ANAT Week 7.docx

Week 7 **Module: Endocrine System** ============================ **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the basic components and function of the endocrine system **LO2**: Describe the location, function and gross anatomy o...

Week 7 **Module: Endocrine System** ============================ **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the basic components and function of the endocrine system **LO2**: Describe the location, function and gross anatomy of the following najor endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands **LO3**: Describe the locatino of the pancreas and the function and histology of its endocrine portion **LO4**: Provide examples of other organs containing endocrine cells **Components and Function** --------------------------- ***LO1: Describe the basic components and function of the endocrine system*** The endocrine system is one of the two major body-controlling systems, with the other being the nervous system. These two systems work together to regulate the body\'s functions and maintain homeostasis. The nervous system exerts rapid control via nerve impulses, while the effects of the endocrine system are via hormones and are both slower and more prolonged. The following body processes are regulated by hormones: - - - - - The main components of the endocrine system are the purely endocrine organs, called glands, and their secretions, called hormones. Hormones are molecules that act as chemical messengers in the body, interacting with specific target cells via receptors to change thier metabolic activity. Recall from the Epithelium module that endocrine glands lack ducts adn release their hormones directly in the bloodstream. Therefore, endocrine glands are well vascularised. They are scattered around the body and include the following: - - - - - These will be the focus of this module. In addition to the endocrine glands mentioned above, the hypothalamus is classified as a neuroendocrine organ. Recall that the hypothalamus is a region in the diencephalon of the barin. It is the control centre of the endocrine system, regulating most of its activity. There are also scattered endocrine cells located in other organs of the body, such as the heart, thymus, gastrointestinal tract, pancreas, kidneys and gonads (testes in male and ovaries in female), among others. We will look at the endocrine portion of the pancreas in this module. **Pituitary Gland or Hypophysis** --------------------------------- ***LO2: Describe the location, function and gross anatomy of the following major endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands*** The pituitary gland, or hypophysis, is a small, oval-shaped gland that is located inferior to the hypothalamus and is connected to it by a thin stalk called the infundibulum ('infundibulum' = funnel). It sits in a small fossa, or depression, in the sphenoid bone called the hypophyseal fossa. Together with its connections, the pituitary gland is the main endocrine interface between the central nervous system and the rest of the body. It acts as the body's "master gland", as it tells the other glands of the body what to do. Structurally and functionally, the pituitary gland can be divided into an anterior pituitary gland and a posterior pituitary gland. Click on the hotspots on the image below to learn more about these two parts of the pituitary gland and the hormones they release. **Pineal Gland** ---------------- ***LO2: Describe the location, function and gross anatomy of the following major endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands*** The pineal gland is a small, cone-shaped gland attached to the posterior region of the epithalamus. Recall that the epithalamus is a region in the diencephalon of the brain. The pineal gland produces a hormone called melatonin, which regulates circadian rhythm (i.e. the 24-hour body clock). **Thyroid Gland** ----------------- ***LO2: Describe the location, function and gross anatomy of the following major endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands*** The thyroid gland is a butterfly-shaped gland located in the neck, inferior to a cartilage of the larynx called the thyroid cartilage and anterior to the trachea (we will look at the larynx and trachea in another module). It is composed of two lobes (right and left) that are connected by a narrow band called the isthmus. It produces a hormone called thyroid hormone (TH), which comes in two forms -- T3 and T4. This hormone increases the metabolic processes of the body by acting on many body cells. The thyroid gland also produces a hormone called calcitonin, which decreases blood calcium levels and is involved in calcium homeostasis in the body. **Parathyroid Glands** ---------------------- ***LO2: Describe the location, function and gross anatomy of the following major endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands*** The parathyroid glands are small nodules located on the posterior surface of the thyroid gland. There are usually four, although this can vary in some people. They produce a hormone called parathyroid hormone (PTH), which increases blood calcium levels. Together with calcitonin from the thyroid gland, parathyroid hormone is a key hormone for calcium homeostasis in the body. Adrenal Glands or Suprarenal Glands LO2: Describe the location, function and gross anatomy of the following major endocrine glands: pituitary gland, pineal gland, thyroid gland, parathyroid glands and adrenal glands The adrenal glands, or suprarenal glands ('supra' = above, 'renal' = kidney), are paired, pyramidal-shaped glands located on the superior aspect of the kidneys. Internally, each adrenal gland has an outer region called the adrenal cortex and an inner region called the adrenal medulla. Click on the hotspots on the image below to learn more about these two regions and the hormones they produce. **Pancreas** ------------ ***LO3: Describe the location of the pancreas and the function and histology of its endocrine portion*** The pancreas is a mixed gland with both endocrine and exocrine functions. In this module, we we are going to focus on its endocrine function. We will look at its exocrine function and look more closely at its specific location and gross anatomy in another module. The pancreas is located in the abdominal cavity, posterior to the stomach and in between a part of the small intestine called the duodenum and the spleen. The exocrine portion of the pancreas makes up the majority of the pancreatic tissue and consists of secretory portions called pancreatic acini. Scattered amongst the clusters of pancreatic acini are small clusters of endocrine cells called pancreatic islets, or islets of Langerhans. These produce several hormones, including insulin and glucagon. These two hormones work together to regulate blood glucose levels -- insulin decreases blood glucose levels, while glucagon increases blood glucose levels. **Other Hormone-Producing Organs** ---------------------------------- ***LO4: Provide examples of other organs containing endocrine cells*** In addition to the major endocrine organs that we have looked at, it is important to be aware that there are many other organs that produce hormones and contribute to the endocrine functions of the body. These include the heart, thymus, gastrointestinal tract, kidneys and gonads (testes in male and ovaries in female), among others. Tissues of the body such as adipose tissue, bone and skeletal muscle also produce hormones. We look at the thymus in another module on the lymphatic system and we will look at the gonads in future modules on the reproductive system. **Endocrine System Video** -------------------------- Now that we have finished looking at the endocrine system, you may like to watch the video below to learn more about it (please note that any details that were not covered in this module are not examinable). **Module: Special Senses Part 1 - Introduction, Olfaction, Gustation** ====================================================================== **General Classification of Senses** ==================================== ***LO2**: Understand the general classification of senses* There are **two basic groups of senses**: general and special senses. The **general senses** have receptors distributed over a large part of the body. They are divided into two groups: the somatic senses and the visceral senses. The **somatic senses** provide sensory information about the body and the environment. The visceral senses provide information about various internal organs, primarily involving pain and pressure. **Special senses** are more specialized in structure and are localized to specific parts of the body. The special senses are smell, taste, sight, hearing, and balance. Now, let's have a closer look at the special senses of smell and taste. **Olfaction** ============= ***LO3**: Briefly describe the receptors for olfaction and the olfactory pathway* The nose contains up to 100 million receptors for the sense of smell of olfaction. Because some nerve impulses for smell and taste propagate to the limbic system, certain odors and tastes can evoke strong emotional responses or memories. The olfactory epithelium occupies the **upper portion of the nasal cavity**. It contains **olfactory receptor cells** (olfactory neurons), which are **first-order neurons** of the olfactory pathway. Olfactory receptor cells are bipolar neurons. Their distal processes (dendrites) have cilia, **olfactory hairs** that project into the olfactory epithelium and get stimulated by inhaled chemicals, the odorants. The proximal processes (axons) of olfactory sensory neurons extend from olfactory epithelium to the olfactory bulb. There are about 40 bundles of these short, slender, unmyelinated axons coursing through about 20 holes in the **cribriform plate of ethmoid bone**. These bundles of axons collectively form the **right and left olfactory nerves** that are referred to as Cranial Nerve I (**CN I**). The olfactory nerves terminate in the brain in paired masses of grey matter called the **olfactory bulbs**, which are located below the frontal lobes of the cerebrum. Within the olfactory bulbs, the axons of olfactory neurons (the first-order neurons) form **synapses** with the **second-order neurons of the olfactory bulb**. The axons of olfactory bulb neurons form the olfactory tract. Some of the axons of the olfactory tract project to the **primary olfactory area** in the **temporal lobe** of the cerebral cortex, where conscious awareness of smell begins. Other axons of the olfactory tract project to the limbic system and hypothalamus. These connections account for emotional and memory-evoked responses to odors. The primary olfactory cortex will process the olfactory information allowing us to perceive the odour. That information will also be sent to the orbitofrontal cortex in the frontal lobe where smells can be analysed and compared to the other smells so we can identify what that smell is. By taking the path to the limbic system this is how we have an emotional response to smells and have memories elicited by those smells. **Gustation** ============= ***LO3**: Briefly describe the receptors for gustation and the gustatory pathway* Taste or gustation allows us to feel tastes such as sour, sweet, bitter, salty and umami (meaty/savory). Odors from the mouth passes into the nasal cavity where they stimulate olfactory receptors as well. The **receptors for taste** sensations are located in the **taste buds**. Most of these are on the **tongue** but there are some taste receptors in the roof of the mouth, pharynx, and epiglottis (of larynx). In the tongue, test buds are located inelevations called the **papillae**, which provide a rough texture to the upper surface of the tongue. There are several types of papillae: vallate (wall-like), fungiform (mushroom-like), filiform (thread-like), and foliate (leaf-like) You are not required to memorize each type but just be aware of different morphology of the papillae. However, the **vallate papillae** are easy to identify on models and on your own tongue, as they are the biggest out of all papillae. They extend in a **V-shaped line** across the root of the tongue. This V-shaped line divides the upper surface of the tongue into two parts: the anterior two-thirds and the posterior third parts. The taste from each part is supplied by a different cranial nerve. The taste buds contain **gustatory receptor cells** that have gustatory hair processes projecting to the external surface through the taste poor (an opening in the taste bud). Unlike the olfactory receptor cells (which are special neuronal cells), the gustatory receptor cells are **separate cells**. They **do not have axons** but rather synapse with dendrites of first-order neurons of the gustatory pathway. Now, let's look at the gustatory pathway in brief. Click on each number on the diagram below to follow the footsteps of the 'tastant', a chemical molecule that stimulate gustatory receptor cell. Please note that you are not required to memorise the precise pathway and the location of neurons, but rather understand the concept and note how cranial nerves and the thalamus participate in the process. **Module: Special Senses Part 2 - The eye** =========================================== **Eyeball Structure** ===================== ***LO3**: Describe the layers of the eyeball* **Eyeball Structure** --------------------- Study the picture below and note that the eyeball itself is made up of many structures designed to support and protect the photoreceptor cells, as well as gather, focus and process light into images. This picture has many labels as the eyeball structure is amazing and complex. You can return to this image again, once you learn more about the eyeball structure. Let's make this picture less complicated. We will start studying the eye from looking at its three layers or tunics. **Layers of the Eyeball** ------------------------- The adult eyeball measures \~2.5 cm in diameter and is divided into three layers: **Fibrous layer (fibrous tunic)** --------------------------------- The external layer that consists of two parts: **cornea** and **sclera** **Vascular layer (vascular tunic)** ----------------------------------- **The middle layer that consists of three parts: choroid, ciliary body and iris** **Retina** ---------- **The internal layer that consists of a pigmented layer and a neural layer** **Let\'s now look at the fibrous layer in more detail.** ### **Fibrous Tunic** **The fibrous tunic is the outer coat of the eyeball. It consists of an anterior cornea and posterior sclera.** **The cornea is a transparent fibrous coat that covers the coloured iris. It is curved and helps focus light rays onto the retina.** **The sclera, the white of the eye, is a coat of dense connective tissue that covers all the entire eyeball except the cornea. It gives shape to the eyeball, makes it more rigid and protects its inner parts.** **An epithelial layer called the conjunctiva covers the sclera, but not the cornea, and lines the inner surface of the eyelids. Inflammation of conjunctiva is called conjunctivitis.** **Now, let's look at the vascular tunic found immediately deep to the fibrous tunic.** ### **Vascular Tunic** **The vascular tunic (also called 'uvea') is the middle layer of the eyeball and is composed of the choroid, ciliary body, and iris.** **The choroid is a thin membrane that lines most of the internal surface of the sclera. It contains many blood vessels. It also contains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color. Melanin absorbs stray light rays, which prevents reflection and scattering of the light within the eyeball.** **At the front of the eye, the choroid becomes the ciliary body. The ciliary body consists of the ciliary processes, folds on the inner surface of the body whose capillaries secrete a fluid called aqueous humor, and the ciliary muscle a smooth muscle that alters the shape of the lens for viewing objects up close or at a distance.** **The lens, a transparent structure that focusses light rays onto the retina, is constructed of many layers of elastic protein fibers. Zonular fibers *or* suspensory ligaments attach the lens to the ciliary muscle and hold the lens in position.** **The iris (=colored circle) is the colored part of the eyeball. It includes both circular and radial smooth muscle fibers (sphincter and dilator pupillae muscles, accordingly). The hole in the center of the iris, through which light enters the eyeball, is the pupil. The sphincter and dilator pupillae muscles regulate the amount of light entering the pupil by decreasing or increasing the size of the pupil, accordingly.** **This picture below shows the anatomy of the ciliary body of the vascular tunic in more details. Please note how the ciliary body includes the muscles and processes. Attached to the processes are the suspensory ligaments (or zonular fibers) that stretch the lens to focus the image of objects based on how close or far it is.** **Now, let's see what happens when the ciliary muscles contract.** **As we discussed, the ciliary body is hidden behind the iris. The ciliary body is home to the ciliary muscle, the contraction of which causes the lens to assume a more rounded shape. This is because the lens is suspended by fine ligaments called zonules, which attach to the ciliary body. When ciliary muscle contracts, the anchoring point of the zonules moves inward, relaxing tension on zonules and natural elasticity of the lens causes it to take on a more spherical shape. This is how our focus shifts, a process called accommodation.** **As we mentioned above, the iris contains muscles that regulate the amount of light entering the pupil:** 1. 2. **Since these are smooth muscles, they are innervated by the autonomic nervous system.** ### **Retina** **The retina is the third and inner coat of the eyeball. It lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway. It has two layers:** **Neural Layer** ---------------- **The neural layer is a multilayered outgrowth of the brain. It includes the three distinct layers of retinal neurons -- the photoreceptor layers, the bipolar cell layer, and the ganglion cell layer.** **Pigmented Layer** ------------------- **Photoreceptors are specialised cells that are found within the neural layer of the retina. Photoreceptors begin the process by which light rays are ultimately converted to nerve impulses. There are two types of photoreceptors:** **Rods** -------- **Rods allow us to see shades of grey and dim light, such as moonlight.** **Cones** --------- **Cones are stimualted by brighter light giving rise to highly acute, colour vision.There are several types of cones, and colour vision results from stimulation of various combinations of different types of cones.** **The complete loss of cone vision causes a person to become legally blind. In contrast, a person who loses rod vision mainly has difficulty seeing in dim light and thus should not drive at night (night blindness). A person with an absence of deficiency of one of the three typs of cones from the retina cannot distinguish some colours from others and is said to be colour-blind.** **Cones are most densely concentrated in the fovea centralis, a small depression in the centre of the macula lutea, or yellow spot, in the exact centre of the retina. The fovea centralis is the area of highest visual acuity or resolution (sharpness of vision) because of its high concentration of cones. The main reason that you move your head and eyes while looking at something -- is to place images of interest on your fovea. Rods are absent from the fovea centralis and macula lutea and increase in numbers toward the periphery of the retina.** **From photoreceptors, information flows to bipolar cells and then from bipolar cells to the ganglion cells.** **The axons of ganglion cells extend posteriorly to a small area of the retina called the optic disc (blind spot), where they all exit as the optic nerve (CN II). Because the optic disc contains no rods or cones, we cannot see an image that strikes the blind spot.** **Ophthalmoscopy is an examination of the back part of the eye (fundus). Let's check what an optometrist can see on an ophthalmoscopic view of your eye** ### **Ophthalmoscopic View** **Ophthalmoscopy is often done as part of a routine physical eye examination. It visualises the posterior part or the fundus of the eye, which includes the retina, optic disc, choroid and blood vessels.** **The retina is the only portion of the central nervous system visible from the exterior. On the picture above, note the macula of retina, a spot containing cone cells. It is located on a direct pass of light through the eyeball. The fovea is located in the center of the macula (not labelled). This is the area of highest visual acuity.** **The fundus of the eye is also the only location in the human body, where the vasculature can be visualized. Viewing the fundus is a great way to get a sense for the patient's overall vasculature. Located on the medial (nasal side) of the fundus is the blind spot or the optic disc. This is where the axons of ganglionic cells form the optic nerve, which exits the eyeball. Note the branches of retinal vessels as they radiate from the center of the optic disc.** **Eye Segments** ================ ***LO4**: Name the two segments of the eye* **Eye Segments** ---------------- The lens divides the interior of the eyeball into two cavities or segments: **Anterior Segment** -------------------- Located anterior to the lens **Posterior Segment** --------------------- Located posterior to the lens The pressure in the eye, called **intraocular pressure**, is produced mainly by the **aqueous humor** with a smaller contribution from the vitreous body. Intraocular pressure maintains the shape of the eyeball and keeps the retina smoothly pressed against the choroid, so the retina is well nourished and forms clear images. Normal intraocular pressure (about 16 mm Hg) is maintained by a **balance between production** and **drainage** of the aqueous humor. Now let's consider what is involved in the visual pathway. **Visual Pathway** ------------------ After stimulation by light, the rods and cones trigger electrical signals that create a cascade of events associated with the visual pathway. Click on hot spots to reveal major stages of the visual pathway. Please note that you are not required to know the names of the brain nuclei involved for this unit. However, note the location of the visual cortex. Because of crossing at the optic chiasm, the right side of the brain receives signals from both eyes for interpretation of visual sensations from the left side of an object, and the left side of the brain receives signals from both eyes for interpretation of visual sensations from the right side of an object. **Module: Special Senses Part 3 - The ear** =========================================== **Learning Outcomes** ===================== **By the end of this module, you should be able to:** **LO1**: Describe the structure of the ear **LO2**: Describe the structure of the external ear **LO3**: Describe the structure of the middle ear **LO4**: Describe the structure of the internal ear **LO5**: Name receptor for equilibrium and specify their location **LO6**: Name receptors for hearing and briefly outline the auditory pathway to the brain **Structure of the Ear** ======================== ***LO1**: Describe the structure of the ear* **Structure of the Ear** ------------------------ The ear is a marvelously unique and sensitive structure. Its sensory receptors can convert sound vibrations into electrical signals 1000 times faster than photoreceptors can respond to light. Beside receptors for sound waves, the ear also contains receptors for equilibrium (balance). The ear is divided into three main regions, the external, middle and internal ear. Click on each hot spot to learn about their general functions. Let's look at the external ear first. **External Ear** ================ ***LO2**: Describe the structures of the external ear* **External Ear** ---------------- The external (outer) ear collects sound waves and passes them inward. These waves travel from the auricle, external acoustic meatus (or external auditory canal) and towards the tympanic membrane ('eardrum' in lay language). Click on each part to reveal more information. **Temporal Bone** ================= ***LO2**: Describe the structures of the external ear* ***LO3**: Describe the structures of the middle ear* ***LO4**: Describe the structures of the internal ear* **Temporal Bone** ----------------- Before we move onto further details on the ear anatomy, let's look at the basic features of the **temporal bone**. On the lateral side of the temporal bone, note the **external acoustic meatus** (or external acoustic canal). It is located anterior to the bulky mastoid process you can feel behind your ear. On the internal surface of the skull base, note the **petrous part of the temporal bone**, which forms a prominent ridge separating the middle and posterior cranial fossae. It contains the **middle** and **inner ear structures**. The **internal acoustic meatus** is an opening in the petrous part of the temporal bone. The vestibulocochlear nerve (CN VIII) together with the facial nerve (CN VII) travel via this foramen. Now we are going to zoom into the temporal bone to study anatomy of the middle ear. ***LO3**: Describe the structures of the middle ear* **Middle Ear** -------------- The middle ear, also called the **tympanic cavity**, is a small, **air-filled cavity** within the **petrous part** of the **temporal bone**. The tympanic cavity is laterally bounded by the tympanic membrane and medially bounded by a wall of bone between it and the inner ear. It contains the **auditory ossicles**. There are **two small openings** that penetrate the medial bony wall: superior **oval window** and inferior **round window**. The **oval window** is covered by the stapes, while the **round window** is enclosed by the **secondary tympanic membrane**. An opening in the anterior wall of the middle ear leads directly onto the **auditory tube**, commonly know as the **eustachian tube**. This tube connects the middle ear with the pharynx (throat). When the auditory tube is open, air pressure can equalize on both sides of the tympanic membrane. Otherwise, abrupt changes in air pressure on one side of the eardrum might cause it to rupture. During swallowing and yawning, the tube opens, which explains why yawning can help equalize the pressure changes that occur while flying in an airplane. Extending across the middle ear cavity are three tiny bones called the **auditory ossicles** that are named for their shapes (from lateral to medial): **malleus**, **incus and stapes** (commonly called the hummer, anvil, and stirrup, accordingly). Note how the **malleus** is attached to the **tympanic membrane** on one side and **articulates with the incus** on the other side. The **incus articulates with the stapes**, which then **encloses the oval window**. Therefore, the ossicles forms a chain interconnecting the tympanic membrane and the oval window. **Two tiny skeletal muscles** control the amount of movement of the ossicles: stapedius (attaches to the stapes) and tensor tympani (attaches to the malleus). These muscles **prevent damage** from excessively loud noises as they restrict the movements of the given ossicles. **Inner Ear** ============= ***LO4**: Describe the structures of the internal ear and name receptors for equilibrium* **Inner Ear** ------------- The **inner ear** contains the **vestibulocochlear organ** concerned with the reception of sound and the maintenance of balance. It is often referred to as the **labyrinth**. There are two main divisions to the inner ear: the **outer bony labyrinth** and the **inner membranous labyrinth**. ### **Bony Labyrinth** **The bony labyrinth** is a series of **cavities in the petrous part** of the temporal bone. It is filled with fluid called the **perilymph**. **The membranous labyrinth** is located **inside the osseous labyrinth** surrounded by the perilymph. The membranous labyrinth is a series of sacs and tubes that contain the fluid called the **endolymph**. Endolymph and perilymph don't mix. Let's look at each labyrinth in more detail. **Bony Labyrinth** ------------------ We will start from the bony labyrinth. This labyrinth is a space within the petrous part of the temporal bone surrounded by a layer of very dense bony tissue. The illustration above shows a cast of this space. There are three main parts in the bony labyrinth: - - - Press on hot spots to learn more about each part of the bony labyrinth. Now, let's focus on the membranous labyrinth. **Membranous Labyrinth** ------------------------ The **membranous labyrinth** consists of series of communicating sacs and ducts that are **suspended in the bony labyrinth** and surrounded by the perilymph. The membranous labyrinth contains **endolymph**, a watery fluid similar in composition to intracellular fluid. This labyrinth includes the **utricle**, **saccule**, **semicircular ducts and cochlear ducts**. The utricle and saccule are housed within the bony vestibule, while semicircular ducts and cochlear ducts are located within the corresponding bony canals. Let's zoom into the cross section through the cochlea to see the **organ of Corti** *or* **Spiral organ.** **Inner Ear** ------------- **On the image below, you can see the cross-section through the cochlea. The membranous cochlear duct filled with endolymph is suspended inside the middle of the osseous canal. This arrangement creates two subdivisions with the osseous canal: scala vestibuli and scala tympani. These two 'scala' are filled by perilymph. The scala vestibuli is continuous with scala tympani at the very tip of the cochlea.** **The receptor organ for hearing is called the organ of Corti *or* spiral organ, and it is located within the membranous cochlear duct. The hair cells of the spiral organ allow for transduction of auditory signals into nerve impulses. In this unit, you are not required to know precise structure of the spiral organ. The illustration above is provided to facilitate deeper understanding.** **Auditory Pathway** ==================== ***LO6: Name receptors for hearing and briefly outline the auditory pathway to the brain*** **Auditory Pathway** -------------------- **Now, let's discuss the auditory pathway. Audition is the sequence of vibrations from one medium to the next: from sound waves in the air to vibration in the bone, to the vibrational or pressure waves in fluid until it is eventually converted to electrical energy (transduction) so the information can be carried along a nerve to be interpreted by the brain.** **Click on hot spots to follow the steps of the sound conduction pathway.** **Sensory neurons in the cochlear branch of each vestibulocochlear nerve (CN VIII) terminate in the medulla oblongata on the same side of the brain. From the medulla, axons ascend to the midbrain, then to the thalamus, and finally to the primary auditory area in the temporal lobe. Because many auditory axons cross to the opposite side, the right and left primary auditory areas receive nerve impulses from both ears.** **You do not need to know specific nuclei participating in this pathway.**