ANAT Week 6.docx

Week 6 **Module: Nervous System 4 - Introduction to Cranial Nerves** ============================================================= **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Identify the cranial nerves by name and number **LO2**: Ident...

Week 6 **Module: Nervous System 4 - Introduction to Cranial Nerves** ============================================================= **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Identify the cranial nerves by name and number **LO2**: Identify where the cranial nerves emerge from the brain **LO3**: Name the foramina in the skull that the cranial nerves traverse **LO4**: Describe the basic functions of the cranial nerves **Introduction** ---------------- ***LO1: Identify the cranial nerves by name and number*** Cranial nerves, like spinal nerves, are part of the peripheral nervous system. There are 12 pairs of cranial nerves - each cranial nerve is paired and is present on both sides. They arise from the inferior (ventral) surface of the brain. Most of them arise from the brainstem. Most cranial nerves have cranial nerve nuclei located within the brainstem. Please recall that nuclei are collections of specialised neuronal cell bodies within the central nervous system. Cranial nerves leave the central nervous system through openings in the skull - the cranial foramina. Study the image below showing the base of the skull and the brain within the cranial cavity and note the cranial nerves as they arise from the brain and traverse the foramina in the base of the skull Now, let's discuss the names of the cranial nerves. ### **Names and Numbers** Each cranial nerve has two names. Firstly, the cranial nerves are numbered according to their positions, beginning with the most anteriorly placed nerve, and using Roman numerals preceded by the prefix 'CN' (standing for 'Cranial Nerve'). For example, CN I (Cranial Nerve I), CN II (Cranial Nerve II) and so on until CN XII. The full names of most cranial nerves generally have some relation to their functions. For example, CN I is called the olfactory nerve (responsible for olfaction, which is the sense of smell), and CN II is called the optic nerve (responsible for vision). Look at the table below and read the name of each cranial nerve. Click on the hotspot next to each name to reveal its meaning. **Cranial Nerves Emerging from the Brain** ------------------------------------------ ***LO2: Identify where the cranial nerves emerge from the brain*** Study the image below and identify each cranial nerve as it arises from the brain. Start from the top of the image, which corresponds to the most anterior (rostral) aspect of the inferior (ventral) surface of the brain. Note that you will be following the numbers consecutively, starting from I through to XII, as you go from anterior (rostral) to posterior (caudal). Please note that, in a simplistic way, we say that the cranial nerves belong to the peripheral nervous system. However, on a structural level, the olfactory (CN I) and optic (CN II) nerves are more accurately considered part of the central nervous system, as embryologically they are derived from the forebrain and cerebrum. Their myelin is also produced by the oligodendrocytes of the central nervous system. Using the labelled image above that you have just studied to guide you, try to identify the cranial nerves on the cadaveric image of the ventral surface of the brain below. Drag and drop the cranial nerve numbers to the corresponding nerves labelled on the cadaveric image. Please note that in this unit, in the practical classes, we will only identify some of the cranial nerves on brain models. **Cranial Foramina** -------------------- ***LO3: Name the foramina in the skull that the cranial nerve traverse*** Study the image of the base of the skull shown below and note the foramina (openings) that the cranial nerves traverse on their way out of the skull. You will notice once again that the cranial nerves exit the skull in an 'orderly' fashion, according to their numbers (i.e. from CN I to CN XII, anterior to posterior). We will identify some of these foramina during the practical classes. At this point, you may already recall some of them that we have identified on the skull in the Skeletal System 2 module. **Basic Functions** ------------------- ***LO4: Describe the basic functions of the cranial nerves*** The functions of the cranial nerves are versatile, complicated and very individual to the particular cranial nerve. Many of these nerves naturally innervate structures of the head and neck. However, there are cranial nerves that, in addition to innervating head and neck structures, travel far distances. For example, the vagus nerve (CN X) innervates structures in the thorax (e.g. heart) and abdomen (e.g. part of the gastrointestinal tract). In fact, the vagus nerve has many branches and it is responsible for functions such as heart rate, gastrointestinal peristalsis, sweating, speech and keeping the larynx open for breathing, plus many more. No cranial nerves are the same and this is why it takes time to understand how these nerves operate. In an introductory anatomy unit such as this one, you are only required to understand the major concepts about the cranial nerves. Each cranial nerve is composed of many axons. Some cranial nerves are composed of motor axons only (e.g. CN XII), some are composed of sensory axons only (e.g. CN I) and some are composed of both sensory and motor axons (e.g. CN V). Therefore, functionally, in a simplistic way, the cranial nerves can be classified as either motor, sensory or mixed (both motor and sensory). The motor functions of the cranial nerves include both somatic and autonomic (parasympathetic only) functions. For example, the accessory nerve (CN XI) innervates the sternocleidomastoid and trapezius muscles (somatic motor function), while the vagus nerve (CN X) innervates smooth muscles and glands of the larynx, heart, lungs and most abdominal organs (autonomic motor function). Click on the hotspot next to the name of each cranial nerve on the image below to see its main functions. Please note that **you are not required to know every function**. Only note the amazing array of jobs that our cranial nerves do. You will study these in more detail later in advanced anatomy units. If you would like to learn a fun way to remember the cranial nerve functions, watch the video below (6 mins). Please note that many details in this video are not examinable - they are for your interest only. **Module: Nervous System 5 - Spinal Cord and Spinal Nerves** ============================================================ **By the end of this module, you should be able to:** **LO1**: Describe the gross anatomy of the spinal cord and its protective structures **LO2**: Describe the cross section of the spinal cord **LO3**: Name the spinal nerves, describe how they emerge from the spinal cord through intervertebral foramina and describe the typical structure of a spinal nerve **LO4**: Define a nerve plexus and identify the cervical, brachial, lumbar and sacral plexuses **LO5**: Describe what a simple spinal reflex is, name a few (stretch, tendon, withdrawal) and discuss their importance **What is the spinal cord?** ============================ ***LO1**: Describe the gross anatomy of the spinal cord and its protective structures* The spinal cord is structurally and functionally integrated with the brain and together, they constitute the central nervous system. The spinal cord serves as a connection between the brain and the rest of the body, and in some instances, it works independently of the brain. The spinal cord serves as a pathway for the: - - - While the brain occupies the cranial cavity and is protected by the cranial bones, the spinal cord occupies the vertebral canal made up of vertebrae. The spinal cord is cylindrical in shape. It commences at the foramen magnum at the base of the cranium and during development fills the entire vertebral canal. At birth the spinal cord is shorter than the vertebral canal and terminates at the upper border of the L3 vertebra. The vertebral column grows faster than the spinal cord and in adults the spinal cord terminates at the level of L1 vertebra. **Spinal Cord Segments** ------------------------ The spinal cord appears to be segmented because 31 pairs of spinal nerves emerge from it in regular intervals. The spinal segments are subdivided into: - - - - - Note that although the spinal segments share the same names as the parts of the vertebral column, they do not correspond to the vertebra of the same name. For example, the lumbar part of the spinal cord is short and corresponds to the lower thoracic vertebrae and only a shy of L1 vertebra, and not the rest of the lumbar vertebrae. Why? Remember the spinal cord finishes at the lower border of the vertebra L1**‼** This is due to the faster and continued growth of the vertebral column after the spinal cord growth has ceased. The spinal cord ends at the pointed **conus medullaris**. The rest of the vertebral canal is occupied by the **cauda equina**, (cauda -- tail, equus -- horse; the horse's tail). The spinal nerves of the lumbar, sacral and coccygeal regions of the spinal cord do not leave the vertebral canal at the same level as they leave the cord. Instead, these nerves pass down the vertebral canal forming cauda equina and exit the canal at the lower intervertebral foramina When examined in the anterior view, the spinal cord shows two enlargements **Cervical Enlargement** ------------------------ Due to increased amount of the grey and white matter that supplies nerves to the upper limbs :**Lumbar Enlargement** Just below the lumbar enlargement the spinal cord becomes conical, and this part of the spinal cord is called the **conus medullaris**. The conus medullaris is continuous with the thin band of pia matter the **filum terminale**, which gives support to the spinal cord in longitudinal axis and attaches it to the coccyx. **Meninges** ------------ The neural tissue of the spinal cord, blood vessels and nerves sprouting from the spinal cord are vulnerable to injury within the rigid vertebral canal. Protection of the spinal cord is achieved by the three layers of meninges (meninx -- membrane), which are a continuation of the brain meninges: **Dura (dura -- hard) mater** ----------------------------- The most superficial layer of dense collagen fibres axially orientated **Arachnoid mater (arachne -- spider)** --------------------------------------- Layer of simple squamous cells, and arachnoid trabeculae and elastic fibres **Pia mater (pia -- delicate)** ------------------------------- The dura and arachnoid mater extend around spinal nerves on their exit through the intervertebral foramina to protect the nerves and finally fuse with the connective tissue of the spinal nerve outside of the vertebral canal. Please note that immediately deep to the arachnoid mater is the space called the subarachnoid space. It contains cerebrospinal fluid. This space is continuous with the subarachnoid space inside the skull. **Epidural Block** ------------------ Epidural space is a narrow space between the vertebral canal and the outer layer of dura matter filled with loose areolar tissue. This space is commonly used to inject anaesthetics (epidural block) for example in childbirth to reduce the pain sensations of the birth canal. **Lumbar Puncture** ------------------- Lumbar puncture is a useful clinical procedure where the needle is inserted through the skin between L3/L4 vertebrae (in adults), piercing the muscles and ligaments of the back, epidural space, dura matter and arachnoid mater and into the subarachnoid space from where the sample of the cerebrospinal fluid is taken for biochemical or microbiology testing. The CSF is clear in healthy individuals but change its appearance and consistency in many medical conditions such as: - - - ***LO2**: Describe the cross section of the spinal cord* **Internal Structure of the Spinal Cord** ----------------------------------------- To understand the functional organisation of the spinal cord, you must study and understand its cross-sectional structure. The series of cross sections through the spinal cord on the image below show the grey matter centrally placed (in the shape of letter H or butterfly) and the white matter on the periphery. The grey matter consists of neuronal cell bodies, unmyelinated axons and glial cells. The white matter is composed primarily of myelinated axons. The central canal is the continuation from the 4th ventricle and the central canal of the medulla oblongata. It travels throughout the length of the spinal cord and contains CSF. The central canal is encircled by the grey matter known as the grey commissure. The external surface of the spinal cord has two longitudinal depressions: **Posterior (Dorsal) Median Sulcus** ------------------------------------ In the midline posteriorly (dorsally) a narrow groove **Grey Matter of the Spinal Cord** ---------------------------------- The morphology of the spinal cord changes depending upon the level from which the section is taken. The diameter of the spinal cord in the cervical region, particularly in its lower segments, and the lumbar region, are the largest due to amount of grey matter needed for the supply of the muscles of the upper and lower limbs. These regions correspond to the spinal cord enlargements. The grey matter of the spinal cord can be subdivided into 3 regions called horns: - - - **White Matter of the Spinal Cord** ----------------------------------- The white matter of the spinal cord is external to the grey matter and can also be subdivided into 3 regions called columns: - - - The columns are sub-organised into smaller groups of axons called tracts. We will look at these tracts more closely in another module. Match the labelled structure with the statements below **Spinal Nerves** ================= ***LO3**: Name the spinal nerves, describe how they emerge from the spinal cord through intervertebral foramina and describe the typical structure of a spinal nerve* The spinal nerves originate from the spinal cord and innervate structures below the head and neck (except few branches of the cervical nerves supplying the head and neck structures). Anterior horn of the spinal cord contains large cell bodies of the motor neurons. The axons of these nerves leave the spinal cord via anterior rootles, uniting to form the anterior root of the spinal nerve. The sensory neurons carry sensory information from the periphery and their cell bodies are found in the posterior root ganglion from where the posterior rootlets enter the spinal cord through the posterior horns. The motor and sensory fibres unite and form the mixed spinal nerve, meaning that the spinal nerve carries both sensory and motor axons. As the spinal nerve leaves the vertebral canal through the intervertebral foramen it splits onto: - - NOTE: both anterior and posterior rami contain both motor and sensory fibres (they are mixed nerves). The spinal nerves are formed by the axons of both motor and sensory neurons and called **mixed nerves**. There are 31 pairs of spinal nerves: - - - - - Note that the cervical region consists of 7 cervical vertebrae and 8 cervical nerves. The cervical spinal nerves exit the vertebral canal through the intervertebral foramen above the corresponding vertebra. For example: C4 spinal nerve exits the vertebral canal through the intervertebral foramen between C3 and C4 vertebra. In the thoracic region and below, the spinal nerves exit the vertebral canal inferior to the corresponding vertebra: T2 spinal nerve exits the vertebral canal below the corresponding vertebra so between T2 and T3 vertebra. **Peripheral Nerve** -------------------- The motor and sensory axons of the peripheral nerve are held together by the connective tissue sheath called **epineurium**. Within the nerve, axons are separated into smaller groups - fascicles which are bound with another tissue sheath known as **perineurium**. Each axon within the fascicle is surrounded with another layer of loose areolar connective tissue - **endoneurium**. Connective tissue provides extra support and cushioning for the delicate nervous tissue and channelise the blood vessels around the axons. **Dermatome** ------------- The area of the skin supplied by a single spinal nerve is called a **dermatome**. Dermatomes have a substantial area of overlapping. The diagrams below show approximate dermatomal distribution of the skin. This is very useful in clinical practice as if a numbness of the skin is noted, clinician is able to locate the site of spinal nerve lesion. **Shingles** ------------ Painful rush and blisters of the skin with dermatomal distribution are common symptoms and signs in shingles. After the childhood infection with chickenpox virus, the virus stays inactivated (latent) in the root ganglia of the spinal nerves. Reactivation of the virus during the adulthood causes proliferation of the virus through the sensory axons of the dermatome and cause changes on the skin. **Myotome** ----------- A myotome is a portion of a skeletal muscle innervated by a single spinal cord level or by a single nerve. Myotomes are more difficult to test than dermatomes because each muscle is usually innervated by nerves deriving from more than one spinal cord level (spinal nerve). For example: biceps brachii is innervated by the musculocutaneous nerve that has a spinal root value C5, C6 and C7 ***LO4**: Define a nerve plexus and identify the cervical, brachial, lumbar and sacral plexuses* A nerve plexus is a network of anterior rami of spinal nerves. These plexuses are either somatic or visceral. The posterior rami of the spinal nerves are short and do not form plexuses. Posterior rami supply the deep muscles and skin of the back. The somatic plexuses are formed of the anterior rami of the spinal nerves: - - - - NOTE: lumbar and sacral plexuses are commonly referred as lumbo-sacral plexus. Visceral plexuses are formed of sympathetic and parasympathetic afferent and efferent components and form: - - - - **Cervical Plexus** ------------------- Cervical plexus (right and left) are formed by the anterior rami of the cervical spinal nerves C1-C4. The plexus delivers 2 sets of nerves: - - **Brachial Plexus** ------------------- The brachial plexuses (right and left) are networks of the anterior rami of C5 --C8 and T1 spinal nerves. The anterior rami of the spinal nerves partially join each other to form the trunks of the brachial plexus, then the trunks divide again and form the anterior and posterior divisions. The divisions than reach axilla and unite to form 3 cords which receive their names according to the relationship with the axillary artery as a posterior, medial and lateral cords. The cords will give terminal branches that innervate muscles within muscle compartments, of the upper limb and muscle that move the shoulder. Main terminal branches of the brachial plexus are: - - - - - **Lumbar Plexus** ----------------- Lumbar plexuses (right and left) are formed by anterior rami of spinal nerves L1-L4 on the posterior wall of the abdominal cavity. The main nerves arising from the lumbar plexus are: - - **Sacral Plexus** ----------------- Sacral plexuses (right and left) are formed by the anterior rami of the spinal nerves L4/L5 and S1-S3/4. The nerves arising from the sacral plexus supply the pelvis, gluteal region, posterior compartment of the thigh and leg muscles and anterior and lateral compartments of the leg. The largest and longest nerve in our body the sciatic nerve is formed by the anterior rami of L4/5 and S1-S3. It consists of two divisions that split into 2 nerves in the popliteal fossa: - - - - **Thoracic Spinal Nerves** -------------------------- Anterior rami of the thoracic spinal nerves form intercostal nerves (T1-T11) and subcostal nerve T12. The thoracic spinal nerves do not form plexuses except the T1 which is the part of the brachial plexus. The intercostal nerves T3-T6 pass through the intercostal spaces, innervate the intercostal muscles and receive sensory information from the overlaying skin. The lower intercostal nerves (T7-&11 and subcostal T12 nerves) supply the intercostal muscles and the anterolateral abdominal muscles and overlying skin. Posterior rami of the thoracic spinal nerves supply deep muscles of the back and skin. LO5: Describe what a simple spinal relfex is, name a few (stretch, tendon, withdrawal) and discuss their importance A reflex is a fast, involuntary sequence of actions in response to a particular stimulus. Reflexes are important to protect us from dangerous situations, before the stimulus is processed by the brain. An example is when you touch a very hot object and immediately withdraw that part of the body from that object to escape a serious burn and this happens even before you are aware that it was harmful event. Some reflexes are: - - When the integration of information and response takes place within the graymatter of the spinal cord, the reflex is called a spinal reflex. Integration within the brain stem is referred as a cranial reflex. The spinal reflexes consist of neurons which are the part of a reflex arc. The *monosynaptic reflex* consists of only two neurons (such as a stretch reflex). In this simple reflex arc the sensory neuron synapses directly onto the motor neuron in the anterior horn of the spinal cord. The *polysynaptic reflex* arc consists of more than two neurons as it involves interneurons. Since there are more than one synapse the response is slower than in monosynaptic reflex. An example is an ipsilateral (same side) withdrawal reflex. Golgi tendon reflex is also polysynaptic reflex. This reflex is protective in the way that prevents skeletal muscle from extensive tension. Golgi tendon organs are nerve endings located within the tendon at the tendon-muscle junction. When the muscle contracts it apply force on the tendon, which activates Golgi tendon organ. The information is sent to interneurons in the spinal cord which then suppress the muscle contraction and subsequently cause relaxation of the muscle, which will prevent injury to muscle fibres. Useful terminology regarding reflex arc: - - - **Polysynaptic Cross-Extensor Reflex** -------------------------------------- More complex polysynaptic contralateral reflex is crossed extensor reflex. This reflex provides withdrawal from the painful stimulus while the muscles on the contralateral side contract to provide postural stability and prevent your falling. **Module: Nervous System 6 - Introduction to Spinal Pathways** ============================================================== **Learning Outcomes** ===================== **By the end of this module, you should be able to:** **LO1**: Describe the organisation and composition of the grey and white matter in the spinal cord **LO2**: Describe the basic components and function of spinal pathway **LO3**: Describe the basic organisation and function of the ascending (sensory) and descending (motor) spinal pathways **LO4**: Describe the corticospinal tracts in the direct motor pathways **LO5**: Briefly describe the indirect motor pathways **Spinal Cord** =============== ***LO1**: Describe the organisation and composition of the grey and white matter in the spinal cord* From the previous module on the spinal cord, recall that the spinal cord serves as a connection between the brain and the rest of the body. This will be the focus of this module, as we are going to look at spinal pathways. However, before we do so, we are going to revise the organisation and composition of the grey and white matter in the spinal cord. The grey matter of the spinal cord is located centrally in the shape of an H and can be subdivided into horns: **Anterior (Ventral Horns)** ---------------------------- Contain cell bodies of somatic motor neurons **Lateral Horns** ----------------- Contain cell bodies of autonomic motor neurons **Posterior (Dorsal) Horns** ---------------------------- Contain cell bodies of interneurons and axons of sensory neurons (remember that the cell bodies of sensory neurons are located outside the spinal cord in the posterior/dorsal root ganglia) The white matter of the spinal cord is located on the periphery and can be subdivided into columns: - - - Another name for these columns is funiculi (singular = funiculus = rope/cord) **Spinal Pathways** =================== ***LO2**: Describe the basic components and function of a spinal pathways* Now that we have revised the spinal cord, we are going to look at spinal pathways, which are how the spinal cord serves as a connection between the brain and the body. Spinal pathways conduct either sensory or motor information between the brain and the body. The brain receives sensory information from the body via ascending pathways and sends motor commands to effectors in the body (e.g. muscles, organs etc.) via descending pathways. Therefore, **ascending** pathways are **sensory** pathways, while **descending** pathways are **motor** pathways. Spinal pathways consist of **tracts** and **nuclei**: **Tracts** ---------- A **tract** is a bundle of axons within the white matter of the spinal cord. Another name for a tract is a fasciculus (fasciculus = bundle). The white columns (funiculi) of the spinal cord are organised into tracts carrying either sensory or motor information. It is important to note that each **column** contains **both** ascending (sensory) and descending (motor) pathways, while the **tracts** within these pathways are more specific and carry **either** sensory or motor information. **Nuclei** ---------- A **nucleus** is a collection of neuronal cell bodies within the central nervous system. Recall the difference between this and a ganglion, which is a collection of neuronal cell bodies outside of the central nervous system (e.g. posterior/dorsal root ganglion). Before we look at ascending and descending pathways individually, there are a few important concepts relating to spinal pathways in general that we need to understand: 1. - - 2. 3. 4. **Ascending (Sensory) Pathways** ================================ ***LO3**: Describe the basic organisation and function of the ascending and descending spinal pathways* Now that we have introduced some major concepts relating to spinal pathways, we are going to start by looking at the ascending pathways, which are sensory pathways. The image below shows the major ascending pathways in the spinal cord. Please note that you do not need to know the names of these individual pathways or their components. The ascending pathways conduct sensory information from the body to the brain, including information on: - - - - - Depending on the stimuli they process, these pathways can be divided into: **Somatosensory Pathways** -------------------------- Process stimuli received from receptors in the skin, skeletal muscles, joints etc. (i.e. somatic structures) Ascending pathways conduct sensory information via either two or three neurons, depending on the pathway. These are called first, second and third order neurons. Click on the hotspots on the image below to learn more about these three neurons. The image above shows the posterior (dorsal) column-medial lemniscus pathway as an example of a major ascending pathway in the spinal cord. In this pathway, the cell body of the second order neuron is located in the medulla oblongata of the brainstem and its axon also decussates here. However, the location of the cell body of the second order neuron and the point of decussation will depend on the pathway. It is also important to note that while most spinal pathways decussate, this does not occur in all pathways. **Descending (Motor) Pathways** =============================== ***LO3**: Describe the basic organisation and function of the ascending and descending spinal pathways* Now we are going to look at the **descending** pathways, which are motor pathways. The image below shows the major tracts within the descending pathways in the spinal cord. Please note that **you do not need to know the names of all these individual tracts** -- we will mainly be focusing on the corticospinal tracts. The descending pathways conduct somatic motor information from the brain to the body to innervate skeletal muscles. These pathways can be divided into: **Direct Pathways** ------------------- These originate from pyramidal cells in the primary motor cortex, which is located in the precentral gyrus of the brain, and travel through the pyramids of the medulla oblongata in the brainstem. For this reason, the direct pathways are also called the pyramidal pathways. They control conscious body movements **Indirect Pathways** --------------------- These originate from cells in the brainstem and do not travel through the pyramids of the medulla oblongata. For this reason, the indirect pathways are also called the **extrapyramidal pathways**. They control subconscious (i.e. reflexive) body movements. Descending pathways conduct motor information via two neurons, called upper and lower motor neurons. We are going to look at these neurons in the context of the direct pathways, and more specifically, the corticospinal tracts within these pathways. Click on the hotspots on the image below to learn more about the upper and lower motor neurons in the corticospinal tracts. The corticospinal tracts conduct motor information to the skeletal muscles of the limbs and trunk. However, there are also other tracts in the direct pathways that conduct motor information to the skeletal muscles of the head and neck. The cell body of the lower motor neuron in these tracts is located in a cranial nerve nucleus in the brainstem and its axon travels via a cranial nerve towards the skeletal muscle it innervates. **Indirect Motor Pathways** =========================== ***LO5**: Briefly describe the indirect motor pathways* While the cell bodies of the upper motor neurons in the direct pathways are located in the primary motor cortex, the cell bodies of the upper motor neurons in the indirect pathways are located in brainstem nuclei. The axons of the upper motor neurons in the indirect pathways have a complex route of descent before synapsing either directly with lower motor neurons in the spinal cord, or with interneurons which then synapse with the lower motor neurons. The function of the indirect pathways is to modify and help the pattern of somatic motor activity, such as altering motor neuron sensitivity or activating feedback loops that project to the primary motor cortex. In other words, motor signals conducted via the indirect pathways can alter or regulate the contraction of skeletal muscles by either exciting or inhibiting the lower motor neurons that innervate these muscles. **Module: Nervous System 7 - Autonomic Nervous System** ======================================================= **Learning Outcomes** --------------------- **By the end of this module, you should be able to:** ===================================================== **LO1: Compare and contrast the functions and structural organisation of the somatic and autonomic nervous systems** ==================================================================================================================== **LO2: Compare and contrast the functions and structural organisation of the parasympathetic and sympathetic divisions of the autonomic nervous system** ======================================================================================================================================================== **LO3: Describe the effects of the parasympathetic and sympathetic divisions on organs and body systems** ========================================================================================================= **Somatic vs Autonomic Nervous System** --------------------------------------- ***LO1: Compare and contrast the functions of the somatic and autonomic nervous systems*** In this module, we will focus on the autonomic nervous system, or ANS. As discussed in the Nervous System 1 module, the nervous system can be divided into two structural parts: the central (CNS) and peripheral (PNS) nervous systems. The PNS acts as the intermediary between the CNS and the rest of the body, including the skin, internal organs and skeletal muscles. There are three main functional parts of the PNS: (1) somatic nervous system (SNS); (2) autonomic nervous system (ANS); and (3) enteric nervous system (ENS). The ENS will be studied later in advanced anatomy units. Click on the hotspots on the image below and note that both the SNS and the ANS have afferent and efferent components. As you can see, while the SNS deals with voluntary body processes, the ANS regulates involuntary processes such as heart rate and respiration. To accomplish such different functions, the SNS and ANS differ significantly in several major aspects, including their sensory inputs, target/effector organs, pathways, neurotransmitters and the responses of the target/effector organs to these neurotransmitters. Let's look at this in more detail. Let's compare and contrast the two major PNS subdivisions - the SNS and the ANS. Study the table below and click on the hotspots to learn more about the differences between the SNS and the ANS. Now, let's focus on the difference in the neuronal pathways between the somatic and autonomic nervous systems (one-neuron versus two-neuron pathways). There are a few key differences in the neurons and neuronal pathways between the somatic (SNS) and autonomic (ANS) nervous systems. Compare and contrast the two images below representing the SNS and ANS pathways. Then, click on the hotspots on these images to learn more about these pathways. Now that we understand the difference in the structural and functional organisation of the SNS and the ANS, let\'s move onto the differences between the two divisions of the ANS. **Sympathetic and Parasympathetic Divisions of the ANS** -------------------------------------------------------- ***LO2: Compare and contrast the functions and structural organisation of the parasympathetic and sympathetic divisions of the autonomic nervous system*** The ANS consists of the sympathetic and parasympathetic divisions, which differ from each other both structurally and functionally. In simple terms, the **sympathetic division** enables the body to cope with stress and is commonly described as being activated in conditions of 'fight, flight or fright'. The **parasympathetic division** acts as the 'housekeeper' of the body. The parasympathetic division is most active when an individual is calm and relaxed, and thus, is sometimes referred to as being the 'resting and digesting' commander. Both ANS divisions have preganglionic and postganglionic neurons. The **preganglionic neuron** is the **first neuron** in the pathway, running **from the spinal cord to an autonomic ganglion** to synapse with the second neuron. The **postganglionic neuron** is the **second neuron** in the pathway, running **from the autonomic ganglion to the target/effector**. Note that most targets/effectors of the ANS receive dual innervation, meaning they receive innervation from both the parasympathetic and sympathetic divisions. Let's compare and contrast the structural organisation of these two divisions. Study the two images below that represent the neuronal chain for each division. Then, click on the hotspots to learn more about these neuronal chains. Next, we are going to look in more detail at the overall structural organisation of both the sympathetic and parasympathetic divisions of the ANS. ### **Sympathetic Autonomic Nervous System** The image below shows the organisational plan of the sympathetic autonomic nervous system. In this unit, we focus on the major concepts only. You don't need to remember every detail on this diagram. Let's point out the most important constituents of the sympathetic nervous system as follows: **Location of the cell bodies of preganglionic neurons** -------------------------------------------------------- The **cell bodies of the preganglionic neurons** of the sympathetic nervous system are located in the thoracic and upper lumbar segments of the spinal cord (T1-L3), so we can use the term '**thoracolumbar output**' to describe their site of origin. The axons of these neurons (preganglionic fibres) leave the spinal cord via the ventral roots of the spinal nerves and travel only a short distance to the **sympathetic ganglia**. **Location of the ganglia (cell bodies of postganglionic neurons)** ------------------------------------------------------------------- The sympathetic ganglia are the locations of the **cell bodies of the postganglionic neurons** of the sympathetic nervous system. This is where the synapse between the pre- and post-ganglionic neurons occurs. In the sympathetic system, most ganglia are located along the vertebral column, in the two paravertebral chains ('para' = next to) called the right and left **sympathetic chain (or sympathetic trunk)**. The sympathetic trunk is a vertical series of ganglia that are interconnected by nerve fibres to form a trunk or chain. It is located on both sides of the vertebral column, extending from the site of its origin in the thoracic and upper lumbar regions up to the neck and down to the pelvis. Therefore, the sympathetic trunk extends all the way from the C1 vertebra down to the coccyx. Thus, there are cervical, thoracic, lumbar, and sacral ganglia. The cervical sympathetic trunk has three adjacent ganglia: superior cervical ganglion, middle cervical ganglion, and inferior cervical ganglion. However, in most people, the inferior cervical ganglion is combined with the first thoracic ganglion, forming the cervicothoracic ganglion, commonly referred to as the stellate ganglion (\'stellate\' = star-shaped). The sympathetic trunk is connected to the spinal nerves via small branches called **rami communicans** (or rami communicantes). The preganglionic fibres that are destined for abdominal and pelvic organs don't synapse in the sympathetic trunk. Instead, these fibres travel to the **prevertebral ganglia** ('pre' = in front of) to synapse. These nerves are called the **splanchnic nerves** ('splanchnic' = related to viscera). The prevertebral ganglia are located in the abdominal cavity around the origin of the major branches of the abdominal aorta. After synapsing in either the para- or pre-vertebral ganglia, the postganglionic fibres (axons of postganglionic neurons) leave the ganglia to travel towards their targets/effectors. **Autonomic nerve plexuses** ---------------------------- The **postganglionic fibres form multiple plexuses** (webs of intertwining nerves) that are scattered throughout the body. These plexuses and their branches mostly accompany body vessels as they travel to their targets/effectors. **Targets/Effectors** --------------------- As the sympathetic nervous system is part of the autonomic nervous system, its targets/effectors are **involuntary organs and tissues**. These include smooth muscle in the walls of hollow organs, cardiac muscle and glands. Click on the hotspot on the image below to see an image of the sympathetic trunk on a human cadaveric specimen. ### **Parasympathetic Autonomic Nervous System** The image below shows the organisational plan of the parasympathetic autonomic nervous system. In this unit, we focus on the major concepts only. You don't need to remember every detail on this diagram. Let's point out the most important constituents of the parasympathetic nervous system as follows: **Location of the cell bodies of preganglionic neurons** -------------------------------------------------------- The cell bodies of the preganglionic neurons of the parasympathetic nervous system are located in the brainstem and the sacral spinal cord (S2-S4), so we can use the term **'craniosacral output'** to describe their site of origin. The axons of the preganglionic neurons (preganglionic fibres) originating in the brainstem travel through the cranial nerves. There are **4 cranial nerves** containing parasympathetic fibres. Three of these nerves only send fibers to the head and neck: oculomotor (CN III), facial (CN VII) and glossopharyngeal (CN IX). The **vagus nerve (CN X)**, however, carries parasympathetic fibers to the organs of the thorax and abdomen. It has an incredibly long course, hence the name vagus (\'vagus\' = wanderer). The preganglionic neurons from the **sacral region** originate at the **S2-S4** levels of the spinal cord. They are called **pelvic splanchnic nerves** and are responsible for innervating pelvic organs and the distal aspect of the gastrointestinal tract. The axons of these neurons (preganglionic fibres) leave the spinal cord and travel long distances to the **parasympathetic ganglia** in the walls of their targets/effectors. **Location of the ganglia (cell bodies of postganglionic neurons)** ------------------------------------------------------------------- The **parasympathetic ganglia** are the locations of the **cell bodies of the postganglionic neurons** of the parasympathetic nervous system. This is where the synapse between the pre- and post-ganglionic neurons occurs. The parasympathetic ganglia of cranial nerves III, VII and IX are found in a few specific locations within the head. The ganglia for the vagus (CN X) and pelvic splanchnic nerves are situated either very close to, or inside the walls of, their targets/effectors. Therefore, while parasympathetic preganglionic fibres are long, the postganglionic fibres (axons of postganglionic neurons) are very short. **Autonomic nerve plexuses** ---------------------------- The **postganglionic fibres form multiple plexuses** (webs of intertwining nerves) that are scattered throughout the body. As the vagus nerve travels a long distance from its skull exit to the thoracic and most abdominal organs, its fibres branch along its path to join several different plexuses. These plexuses travel to specific organs such as the heart, lungs and gastrointestinal tract. The pelvic splanchnic nerves also form plexuses that supply the distal aspect of the gastrointestinal tract and the pelvic organs. Note that **most autonomic nerve plexuses end up containing both sympathetic and parasympathetic fibres** **Targets/Effectors** --------------------- **As the parasympathetic nervous system is part of the autonomic nervous system, its targets/effectors are involuntary organs and tissues. These include smooth muscle in the walls of hollow organs, cardiac muscle and glands.** **Physiological Effects of the ANS** ------------------------------------ ***LO3: Describe the effects of the parasympathetic and sympathetic divisions on organs and body systems*** **The table below summarises the effects of the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS). Normally, these two divisions are balanced according to the body's changing needs. An autonomic \'tone\' of the ANS represents its normal background activity, balancing the prevalence of either sympathetic or parasympathetic influences.** **In simple terms, the sympathetic nervous system and the parasympathetic nervous system are referred to as antagonistic, as they typically have opposite effects at their targets/effectors. The sympathetic nervous system predominantly mediates the 'fight, flight or fright' response, while the parasympathetic nervous system typically mediates the body\'s 'rest and digest' activities.** **Many targets/effectors of the ANS have dual innervation. This means that they are supplied by both sympathetic and parasympathetic neurons. In a sense, this dual innervation provides both a 'break' and an 'accelerator' for changing the activity of our internal organs, offering a good amount of control. Both the sympathetic nervous system and parasympathetic nervous system influence our internal organs simultaneously. At all times, the heart is receiving signals from the sympathetic nervous system which increase heart rate, as well as signals from the parasympathetic nervous system which decreases heart rate. However, this seesaw-like balance can shift quickly in either direction, such as inducing a sympathetic response if a fearful stimulus is encountered.** **Note that some organs have only sympathetic innervation, such as sweat glands, the adrenal medulla, arrector pilli muscles and many blood vessels. These organs are controlled by the regulation of the 'tone' of the sympathetic nervous system.** ### **Visceral Reflex** **Now, let's consider how the ANS carries out involuntary actions (i.e. without our conscious intent or awareness). The visceral targets/effectors of the ANS only adjust their activity to the body's changing needs. Visceral reflexes are unconscious, automatic, stereotyped responses to stimulation, involving visceral receptors and targets/effectors and somewhat slower responses. Study the image below, which shows a visceral reflex arc in response to high blood pressure. Click on the hotspots on the image to learn more about the components of this reflex arc.** ### **Visceral Sensory Information** **Technically, the ANS produces motor responses, but it also responds to visceral sensory input. Senses associated with the viscera are unconscious. For example, there are specific receptors that sense if there is enough oxygen in your blood, but you do not have a conscious perception of your oxygen levels. If oxygen is too low, that sensory input will eventually cause an increase in your respiratory rate.** **Although visceral senses are primarily not a part of conscious perception, these sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. For example, if you inhale especially cold air, you can feel it as it enters your larynx and trachea.** **When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations from the heart will be felt as pain in the left shoulder and left arm. This irregular projection pattern of the conscious perception of visceral sensations is called referred pain. Depending on the organ affected, the referred pain will project to different areas of the body.** **The location of referred pain is not random. The most broadly accepted theory for this phenomenon is that the visceral sensory fibres from the affected organ enter at the same level of the spinal cord as the somatosensory fibres from the referred pain location. The visceral sensory fibres from the heart would enter the spinal cord at the same level as the somatosensory fibres from the shoulder and arm, so the brain misinterprets the sensations from the heart region as being from the shoulder and arm regions.** **Please note that you do not need to memorise the referred pain areas shown on the image below. Just focus on understanding the concept of referred pain and the fact that the ANS involves visceral sensory input as well as motor output.**