Brainstem Cranial Nerves PDF

*Brainstem 1.* *Cranial Nerves*  **Brainstem** divided into three parts: Midbrain, Pons and Medulla oblongata. - Mammillary body belongs to the hypothalamus. **Cranial Nerve I: Olfactory Nerve.** **Function:** Special sensory for olfaction (sense of smell). **Pathway:*...

*Brainstem 1.* *Cranial Nerves*  **Brainstem** divided into three parts: Midbrain, Pons and Medulla oblongata. - Mammillary body belongs to the hypothalamus. **Cranial Nerve I: Olfactory Nerve.** **Function:** Special sensory for olfaction (sense of smell). **Pathway:** The olfactory nerve pathway begins in the **nasal epithelium**, located in the upper part of the **nasal cavity**, where olfactory sensory neurons detect **odour molecules**. These neurons send their **axons** through tiny **openings** in the **cribriform plate** of the **ethmoid bone** to reach the **olfactory bulb**, a structure situated at the base of the brain. Within the olfactory bulb, the sensory neurons form **synapses** with **specialized neurons**, allowing initial processing of **smell information**. From there, axons from the olfactory bulb **bundle** together to form the **olfactory tract**, which carries the processed signals to **higher brain regions**, including the olfactory cortex, amygdala, and other associated areas. These regions further interpret the signals, associating them with memory, emotion, and perception of smell. **Target structures:** Olfactory cortex, piriform cortex, and limbic system**.** **Notes:** The olfactory system is highly developed in animals like dogs, which rely on it for environmental exploration, unlike humans who depend more on vision and touch.  **Cranial Nerve II: Optic Nerve** **Function**: Special sensory nerve responsible for **vision**, transmitting visual signals from the retina to the brain. **Functional Components: Sensory**: Detects light and processes visual information. - It does not directly control muscles or movement of the eye but plays a critical role in providing input for vision-related reflexes and perception. **Pathways:** The **optic nerve** pathway begins at the **retinal ganglion** cells in the **retina**. **Nerve fibres** from these cells converge to **form** the **optic nerve**, which **exits** the orbit through the **optic canal**. The **optic nerves** from both eyes **meet** at the **optic chiasm**, where fibres from the **nasal** (medial) halves of each retina **cross** over to the **opposite** side. After the chiasm, the fibres continue as the optic tract and **project** to the **lateral geniculate nucleus (LGN)** in the **thalamus**. From the LGN, the visual **signals** are relayed via the **optic radiation** to the **primary visual cortex** in the **occipital lobe**, where they are processed to form **visual perception**. **Nuclei and Targets:** The main **destination** of **visual signals** is the **primary visual cortex**, located in the **occipital lobe.** Additional projections reach: **Superior colliculus** (for visual reflexes), **Pretectal area** (pupillary light reflex) and **Suprachiasmatic nucleus** (regulation of circadian rhythms). **Extraocular Muscles** The movement of the eye is controlled by **extraocular muscles**, which are essential for tracking, focusing, and stabilizing vision. These muscles are innervated by other cranial nerves. **Six Extraocular Muscles** [**Rectus muscles (4)**:] Straight muscles that control linear movements. **Superior rectus**: Elevates the eye (looks upward). **Inferior rectus**: Depresses the eye (looks downward). **Medial rectus**: Moves the eye medially (towards the nose). **Lateral rectus**: Moves the eye laterally (away from the nose). [**Oblique muscles (2)**:] Control rotational and finer adjustments. **Superior oblique**: Depresses and intorts the eye (downward and inward). **Inferior oblique**: Elevates and extorts the eye (upward and outward). **Cranial Nerve III: Oculomotor Nerve** **Function**: The oculomotor nerve is responsible for controlling most of the **eye\'s movements**, **elevation of the eyelid**, **pupil constriction**, and **lens accommodation** for focusing. **Origin**: Arises from the **interpeduncular fossa**, a depression located between the **cerebral peduncles** of the midbrain. **Nuclei Location**: The nuclei associated with the oculomotor nerve are situated in the **midbrain.** **Oculomotor Nucleus** controls **four extraocular muscles**: **Superior rectus**, **Inferior rectus**, **Medial rectus**, **Inferior oblique** - Also responsible for the **elevation of the eyelid** through the **levator palpebrae superioris muscle**. **Edinger-Westphal Nucleus** mediates **pupillary constriction** (reducing pupil size in bright light). Controls **accommodation** by adjusting the shape of the lens for near vision. **Target Structures: Extraocular muscles**: For precise eye movements. **Pupil and lens**: For visual focus and light regulation. **Cranial Nerve IV: Trochlear Nerve** **Nucleus Location**: The trochlear nerve originates from its **nucleus**, which is situated in the **midbrain**, just below the inferior colliculus. **Function:** The trochlear nerve is a **motor nerve** that innervates the **superior oblique muscle**, one of the extraocular muscles of the eye. This muscle is responsible for: **1. Depression**: Moving the eye downward, especially when it is in an adducted position (looking toward the nose). **2. Intorsion**: Rotating the eye inward (toward the nose). **Cranial Nerve VI: Abducent Nerve** **Nucleus Location:** The abducens nerve originates from the **abducens nucleus**, which is situated in the **caudal pons**, near the floor of the fourth ventricle. **Function:** The abducent nerve is a **motor nerve** that innervates the **lateral rectus muscle**, an extraocular muscle responsible for **abduction of the eye**: Moving the eyeball outward, away from the midline (lateral movement). **Cranial Nerve V: Trigeminal Nerve** **Function:** The trigeminal nerve is a **mixed nerve** responsible for: 1. **Sensory innervation**: Provides general sensation (touch, pain, temperature) from the face, scalp, oral and nasal cavities, and anterior two-thirds of the tongue. 2. **Motor innervation**: Supplies the **muscles of mastication** (chewing) and other small muscles. **Emergence**: The trigeminal nerve arises from the **ventral surface of the pons**. **Nuclei**: Several nuclei contribute to its formation and span the brainstem: **Mesencephalic nucleus**: Processes proprioceptive input from facial muscles. **Main sensory nucleus**: Processes touch and pressure sensations. **Spinal nucleus**: Processes pain and temperature sensations. **Motor nucleus**: Controls the muscles of mastication. The **trigeminal nerve** is divided into **three** main branches: 1. **Ophthalmic Division (V1)**: Purely sensory. Provides sensation to the forehead, scalp, cornea, upper eyelid, and nose. 2. **Maxillary Division (V2)**: Purely sensory. Provides sensation to the cheeks, upper lip, teeth, nasal cavity, and palate. 3. **Mandibular Division (V3)**: Mixed sensory and motor. **Sensory:** Provides sensation to the lower lip, jaw, lower teeth, and anterior tongue. **Motor:** Controls the muscles of mastication, including the temporalis, masseter, and pterygoid muscles.  **Cranial Nerve VII: Facial Nerve** **Nuclei:** The facial nerve arises from several nuclei in the brainstem, each responsible for specific functions: 1. **Facial Motor Nucleus**: Provides **motor innervation** to the muscles of **facial expression**, allowing for facial movements like smiling, frowning, and blinking. 2. **Nucleus of the Tractus Solitarius (NTS)**: Processes **taste sensations** from the **anterior two-thirds of the tongue** via the chorda tympani branch of the facial nerve. 3. **Superior Salivatory Nucleus** and **Lacrimal Nucleus**: Control **parasympathetic functions** such as Innervating the **submandibular** and **sublingual salivary glands**, stimulating saliva production. Innervating the **lacrimal gland**, stimulating tear production and innervating the **nasal glands** and **glands of the palate**, aiding in mucus and glandular secretions. The facial nerve is a **mixed nerve** with the following roles: **Motor**, **Sensory** and **Parasympathetic**. **Pathway:** Originates from the **brainstem (pons)**. Exits the skull via the **stylomastoid foramen** to reach the face. Branches into several divisions to innervate facial muscles, salivary glands, and glands in the nasal cavity and palate.  **Cranial Nerve VIII: Vestibulocochlear Nerve** **Location:** The vestibulocochlear nerve is situated **lateral** to the facial nerve (**CN VII**) along the **inferior pontine sulcus**, emerging at the junction between the pons and medulla. The nerve consists of **two distinct components**, each serving a specific sensory function: 1. **Vestibular Nerve**: Transmits information about **balance** and **spatial orientation**. Conducts nerve impulses from the **vestibular apparatus** in the inner ear, including the semicircular canals, utricle, and saccule, which detect head position and motion. 2. **Cochlear Nerve**: Transmits signals related to **hearing**. Conducts impulses from the **cochlea**, where sound waves are converted into neural signals. The vestibulocochlear nerve is a **purely sensory nerve**.  **Cranial Nerve IX: The** **Glossopharyngeal Nerve** Sensory, motor and parasympathetic. The **glossopharyngeal nerve** originates from multiple nuclei within the brainstem, each associated with specific functions: 1. **Nucleus Ambiguus**: Provides motor innervation to a **pharyngeal muscle** (stylopharyngeus), which is involved in **swallowing**. 2. **Inferior Salivatory Nucleus**: Supplies **parasympathetic innervation** to the **parotid salivary gland**, enabling salivation. 3. **Nucleus of the Tractus Solitarius (NTS)**: Processes sensory input, including **Taste** from the **posterior one-third** of the tongue. And information from the **carotid sinus** (monitoring blood pressure) and the **carotid body** (monitoring blood composition, like oxygen and carbon dioxide levels). 4. **Spinal Nucleus of CN V**: Handles **general sensation** from the posterior tongue, pharynx, and other associated regions.  **Cranial Nerve X: Vagus Nerve** Mixed nerve with motor, sensory, and parasympathetic functions, originating from various nuclei in the brainstem. **Nucleus Ambiguus**: Provides motor innervation to the muscles of the **pharynx** (involved in swallowing) and the **larynx** (responsible for speech). **Dorsal Nucleus of the Vagus**: Supplies **parasympathetic innervation** to involuntary muscles of the **thoracic and abdominal viscera**. **Nucleus of the Tractus Solitarius (NTS)**: Processes **visceral sensation** (e.g., from the thoracic and abdominal organs) and **taste** from the epiglottis. **Spinal Nucleus of CN V**: Handles **common sensation**, such as pain, temperature, and touch, from areas like the external ear and surrounding regions. **Cranial Nerve XI: Accessory Nerve** The **accessory nerve** is primarily a motor nerve with two distinct roots, each contributing to specific functions: **Cranial Root**: Originates from the **nucleus ambiguus** in the brainstem. **Functions** as part of the Vagus nerve (CN X) to assist in motor control of certain muscles of the pharynx, larynx, and soft palate. **Spinal Root**: Arises from the **spinal nucleus of the accessory nerve** located in the upper cervical spinal cord segments (C1-C5). Exits the spinal cord and ascends into the cranial cavity through the foramen magnum. Joins briefly with the cranial root before separating and exiting the skull via the **jugular foramen**. **Function:** The **spinal root** innervates two key muscles: - **Trapezius Muscle**: Controls shoulder elevation and movement of the scapula. - **Sternocleidomastoid Muscle**: Enables rotation and flexion of the neck.  **Cranial Nerve XII: Hypoglossal Nerve** The **hypoglossal nerve** is a **motor nerve** responsible for controlling the movements of the tongue. **Nucleus**: Arises from the **hypoglossal nucleus** located in the medulla oblongata. **Function**: Innervates all intrinsic and most extrinsic muscles of the tongue, including: **Intrinsic muscles**: Shape the tongue (e.g., for speech and swallowing). **Extrinsic muscles**: Control tongue movements (e.g., protrusion, retraction, and side-to-side movement). The only extrinsic tongue muscle not innervated by CN XII is the **palatoglossus**, which is controlled by the Vagus nerve (CN X).   *Brainstem 2.* **Function of the brainstem:** - Serves as a **conduit** for the ascending and descending tracts to reach different parts of the central nervous system. - Contains important **reflex centres** (e.g. respiration, cardiovascular system and consciousness). - Contains the **nuclei** of cranial nerves III-XII. (3-12). **Number 2 is the Medulla** as **anterior median fissure** (down the middle). **Pyramids** (lateral to the fissure). **Olives** (lateral to the pyramids). **Number 1 is Pons** as you can see the **anterior bulge** (basilar part of the pons). **Number 3 is the Midbrain** evident by the **cerebral peduncle** either side and the **interpeduncular fossa** in the middle as a shallow depression. **[Medulla:]** In a transverse section the myelin sheath is stained dark so white matter is darker.   **Medial Lemniscus:** Located dorsal to the pyramidal tract (blue in the image). Transmits conscious proprioception, vibration sense, and fine touch from the contralateral side of the body, as part of the dorsal column-medial lemniscus pathway. **Spinothalamic Tract:** Located lateral to the medial lemniscus (light blue in the image). Conveys pain, temperature, and crude touch sensations from the contralateral side of the body as part of the anterolateral system. **Inferior Olivary Nucleus:** Found lateral to the pyramidal tract (orange structure in the ventrolateral medulla). A key relay centre involved in motor learning and coordination. Sends signals to the cerebellum through the inferior cerebellar peduncle. **Inferior Cerebellar Peduncle:** Located dorsolaterally (yellow structure in the image). Connects the medulla to the cerebellum. Transmits proprioceptive input and information from the vestibular nuclei to the cerebellum. **4th Ventricle:** The 4th ventricle (centred dorsal cavity in the image) lies dorsally in the rostral medulla. Forms the floor of the ventricle and contains various nuclei involved in autonomic and motor control. **Nucleus of the Solitary Tract (NTS):** Composed of nerve fibres from cranial nerves VII (facial), IX (glossopharyngeal), and X (Vagus). Functions as the primary visceral sensory nucleus, receiving taste inputs as well as visceral afferent signals. **Spinal Trigeminal Tract and Nucleus:** Located laterally in the medulla. Conveys pain, temperature, and crude touch sensations from the ipsilateral face. Fibers terminate in the spinal trigeminal nucleus, which extends caudally from the mid-pons to the cervical spinal cord. **Vestibular Nuclei:** Includes the superior, inferior, medial, and lateral vestibular nuclei (pink structure in the image). These nuclei are critical for maintaining balance and spatial orientation by receiving input from the vestibular apparatus. **Vestibulocochlear Nerve (CN VIII):** Shown emerging dorsally in this image, it transmits auditory and vestibular information to the brainstem. The vestibular portion communicates with the vestibular nuclei, while the cochlear portion synapses in the cochlear nuclei. **Hypoglossal Nucleus:** Typically, ventral and medial to the 4th ventricle in this region. Origin of cranial nerve XII (hypoglossal nerve), which innervates the tongue muscles. **Nucleus Ambiguus (NA):** Contributes motor fibres to cranial nerves IX (glossopharyngeal), X (Vagus), and XI (accessory). Innervates muscles of the pharynx, larynx, and soft palate, essential for swallowing and phonation. **Pyramidal tract**: Is located in the ventral medulla and contains corticospinal and corticobulbar fibres, responsible for voluntary motor control of the contralateral body. The corticospinal tract originates from layer 5 pyramidal-shaped neurons in the cerebral cortex. **Corticobulbar tract:** The corticobulbar tract originates in the cortex and terminates in the lower motor neurons (LMNs) of cranial nerve nuclei, which control the skeletal muscles of the head and neck, including those for facial expression and mastication. **Pyramidal tract/corticospinal tract:**  **Neurons in the Cortex**: Originates from **pyramidal cells** located in **layer V of the cortex**, named for their pyramid-shaped cell bodies. **Pathway through the Medulla**: At the junction between the medulla and spinal cord, most corticospinal fibres **cross the midline** at the **pyramidal decussation** (also called the decussation of the pyramids). **Spinal Cord Level**: Fibers that do not decussate in the medulla will cross the midline at their respective spinal cord levels. **Synapse and Function**: In the spinal cord, corticospinal fibres synapse with **interneurons** (also called interneuronal connections). These interneurons, in turn, synapse with **motor neurons** in the ventral horn of the spinal cord, which innervate skeletal muscles. Responsible for **voluntary motor control** of the body. **Corticobulbar (corticonuclear) tract:** The corticobulbar tract originates in the **cortex.** It terminates in the **lower motor neurons (LMNs)** located in the cranial nerve nuclei. Innervates the **skeletal muscles of the head and neck**, including **Muscles of facial expression** (e.g., via **CN VII**), **Muscles of mastication** (chewing). **Example**: **Cranial Nerve VII (Facial Nerve)**: Controls muscles of **facial expression**.  **Two major sensory pathways that transmit different sensory modalities to the brain:** **Spinothalamic Pathway (Anterolateral System):** **Sensory Modalities**: Pain, temperature, and crude touch. **First Order Neuron**: Located in the **dorsal root ganglion**. Its central process enters the spinal cord and synapses with the **second-order neuron** in the **dorsal horn** of the spinal cord. **Second Order Neuron**: Located in the **spinal cord** (dorsal horn). After synapsing with the first-order neuron, it decussates (crosses over) to the opposite side (contralateral side) at the **level of the spinal cord**. It then ascends in the **spinothalamic tract** to the **thalamus**. **Third Order Neuron**: Located in the **thalamus**. It projects to the **somatosensory cortex** (postcentral gyrus) in the **parietal lobe** of the brain, where sensory information is processed. **Dorsal Column Pathway (Medial Lemniscal Pathway):** **Sensory Modalities**: Discriminative touch, vibration, and conscious proprioception. **First Order Neuron**: Also located in the **dorsal root ganglion**. Its central process enters the spinal cord and ascends **ipsilaterally** (on the same side) in the **dorsal columns** (fasciculus gracilis and fasciculus cuneatus) without crossing the midline initially. **Second Order Neuron**: Located in the **medulla** (nucleus gracilis and nucleus cuneatus). After synapsing with the first-order neuron in the medulla, the axons of the second-order neurons cross (decussate) to the opposite side (contralateral side) at the **medulla** level and ascend in the **medial lemniscus**. **Third Order Neuron**: Located in the **thalamus**. It projects to the **somatosensory cortex** (postcentral gyrus) in the **parietal lobe** of the brain, where the sensory information is interpreted. Key Differences: Modalities, Decussation, Ipsilateral vs. Contralateral Ascension, Location of Second Order Neuron. In **Brown-Sequard syndrome**, which is caused by a hemi section (partial damage) of the spinal cord, you get a distinctive pattern of sensory and motor deficits due to damage to different pathways: 1. **Spinothalamic tract (pain and temperature)**: - **Contralateral loss**: The spinothalamic tract crosses to the opposite side of the body at the spinal cord. So, if the left side of the spinal cord is damaged, you lose pain and temperature sensation on the **right side** of the body below the injury. 2. **Dorsal column/medial lemniscus pathway (touch, vibration, proprioception)**: - **Ipsilateral loss**: The dorsal column pathway carries sensations of fine touch, vibration, and proprioception (sense of body position). This pathway stays on the same side until it reaches the brainstem, so if the left side of the spinal cord is damaged, you lose these sensations on the **left side** of the body below the injury. 3. **Corticospinal tract (motor control)**: - **Ipsilateral weakness or paralysis**: The corticospinal tract controls voluntary muscle movements. Like the dorsal column pathway, it travels down the same side of the spinal cord before crossing at the brainstem. So, damage to the left side of the spinal cord causes **weakness or paralysis** on the **left side** of the body below the level of the lesion. [Pons:] The **cortex** sends motor signals through the **corticopontine fibres** to the **pontine nuclei** located in the **pons**. These fibres synapse with the pontine nuclei, which then send out **transverse pontine fibres**. These fibres cross the **midline** (decussate) and travel to the **contralateral cerebellum** via the **middle cerebellar peduncle**. The **cerebellar peduncles** are pathways that connect the **cerebellum** to the brain, and the middle cerebellar peduncle specifically carries information from the pons to the cerebellum, allowing coordination and fine-tuning of motor movements.  [Midbrain:] Four swellings on the dorsal side of the midbrain: upper two are **superior colliculi**, lower two are **inferior colliculi**. A transverse section through the **inferior colliculi** reveals the **tectum** of the midbrain. Anterior to the tectum lies the **tegmentum**, and anterior to that is the **crus cerebri**. The **substantia nigra** divides the cerebral peduncle into the **tegmentum** and **crus cerebri**. The **superior colliculi** are involved in the visual pathway, specifically visual reflexes. The **inferior colliculi** are part of the auditory pathway. The midbrain contains **ascending and descending tracts**, **cranial nerve nuclei**, and **dopaminergic neurons** (important for motor and reward systems). The **cerebral aqueduct** is found only at the midbrain level, connecting the third and fourth ventricles. **Hydrocephalus:** The **cerebral aqueduct** is a common site of **blockage** (tumor), leading to hydrocephalus where the CFS cannot flow down through the cerebral aqueduct then there's a buildup of CFS above the blockage then the ventricle above it gets expanded putting pressure on the brain. If the midbrain is cut at the level of the **superior colliculus** , the dark area is the **crus cerebri**, the middle 2/3^rd^ of it is where the **pyramidal tract** is found. Behind that is the **substantia nigra**. Then the **red nucleus** behind that only find in the superior colliculi level. Lateral to that is the **medial lemniscus** and **spinothalamic tract**. Also, the cerebral aqueduct and surrounded by the **periaqueductal grey (PAG).** Ventral to that is the **oculomotor nucleus**. Also have the **superior colliculus**. The red nucleus is pink in fresh specimen as the neurons contains iron. The oculomotor nerve emerges from the interpeduncular fossa.  **Dopaminergic neurones:** The substantia nigra appears darker under normal conditions due to the presence of a pigment called neuromelanin, which is found in dopaminergic neurons. These neurons are involved in motor function and play an important role in motivation and reward. Many drugs of abuse affect this area. In Parkinson's disease, the substantia nigra becomes lighter because the dopaminergic neurons degenerate, leading to a decrease in neuromelanin production, which reduces the darkness of the region. **Stroke:** Two types: **Haemorrhagic**- rupture of blood vessel, bleeding and **Ischemic**- Blockage of blood flow to brain.  **Reticular Formation:** **The term \"reticular\" refers to a net-like structure, as it consists of a network of nerve cells and fibres scattered throughout the centre of the brainstem.** **Functions** of the reticular formation: Regulates arousal and wakefulness, maintaining consciousness and attention. Plays a role in the sleep-wake cycle, including stages of sleep. Motor control: Involved in the coordination of movement, posture, and muscle tone. Modulates sensory input, filtering information to prevent overload and ensure focus. Influences autonomic functions, such as heart rate, blood pressure, and respiration. **Major nuclei of the reticular formation include:** - **Raphe nuclei:** These nuclei primarily release serotonin, influencing mood, sleep, and arousal. - **Cholinergic reticular nuclei:** Involved in the release of acetylcholine, important for alertness and sleep-wake cycles. - **Noradrenergic neurons (Locus coeruleus):** These neurons release norepinephrine and are involved in regulating attention, stress response, and overall alertness. **Raphe nuclei:** Groups of neurons located either in or adjacent to the midline (raphe) of the brainstem. They are particularly active during deep sleep. Responsible for synthesizing and secreting serotonin (5-HT), which plays a crucial role in regulating mood, sleep, and other central nervous system functions.  **RAS** is a key part of the reticular formation located in the brainstem that plays a crucial role in regulating arousal, wakefulness, and alertness. **Location**: Found within the brainstem, particularly in the midbrain, pons, and medulla, as part of the reticular formation. **Function**: The RAS filters sensory input and determines what information is important to bring to conscious attention. It is essential for maintaining wakefulness, alertness, and focus. **Connection**: It connects to various parts of the brain, including the thalamus, cortex, and spinal cord. **Role in Sleep-Wake Cycle**: The RAS is vital for the transition between sleep and wakefulness and for sustaining a state of consciousness. **Damage to RAS**: Can result in reduced alertness, drowsiness, or even coma. Brainstem compression and coma occur when increased pressure or mass effect in the brain compresses the brainstem, which is critical for maintaining consciousness, autonomic functions, and motor control. The brainstem houses vital structures, including the reticular activating system (RAS), cranial nerve nuclei, and centres for regulating heart rate, respiration, and consciousness. Causes of brainstem compression include trauma, tumours and stroke. Compression of the brainstem can disrupt the RAS, leading to a loss of arousal and consciousness, resulting in a coma.