in place as well as regulating the composition of the surrounding extracellular fluid.

Ependymal cells CNS Secrete and move the cerebral spinal fluid.

Microglia CNS They engulf unwanted tissue in the CNS, e.g. microorganisms and damaged tissue.

Oligodendrocytes CNS Each cell forms myelin sheaths around multiple axons in the CNS. Schwann Cells PNS Each cell forms a myelin sheath around a single axon in the PNS.

Myelin Sheaths

The lipid rich membrane of the oligodendrites or schwann cells tightly wrap around a section of an axon several times like a swiss roll. It is this tightly packed membrane that forms the myelin sheath

around an axon, which is now known as a myelinated fibre. Cells line up in rows along the axon and between each adjacent oligodendrite or schwann cell is a tiny gap called a node of Ranvier. The myelin sheath acts like as an insulator between the nodes of Ranvier, only allowing the action potential to leap from node to node rather than to travel along the entire length of the axon. This means that axons with a myelin sheath conduct action potentials quicker along their length than unmyelinated axons. The myelin sheath also prevents the action potential from being passed to adjacent neurons as well as protecting the fibre.

Clinical Considerations

Multiple Sclerosis

It is unclear exactly what causes multiple sclerosis, but it is thought to be an autoimmune disease. The immune system attacks its own cells (oligodendrocytes, schwann cells) resulting in the demyelination of axons throughout the nervous system as well as the formation of scar tissue. Demyelination interferes with the ability of the nerve to send signals, and the scarring can cause damage to the nerves themselves.

Multiple sclerosis causes muscle weakness, double vision, problems with balance and coordination, and problems with memory and problem solving.

Action Potentials

In every part of our body are electrically charged particles known as ions which can be positively or negatively charged. Neurons rely on these differently charged ions to create and conduct electrical impulses (action potentials).

Name Symbol Electrical Charge Sodium ions (Na+) Plus 1 positive charge.Potassium ions (K+) Plus 1 positive charge.Calcium ions (Ca++) Plus 2 positive charge.Chloride ions (Cl-) Minus 1 negative charge.

All cells have a 'resting potential', meaning when at rest the overall charge of ions inside the cell are negative compared to the ions outside the cell in the extracellular fluid. The difference in charge across the cell membrane of a neuron creates a potential electrical difference of about -70 milivolts (mV). The cell membrane maintains this resting potential by selectively allowing some ions to pass into the cell via special channels or gates and by blocking the entry of other ions. Due to the electrochemical gradients Na+ slowly diffuses into the neuron and K+ slowly diffuses out of the neuron. Because of this natural diffusion the resting neuron must actively pump Na+ out of the cell and take K+ in to maintain its resting potential of -70 mV.

When a neuron is stimulated a section of its membrane becomes depolarised by the exchange of ions across it. A section of the cell membrane opens its sodium channels allowing sodium ions to move inside the cell. The sodium ions are positively charged and are attracted into the cell by the negatively charged ions inside, as well as the lower sodium concentrations. The influx of positive

ions reverses (depolarises) the resting potential and the inside of the neuron becomes more positively charged. When depolarisation reaches a certain level or threshold, i.e. the voltage inside the cell reaches at least -55 milivolts, it triggers the opening of more sodium channels which in turn triggers the opening of sodium channels in the adjacent cell membrane. Thus depolarisation is spread along the entire cell membrane in a wave; this is an action potential and conducts the nerve impulse along the axon. Once the action potential reaches the end of the axon the action potential is converted to a chemical signal by the release of a neurotransmitter. The inside of the cell continues to depolarise until the voltage peaks at about +35 milivolts, at which point the cell membrane closes its sodium channels so that no more Na+ can enter, and opens its K+ channels to allow the positively charged K+ ions to leave the cell. This process reverses the depolarisation (repolarisation) and allows the neuron to return to its original resting potential of -70 milivolts.

SELF-TEST Complete the following questions before you go onto the next section:

Draw and label a typical neuron.

What is the significance of myelin sheaths.

Describe how an action potential is propagated.

NERVOUS SYSTEM: CENTRAL NERVOUS SYSTEM CHAPTER OBJECTIVES

When you have completed this chapter you should be able to:

Describe the difference between the CNS and PNS.

Locate and identify the forebrain, midbrain and hindbrain .

Identify the major gyri and sulci of the brain.

Describe the main parts of the forebrain and describe its function.

Describe the main parts of the midbrain and describe its function.

Describe the main parts of the hindbrain and describe its function.

Describe the main parts of the spinal cord.

The central nervous system (CNS) consists of the brain and spinal cord and the peripheral nervous system (PNS) consists of the spinal nerves and ganglia.

BrainThe brain occupies the cranial cavity and can be divided into three main parts. The forebrain, midbrain and hindbrain. The midbrain and hindbrain are collectively know as the brain stem and contain the nuclei from which the cranial nerves originate.

Name Description Important named parts

ForebrainProsencephalon

Largest part of the brain.

Telencephalon (cerebrum). Diencephalon (thalamus, hypothalamus, pineal

body).

MidbrainMesencephalon (brain stem)

1.5 cm in length. Quadrigeminal bodies - cerebral peduncles.

HindbrainRhombencephalon (brain stem)

Pons. Medulla oblongata. Cerebellum.

Forebrain

Telencephalon (Cerebrum)

The cerebrum is the largest part of the brain and is divided into left and right hemispheres by a longitudinal fissure that runs along the median sagittal plane. Inferiorly the hemispheres are connected together by a band of white matter called the corpus collosum. The outer layer of the cerebrum is composed of grey matter and called the cerebral cortex. It is responsible for the analysis of sensory input, memory, learning and cognitive thought. Each hemisphere is greatly folded forming gyri (folds) and sulci (grooves) which increases the surface area of the cerebral cortex. Although the exact location of the sulci and gyri varies between different individuals, there are a number of large gyri and deep sulci which can be identified as constant landmarks. The main ones have been listed below;

Name Description

Longitudinal fissure

A large fissure running from back to front along the median sagittal plane; it divides the cerebrum into left and right cerebral hemispheres.

Central sulcus Descending downwards and forwards from the top of the hemisphere. It divides the frontal and parietal lobes.

Parietal-occipital sulcus

Descending downwards and forwards mainly inside the longitudinal fissure, it divides the parietal and occipital lobes.

Precentral gyrus This is found at the posterior border of the frontal lobe, in front of the central sulcus. It descends downwards and forwards from the top of the hemisphere. Forms the primary motor area (cortex).

Postcentral gyrus This is found at the anterior border of the parietal lobe, behind the central sulcus. It descends downwards and forwards from the top of the hemisphere. Forms the primary sensory area (cortex).

Lateral sulcus Found on the lateral side of the brain it ascends almost horizontally from the front of the brain to the angular gyrus and separates the temporal lobe from the frontal lobe above.

Each hemisphere can be further divided into lobes, their names of which correlate with the surrounding bones that protect them.

Lobe of the Cerebrum

Description Function

Frontal lobe

The largest lobe found at the front of the brain undercover of the frontal bone. It contains the precentral gyrus posteriorly. It is separated from the parietal lobe posteriorly by the central sulcus and from the temporal lobe inferiorly by the lateral sulcus.

The primary motor area (cortex).

Motor association area (motor control).

Brocha's area - motor speech (production)

Cognitive thought and memory.

Personality Primary olfactory cortex.

Temporal lobe

Found at the side of the brain undercover of the temporal bone. It is separated above from the frontal lobe by the lateral sulcus.

Primary auditory area (hearing).

Auditory association area (hearing).

Wernicke area (speech comprehension).

Special senses (hearing, smelling).

Learning and memory (retrieval).

Emotions

Parietal lobe

Found at the top of the brain undercover of the parietal bone. Anteriorly it contains the postcentral gyrus and is separated from the frontal lobe by the central sulcus. Posteriorly it is separated from the occipital lobe by the parietal-occipital sulcus.

Primary sensory area (cortex).

Sensory association area (general senses).

Body orientation. Primary gustatory cortex

(taste).

Occipital lobe

Found at the back of the brain undercover of the occipital bone.

Primary visual area (cortex). Visual association area

(vision) - visual interpretation.

Insula The smallest lobe of the brain found deep in the cerebrum between the lips of the lateral sulcus.

Special senses (taste, hearing).

Visceral sensation.

LOBES AND MAJOR LANDMARKS OF THE CEREBRUM

Diencephalon

The diencephalon consists of two thalami, two hypothalami and a single pineal body.

Thalamus

The thalami are the largest parts of the diencephalon and are located in the centre of the brain in the outer walls of the third ventricle. They are often connected to each other across the third ventricle by a small interthalamic adhesion.

The thalamus receives sensory and motor input as well as influences mood. It receives mostly sensory input including auditory and visual input and relays the signals to the cerebral cortex.

Sensory Nuclei of the Thalamus Sensory input from

Medial geniculate nucleus Auditory

Lateral geniculate nucleus Visual

Ventral posterior nucleus Other sensory input

Pineal body

This is a small pine-cone shaped gland projecting from the posterior of the third ventricle by a mid line stalk. Its role is not fully understood but it is thought to be involved in the sleep-wake cycle and the onset of puberty.

Hypothalamus

The hypothalamus is located at the very bottom of the diencephalon below the thalamus and behind the optic chiasma. It is very important and is often referred to as the 'master gland' as it controls a large number of bodily functions, one of the most important being that of homeostasis. Homeostasis is the maintenance of the bodies physiology, i.e. the maintenance of blood pressure, body temperature, weight and the chemical composition of the body's fluids. Other regulatory roles of the hypothalamus are control of our mood and emotions, autonomic functions, food and water intake, sleep wake cycle and endocrine function.

Name Description Function

Mammillary bodies

A pair of small white bodies protruding from the front of the hypothalamus.

Emotional responses to smells.

Infundibulum A stalk which connects the hypothalamus with the pituitary gland (hypophysis).

Through its connection the hypothalamus regulates the function of the pituitary gland.

Mesencephalon (midbrain)

The smallest part of the brainstem measuring 1.5 cms it consists of the tectum, tegmentum, cerebral peduncles and the substantia nigra. It is responsible for the visual and gustatory response as well as the coordination of movement.

Name Description

Tectum

Roof of the midbrain, consisting of four nuclei which form 4 mounds, collectively know as quadrigeminal bodies, on the dorsal surface of the brain stem.

The 2 superior nuclei are called the superior colliculi the 2 inferior nuclei are called the inferior colliculi. The superior colliculi control the visual response and the inferior colliculi control the auditory response.

TegmentumIs the floor of the midbrain and consists of ascending tracts from the spinal cord to the brain. It controls motor functions.

Substantia nigra

A pigmented lamina located between the tegmentum and cerebral peduncles which helps to coordinate movement.

Cerebral peduncles

Located inferior to the tegmentum and consist of descending (motor) tracts from the cerebrum to the spinal cord and cerebellum.

Cranial nerve nuclei

The nuclei of the trochlear and oculomotor cranial nerves are located in the midbrain.

Brainstem

The midbrain and hindbrain are collectively know as the brain stem. It is the lowest part of the brain and is continuous inferiorly with the cervical spinal cord at the foramen magnum. Its fibres connect the peripheral nervous system (spinal nerves and cranial nerves) to the central nervous system (brain and spinal cord).

The brain stem is extremely important because it contains the nuclei from which most cranial nerves originate as well as the vital centres necessary for survival; breathing, digestion, heart rate, blood pressure and for consciousness (being awake and alert).

Retinacular Formation

The retinacular formation is a series of important nuclei that span the brainstem and receive the majority of the sensory information from the body and the motor signals from the cerebrum. The nuclei also play an integral role in the maintenance of the conscious state.

Rhombencephalon (hindbrain)

The hindbrain consists of the pons superiorly, the cerebellum posteriorly and the medulla oblongata inferiorly. The medulla oblongata is continuous inferiorly with the spinal cord.

Pons

Located in front of the cerebellum, the pons is only 2.5 cm in length and bulges anteriorly. It consists of descending fibres travelling to the spinal cord and ascending fibres to the cerebellum. It also contains the nuclei of four of the cranial nerves and the respiratory centre which controls expiration.

Name Description

Pontine nuclei Located anteriorly in the pons they connect the cerebrum to the cerebellum and coordinate voluntary movement.

Respiratory centre

Controls respiratory (expiration) movements.

Cranial nerve nuclei

The nuclei of the following cranial nerves are located in the posterior part of the pons;

Trigeminal (V) Abducens (VI) Facial (VII) Vestibular cochlear (VIII)

Medulla Oblongata

The medulla oblongata is only 3 cm in length and is the most inferior portion of the brainstem being continuous with the spinal cord inferiorly. It consists of the pyramids and olives and contains ascending and descending nerve tracts, several nuclei and importantly the 'vital centres', which regulate heart rate, respiration and blood vessel diameter. It also contains some non-vital centres involved in swallowing, vomiting, sneezing and coughing.

Name Description Function

Pyramids

Two enlargements on the anterior surface of the length of the medulla; they taper towards the spinal cord. Here the descending nerve tract fibres (corticospinal fibres) cross over to the other side to form the 'pyramidal decussation'.

Conscious voluntary movements.

Olive Two protrusions found on the anterolateral side of the medulla just lateral to the pyramids. It consists of an olivary complex of nuclei.

Balance Coordination of sound from the ear.

Vital The medulla is the centre for several important regulatory reflexes; Heart rate. Respiratory

Centres Cardiac centres Respiratory centres Vasomotor centres

(inspiration). Blood vessel diameter.

Cranial nerve nuclei

The nuclei of the following cranial nerves are located in the medulla oblongata;

Glossopharyngeal (IX) Vagus (X) Accessory (XI) Hypoglossal (XII)

Cerebellum

The cerebellum is the lobe of the brain situated in the posterior cranial fossa. Its surface is folded into folia and consists of two hemispheres connected in the mid line by the vermis. It is separated from the pons and medulla oblongata anteriorly by the fourth ventricle. The cerebellum is responsible for coordination of movement and sends information to the thalamus and cortex.

Grey and white matter

The brain and spinal cord contain both grey and white matter.

In the Brain

The grey matter can be found in the cerebral cortex, the basal ganglia and the limbic system. It is made up of the cell bodies, dendrites and synapses of the neurons and are grouped into functionally important nuclei.

The white matter is made up of the myelinated fibres (axons) which connect the different parts of the brain to each other as well as to the spinal cord.

In the Spinal Cord

The spinal cord is oval in cross section and consists of white and grey matter. The grey matter lies centrally and is arranged into ventral, dorsal and lateral grey horns (anterior and posterior horns). It consists of neurons and neurites, neuroglia and blood vessels. It appears grey because of the abundance of neuronal cell bodies.

The white matter surrounds the grey mater and is white in colour due to the presence of myelin, which insulates the nerve fibres.

Ventricles

Inside the brain are four interconnected cavities filled with cerebral spinal fluid; two lateral ventricles, a single third ventricle and a single fourth ventricle. The two lateral ventricles are the largest ventricles and lie one in each cerebral hemisphere. They are approximately C-shaped (wish bone), each communicating with the thin mid line third ventricle via an intraventricular foramen. The third ventricle communicates inferiorly with the fourth ventricle via the cerebral aqueduct and descends in the mid plane through the midbrain. The fourth ventricle is a small, triangular chamber found between the pons in front and the cerebellum behind. Inferiorly it narrows to form the central canal which descends though the medulla oblongata and spinal cord.

Each ventricle contains a choroid plexus which secretes cerebral spinal fluid (CSF) into the ventricles. The third ventricle contains the thalamus and hypothalamus in its lateral walls and the infundibulum, tuber cinereum and the mammillary bodies in its floor. The corpus collosum forms the roof of the lateral and third ventricles.

Cerebral Spinal Fluid (CSF)

The CSF is a clear fluid produced by the choroid plexuses of the ventricles. It circulates within the ventricles as well as in the subarachnoid space between the pia mater and arachnoid mater surrounding the brain and spinal cord. CSF baths the brain and spinal cord in a chemically stable environment and provides it with nutrients. It also allows the brain to be buoyant and protects the brain from jolting into the cranium.

Meninges

Surrounding the brain and spinal cord are three membranous layers;

In the Brain

Around the brain these three layers are collectively known as the meninges;

Outermost layer;

The outermost layer, the dura mater is dense and consists of two layers, a periosteal layer and a meningeal layer. The periosteal layer adheres to the internal surface of the cranium and for the majority of its course lies directly touching the meningeal layer. At certain locations the periosteal and meningeal layers are pulled away from each other to create a space, a dural sinus. The sinuses are filled with venous blood from the brain via the cerebral veins. They drain blood into the internal jugular vein.

Dural Sinuses

Superior sagittal sinus

Cavernous sinus

Inferior petrosal sinus

Superior petrosal sinus

Transverse sinus

Sigmoid sinus

Occipital sinus

The meningeal layer folds inwards to form two double thickness sheets which help to hold the brain in place. The falx cerebri is the fold of dura which projects vertically into the longitudinal fissure between the cerebral hemispheres. The tentorum cerebelli is the fold of dura which projects horizontally between the cerebellum below and the cerebrum above.

Middle layer;

The middle layer, the arachnoid mater, is thin and transparent and lines the inner surface of the dura mater. It possesses arachnoid trabeculae (granulations) which project into the pia mater and villi which project into the dura mata.

Innermost layer;

The innermost layer, the pia mater lies directly on the surface of the brain. This layer is very thin and transparent and closely follows all of the gyri and sulci. Between the pia mater and the arachnoid mater is the subarachnoid space in which the CSF is circulated. CSF is returned to the blood via the arachnoid trabeculae (granulations).

In summary;

Name Location Description

Pia Mater Innermost layer.

Thin and transparent. Invests the surface of the brain and spinal cord.

Arachnoid Mater Middle layer. Thin and transparent. Project villi through the dura into the venous sinuses to

absorb CSF.

Dura Mater Outermost layer.

Thick and fibrous. Made up of two layers; periosteal and meningeal. Contains the venous sinuses.

Folds inwards to form the falx cerebri and tentorum cerebelli.

CROSS SECTION OF THE SCALP

In the Spine

The spinal cord meninges are a continuation of the cranial meninges and are connected to the foramen magnum. Like the cranial meninges the fibrous dura mater is the thick outermost layer and is connected posteriorly to the posterior longitudinal ligament. The arachnoid mater lines the inner surface of the dura mater and the pia mater lies directly on the spinal cord itself. The pia mater attaches to the dura mater via the denticulate ligament.

DENTICULATE LIGAMENT

The spinal cord terminates at the level of L2, but the dura continues to the level of S2 creating a cistern into which the the lower spinal roots hang.

SELF-TEST

Complete the following questions before you go onto the next section:

Describe the important structures of the midbrain.

Name three structures related to the third ventricle.

Describe the functions of CSF.

Spinal cordThe spinal cord is continuous with the medulla oblongata at the foramen magnum, and descends in the vertebral canal. It consists of 31 segments corresponding to the 31 spinal nerves; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

At the level of the second lumbar vertebrae the spinal cord terminates by tapering to a conus medullaris. From the conus medullaris is a long thin filament called the filum terminale. The vertebral canal below the second lumbar vertebra is filled with the nerve roots from the lumbosacral spine; this bunch of nerve roots resembles a horses tail and so is known as the cauda equina.

Along its course the spinal cord has two enlargements, the cervical enlargement and the lumbosacral enlargement, in the cervical and lumbar regions respectively. These swellings are due to the large spinal nerves which emerge from these parts of the cord to supply the upper and lower limbs.

The spinal cord is made up of a column of grey matter (contains cell bodies) surrounded by a cylinder of white matter (myelinated neurons). The neurons of the grey matter are arranged into ventral, dorsal and lateral horns. The fibres of the white matter travel longitudinally along the spinal cord in designated columns.

SELF-TEST

Complete the following questions before you go onto the next section:

How many segments does the spinal cord have?

Which vertebral level does the spinal cord terminate?

Why does the spinal cord have a number of enlargements along its course?

NERVOUS SYSTEM: PERIPHERAL NERVOUS SYSTEMCHAPTER OBJECTIVES

When you have completed his chapter you should be able to:

Name all 12 cranial nerves and describe their functions.

Describe the anatomy of a typical spinal nerve.

Identify the cervical, lumbar, sacral and coccygeal plexuses.

Name the main branches of each plexus and describe their course and function.

Describe the difference between the parasympathetic and sympathetic nervous systems.

The peripheral nervous system (PNS) consists of the spinal nerves and ganglia and the cranial nerves. The nerves of the PNS contain sensory fibres which relay signals to the central nervous system (CNS) and motor fibres which relay signals from the CNS to the effector muscles/glands.

The PNS can be divided into sensory somatic and autonomic systems. The sensory somatic nervous system is voluntary and relays sensory information, of which we are conscious, from the external environment to the CNS and relays motor signals from the CNS to operate the muscles of the body. The autonomic nervous system is involuntary and relays sensory information about the internals of the body to the CNS and relays motor signals from the CNS to regulate the internal environment of the body, e.g. vessel diameter.

Sensory somatic systemThe sensory somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Cranial nerves

The cranial nerves all originate or terminate in the brain stem. All cranial nerves, apart from the first two, which are purely sensory, contain motor as well as sensory fibres and can be described as 'mixed' nerves. However, for descriptive terms each nerve is usually described in terms of its predominant fibres. The motor (afferent) fibres originate in the brain stem and terminate in muscles or glands, and the sensory (efferent) fibres originate in the sensory organs and receptors and terminate in the brain stem.

Cranial Nerve Fibres Course Function

Olfactory (I) Sensory Origin: mucosa of the nasal cavity. Terminates: olfactory

Smell.

bulb.

Optic (II) Sensory

Origin: retina of the eyeball. Terminates: lateral geniculate body of the thalamus.

Vision.

Oculomotor (III) Predominantly motor

Origin: midbrain.Terminates: extrinsic muscles of the eye.

Extrinsic muscles of the eyeball (superior, medial and inferior rectus and inferior oblique and levator palpebrae superioris).

Parasympathetic: intrinsic muscles of the eyeball (sphincter of the pupil and the ciliary muscle of the lens).

Trochlear (IV) Predominantly Motor

Origin: midbrain Terminates: extrinsic muscle of the eye.

Extrinsic muscle of the eyeball (superior oblique).

Trigeminal (V) Mixed

Origin: middle and upper face and the pons. Terminates: pons and the muscles of mastication.

Sensory (ophthalmic, maxillary and mandibular nerves): scalp, face and mouth.

Motor (mandibular nerve): muscles of mastication (chewing) and soft palate and the middle ear.

Abducens (VI) Motor Origin: pons. Terminates: extrinsic muscle of the eye.

Extrinsic muscle of the eyeball (lateral rectus).

Facial (VII) Mixed

Origin: taste buds and pons. Terminates: thalamus and muscles of facial expression and salivary glands.

Sensory: taste, external ear and palate. Motor (temporal, zygomatic, buccal,

mandibular and cervical nerves): muscles of facial expression and middle ear.

Parasympathetic: salivary and lacrimal glands.

Vestibulocochlear (VIII)

Predominantly sensory

Origin: cochlear and semicircular canals of the inner ear. Terminates: pons and

Hearing. Balance.

medulla oblongata.

Glossopharyngeal (IX)

Mixed

Origin: pharynx, middle ear and tongue and the medulla oblongata. Terminates: medulla oblongata, parotid gland and pharynx.

Sensory: taste, tongue, pharynx, tonsils and middle ear.

Motor: muscles of the pharynx (swallowing).

Parasympathetic: parotid gland.

Vagus (X) Mixed

Origin: viscera, tongue, pharynx and larynx; medulla oblongata. Terminates: medulla oblongata; viscera, tongue, pharynx and larynx.

Sensory: pharynx, larynx, thoracic and abdominal organs and taste.

Motor: soft palate, pharynx, intrinsic laryngeal muscles (voice) and extrinsic tongue muscle.

Parasympathetic: to the thoracic and abdominal viscera - digestive tract, heart and lungs, kidneys, spleen liver and pancreas.

Accessory (X) Motor

Origin: medulla oblongata Terminates: muscles of the neck and swallowing

Neck muscles (sternocleidomastoid and trapezius).

Muscles of swallowing (pharynx and soft palate).

Hypoglossal (XII) Motor

Origin: medulla oblongata Terminates: tongue and hyoid muscles.

Extrinsic and intrinsic muscles of the tongue and hyoid muscles.

SELF TEST

Complete the following questions before you go onto the next section:

Name in order all 12 cranial nerves.

Name the cranial nerves that innervate the extrinsic muscles of the eyeball.

Name the cranial nerves that carry parasympathetic fibres.

Spinal nerves

There are 31 pairs of spinal nerves each arising from dorsal and ventral rootlets from the corresponding 31 segments of the spinal cord. There are 8 pairs of cervical spinal nerves, the first set arising above the atlas (C1) and the last arising below the seventh cervical vertebra (C7). There are 12 pairs of thoracic spinal nerves, 5 sets of lumbar spinal nerves, 5 sets of sacral and spinal nerves and 1 set of coccygeal spinal nerves.

The dorsal roots arise from the posterolateral sides of the cord and carry sensory fibres from the cord. The ventral roots arise from the anterolateral sides of the cord and carry motor fibres from the cord. The dorsal and ventral roots unite to form the mixed (sensory and motor) spinal nerves. Just before the ventral and dorsal roots join, there is a small swelling known as the dorsal root ganglion. It is formed by the cell bodies of the sensory neurons.

With the exception of the 1st cervical, the sacral and the coccygeal spinal nerves, all of the spinal nerves exit the vertebral canal via the intervertebral foramina. The intervertebral foramina are found along the sides of the vertebral column, formed between the pedicles of adjacent vertebrae. The first cervical spinal nerve exits above the atlas, between it and the occipital bone, and the sacral spinal nerves exit via the ventral and dorsal sacral foramina.

After exiting through the intervertebral foramina the spinal nerves soon divide into dorsal and ventral rami (branches). The dorsal branches supply the deep muscle and the skin of the back of the trunk. The ventral branches supply the rest of the body. The ventral branches of the thoracic spinal nerves form the intercostal nerves which supply the muscles of the thoracic cage. The ventral branches of the rest of the spinal nerves interlink to form specialised nerve plexuses which supply the rest of the body.

Cervical Plexus

The ventral branches of the 1st-4th cervical spinal nerves (C1-C4) interlink to form the cervical plexus. The cervical plexus gives off many branches which supply the superficial muscles of the neck and the skin over the neck and back of the head. An important branch of the cervical plexus is the phrenic nerve which travels through the thorax to innervate the movement of the diaphragm; without it we would not be able to breath.

Brachial Plexus

The ventral branches of the 5th-8th cervical and the 1st thoracic spinal nerves (C5-T1) interlink to form the brachial plexus. The brachial plexus supplies the entire upper limb with motor and sensory innervation. The brachial plexus divides into lateral, posterior and medial cords before dividing into a number of large branches;

Brachial plexus (C5-T1)

Origin Course Innervates

Musculocutaneous Nerve (C5-C7)

A branch of the lateral cord.

It descends between biceps and brachialis to the elbow where it

Motor: flexors of the elbow joint; coracobrachialis,

becomes the lateral cutaneous nerve of the forearm.

biceps and brachialis muscles.

Sensory (cutaneous): Skin over the radial (lateral) border of the forearm.

Axillary Nerve (C5-C6)

A branch of the posterior cord.

Descends behind the axillary artery winds around the surgical neck of the humerus.

Motor: deltoid and teres minor muscles.

Sensory: shoulder joint

Sensory (cutaneous): Skin over the shoulder and lateral arm.

Radial Nerve (C5-T1)A continuation of the posterior cord.

It exits the axilla under teres minor and runs around the back of the humerus in the radial groove with the arteria profunda brachii. From here it runs down the lateral side of the forearm to the wrist.

Motor: extensors of the elbow, wrist and hand.

Sensory: elbow, wrist and hand joints.

Sensory (cutaneous): Skin over the dorsum of the hand.

Ulnar Nerve (C7-T1) A continuation of the medial cord.

Runs down the medial side of the arm in front of the medial head of the triceps to reach the elbow. It enters the flexor compartment of the forearm to travel to the wrist.

Motor: a few of the flexors of the wrist and hand and most of the intrinsic muscles of the hand.

Sensory: hand joints. Sensory (cutaneous):

Skin of the ulnar (medial) aspect of the hand.

Median Nerve (C5-T1)

Arises from the medial and lateral cords of the brachial plexus.

It leaves the axilla with the brachial artery and travels to the elbow. In the forearm it travels to the wrist where it enters the carpal tunnel and divides into medial and lateral branches.

Motor: most of the long flexors of the forearm and the thenar muscles.

Sensory (cutaneous): Skin of the elbow, wrist, and radial aspect of the palm of the hand.

DIAGRAM OF THE BRACHIAL PLEXUS

Lumbar Plexus

The ventral branches of the 1st to 4th lumbar spinal nerves interlink to form the lumbar plexus. The lumbar plexus supplies the thigh and skin of the buttocks and genitals;

Lumbar plexus (L1-L4)

Origin Course Innervates

Femoral nerve Posterior divisions of L2-L4.

The largest nerve of the lumbar plexus, it arises with in the substance of the psoas muscle. It passes into the thigh under the

Motor: iliopsoas, sartorius and quadriceps femoris muscles.

inguinal ligament, where it divides into anterior and posterior branches.

Sensory (cutaneous): Skin over the anterior and lateral thigh and the medial leg and foot.

Iliohypogastric Anterior division of T12-L1

Arises within the substance of psoas emerging laterally to travel over quadratus lumborum. It pierces transversus abdominis to travel between it and internal oblique.

Motor: abdominal muscles.

Sensory (cutaneous): Skin of the anterior buttocks and abdominal wall above the pubis.

Ilioinguinal Anterior division of L1

Emerges from the superolateral border of psoas major, passes over quadratus lumborum and pierces transversus abdominis and internal oblique. It continues underneath external oblique and enters the inguinal canal.

Sensory (cutaneous): Skin of the superomedial thigh and genital area.

Lateral femoral cutaneous

L2-L3

Emerges from the lateral aspect of psoas major travels along iliacus to enter the surface of the thigh under the inguinal ligament. Here it divides into anterior and posterior branches.

Sensory (cutaneous): Skin over the lateral aspect of the thigh down to the knee.

Genitofemoral Anterior divisions of L1-L2

Passes through the substance of psoas major and emerges from its anterior surface where it divides into genital and femoral branches.

Sensory (cutaneous): Skin over the femoral triangle and genital area.

Obturator Anterior divisions of L2-L4

It ascends through psoas major to emerge at the pelvic brim medial to the muscle. It descends along the lateral wall of the pelvis to enter the thigh through the obturator foramen.

Motor: Adductors of the thigh.

Sensory: hip and knee joints and cruciate ligaments.

Sensory (cutaneous): Skin of the medial side of the thigh.

Sacral Plexus

The ventral branches of the 4th lumbar to the 4th sacral spinal nerves (L4-S4) interlink to form the sacral plexus. Because there is an overlap between the lumbar and sacral plexuses they are often

described together as the lumbosacral plexus. A large division known as the lumbosacral trunk arises from the ventral branches of the L4-L5 and joins with the ventral branches of S1 to supply the lower limb.

Sacral Plexus

Origin Course Innervates

Sciatic nerve

Anterior divisions of the L4-S3

The largest nerve in the body, it is really two nerves bound together with connective tissue; the medially placed tibial nerve and laterally placed common (fibular) peroneal nerve. It exits the pelvis, through the greater sciatic foramen, underneath piriformis and runs down the back of the leg to the knee where its two components split from one another.

Motor: hamstrings. Sensory: hip joint.

Tibial nerve

Larger of the two divisions of the sciatic nerve (L4-S3)

Arises as a separate division in the popliteal fossa, and descends into the flexor compartment of the leg (calf) in which it travels to the ankle. On reaching the ankle it divides into medial and lateral plantar nerves which enter the sole of the foot.

Motor: flexors of the leg.

Sensory: knee and foot joints.

Sensory (cutaneous): skin over the back of the leg and on the sole of the foot.

Common (fibular) Peroneal nerve

Smaller of the two divisions of the sciatic nerve (L4-S2)

Arises as a separate division in the popliteal fossa and travels laterally around the neck of the fibula to enter the peroneus longus muscle where it divides into deep and superficial branches.

Motor: peroneal muscles and extensors of the ankle.

Sensory: knee, foot and ankle joints.

Sensory (cutaneous): skin over the anterior, lateral and posterior aspect of the leg and skin of the dorsum of the foot and toes.

Superior Gluteal Nerve

Posterior divisions of L4-S1

It exits the pelvis through the greater sciatic foramen, above piriformis to reach the gluteal region.

Motor: gluteus medius and minimus and tensor fasciae latae.

Inferior Gluteal Nerve

Posterior divisions of L5-S2

It exits the pelvis through the greater sciatic foramen, below piriformis to reach the gluteal region.

Motor: gluteus maximus.

Pudendal Nerve

Anterior divisions of S2-S4

It exits the pelvis through the greater sciatic foramen, between piriformis and coccygeus and enters the pelvis again through the lesser sciatic foramen.

Motor: perineal muscles.

Sensory (cutaneous): skin of the perineum.

Coccygeal Plexus

The ventral branches of the 4th to 5th sacral spinal nerves and the coccygeal spinal nerve (S4-Co1) interlink to form the coccygeal plexus. It pierces the anococcygeal ligament to supply sensory innervation to the skin of that region.

SELF TEST

Complete the following questions before you go onto the next section:

Beginning at the spinal cord describe the formation of a spinal nerve.

Which nerve roots contribute to each plexus?

Name 3 nerves from each of the cervical, brachial and lumbar plexuses.

Autonomic nervous systemThe autonomic nervous system (ANS) maintains our internal bodily functions without us being aware of it. It consists of afferent (sensory) fibres that transmit sensory information from the periphery (viscera) to the CNS, and efferent (motor) fibres that send motor signals from the CNS to the periphery. This loop of autonomic sensory and motor fibres allows the body to monitor and control such functions as heart rate, vessel diameter, smooth muscles surrounding organs and the secretions of glands.

The ANS can be divided into two separate systems, sympathetic and parasympathetic, both of which contain motor and sensory fibres. The fibres in both systems arise in the central nervous system (brain and spinal cord) as preganglionic fibres. These synapse in ganglia with postganglionic fibres, which then travel to the target organ.

Most organs are innervated by fibres from both divisions of the ANS; the sympathetic system generally prepares the body for activity while the parasympathetic prepares it for rest.

Sympathetic

The sympathetic system controls our 'fight or flight' responses, i.e. quickly activates our body preparing it for action by increasing the heart rate, dilating the pupils and vessels to the muscles, increases skin sweating and suppresses digestion.

The motor (efferent) sympathetic fibres (preganglionic motor neurons) arise from the intermediolateral column in the thoracic spinal cord and travel to the sympathetic ganglia listed below, where they synapse with postganglionic neurons before reaching their target organs. The sympathetic chain is found on either side of the vertebral column and consists of a number of sympathetic ganglia strung together by sympathetic trunks.

Sympathetic Ganglia

Chain ganglia

Middle cervical ganglia

Superior cervical ganglia

Stellate ganglia

Aortic ganglia

Celiac ganglia

Renal ganglia

Mesenteric ganglia

White and grey rami connect the sympathetic trunks to the spinal nerves. Fibres emerge from the chains to form sympathetic plexuses around the internal organs.

Sympathetic plexuses

Oesophageal plexus

Aortic plexus

Gastric plexus

Cardiac plexus

Superior hypogastric plexus

Inferior hypogastric plexus

The sensory (afferent) sympathetic fibres carry sensory information from the viscera to the CNS. The afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents from the skin, this can cause confusion with referred pain.

Referred pain

Because the afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents from the skin, pain from the viscera can be perceived as originating from elsewhere in the body. This is known as referred pain and must be understood when investigating the condition of a patient, for example, when a person is experiencing a heart attack they will often feel pain in their left shoulder, neck and arm. This is due to the sensory fibres from the heart and those areas of the arm entering the spinal cord at the same level where the signals get confused.

Parasympathetic

The parasympathetic system controls the opposite responses to 'fight or flight' responses, i.e. depresses the activity of the body preparing it for rest by decreasing the heart rate, constricting the pupils and vessels to the skeletal muscles and stimulating digestion (peristalsis and salivary production).

Parasympathetic cells are located in specialised nuclei throughout the brainstem and the sacral spinal cord. Its fibres travel in four of the cranial nerves chiefly the vagus nerve as well as the splanchnic and pelvic nerves. The majority of sensory (afferent) parasympathetic fibres return to the medulla oblongata via the vagus nerve where the fibres then travel in the solitary tract to the solitary nucleus which receives sensory information about the blood pressure, CO2/O2 levels and digestive tract distension.

Parasympathetic nerves Targets

Vagus nerve Heart, stomach, small intestine, parts of the large intestine.

Oculomotor Sphincter of pupil and ciliary muscle.

Facial nerves Lacrimal, nasal, submandibular and sublingual glands.

Glossopharyngeal nerve Parotid gland.

Pelvic splanchnic nerves Parts of the large intestine, rectum, bladder, penis or clitoris.

The motor (efferent) parasympathetic fibres travel to parasympathetic ganglia before reaching their target organ.

Parasympathetic Ganglia Targets

Ciliary ganglia Sphincter of pupil and ciliary muscle.

Pterygopalatine ganglia Lacrimal and nasal glands.

Submandibular ganglia Submandibular and sublingual glands.

Otic ganglia Parotid gland.

Mesenteric ganglia Parts of the large intestine and rectum.

Pelvic ganglia Bladder, penis or clitoris.

Autonomic Reflexes

A reflex is a pathway with an afferent (sensory) signal that evokes an immediate efferent response (motor) without the signal being transmitted to the brain.

e.g. afferent fibres from the viscera baroreceptors and chemoreceptors in the carotid sinus and arch of the aorta are carried to the CNS by the major autonomic nerves where they control heart rate, blood pressure and respiration.

Summary of ANS Function;

Structure Sympathetic Action Parasympathetic Action

Heart Rate Increase. Decrease.

Heart Contraction Increase force. Decrease force.

Blood Vessels Directs blood to the skeletal muscles (away from the skin and viscera).

Directs blood to the to the skin and viscera.

Sweat Production Increases production. Decreases production.

Pupil Aperture Dilates. Constricts.

Trachea and Bronchi

Dilates. Constricts.

Blood Pressure Increases. Lowers.

Salivary Gland Decreases production. Increase production.

Gastrointestinal Tract

Decreases peristalsis, reduced digestion.Increases peristalsis, increases digestion.

Liver Stimulates the conversion of glycogen to glucose.Inhibits the conversion of glycogen to glucose.

Kidney Decreased urine production. Increased urine production.

Bladder Wall relaxed, sphincter closed. Wall contracted, sphincter relaxed.

SELF TEST

Complete the following questions before you go onto the next section:

Describe the functions of the sympathetic and parasympathetic nervous systems.

Name and locate five sympathetic ganglia.

Name and locate four parasympathetic nerves.