🎯 Objectives
The main objective of this lesson is to study:
- The Brain and the Peripheral systems 🧠🔌
- Brain: Forebrain 🔝, Mid brain 🔄, Hind Brain 🔙 functioning of each anatomical location in the CNS. Starting from the posterior located areas up to the anterior-most 📍
- Cerebral Cortex 🧠: its layers 📚, Corpus Callosum 🤝 and the two hemispheres 🌐
📚 Main Purpose
Students will become familiarized with the location of various brain areas 📍, their functions ⚙️ and their relationship to other areas 🔗. The students would also understand how control and modulation of behavior related to each neuroanatomical site takes place 🎮.
🧬 Basic Neuroanatomy Assumptions
Basic Neuroanatomy is based on the following assumptions:
- The cortex is newly evolved 🆕 as compared to the other structures, and it controls all major functions (especially in the higher animals 🦧)
- The higher areas cover the lower areas 📐 as they have grown over these areas. The lower brain areas have to send information to the higher areas and then the commands for actions come back from these areas (the thalamus send all messages it receives to the relevant cortices and receives commands 📡)
- The lower underlying areas are more primitive 🦴 in terms of functions, their functions are more survival functions (the brain stem areas 🏥)
- These lower areas are similar in locations, site and functions in lower and higher order animals (e.g., the hypothalamus, the thalamus) 🐾
Phylogenetically, encephalization (control of the higher brain areas 🧠) has occurred leading to increased ability to interact and respond to the environment (greater adaptability 🌍). The greater the encephalization, the more the functions are controlled by the higher brain areas, therefore if these areas are damaged the animals cannot function at a normal level ⚠️.
🧠 Brain Structure Organization
The following would elaborate how the brain is located with reference to the spinal cord 🦴 and how other areas within the brain and the top of the spinal cord:
Brain 🧠 ————————————————— Spinal Cord 🦴
🔹 Three Major Divisions of the Brain
- Forebrain 🔝: Comprises of the Telencephalon (cortical and some subcortical structures) and Diencephalon (the Thalamus and the hypothalamus)
- Midbrain 🔄: Consists of older structures, the Mesencephalon
- Hindbrain 🔙: The posterior most part of the brain, which has two sections the Metencephalon and the Myelencephalon
🧠 Detailed Brain Areas
We will elaborate these areas further moving from the frontal and dorsal areas to the more caudal ventral areas 📐. The anterior most and the dorsal area is the Telencephalon 🔝, with Myelencephalon at the caudal most end 🔙.
🔝 Telencephalon (Forebrain)
Comprises of the Cerebral Cortex 🧠, the Corpus Striatum 💎, Cingulate Gyrus 🔄, Septum 📊, Amygdala 🔺, Hippocampus 🌊 (these last three are also part of the limbic system 💚). Therefore, this is cerebral cortex which is phylogenetically a newer structure 🆕, and some older primitive structures which underlie the cerebral cortex 🦴.
🎯 Diencephalon
Comprises of the Thalamus which is the junction and the gateway for all sensory-motor and association pathways 🚪📡 and the Hypothalamus which is the primitive survival center 🏥❤️.
🔄 Mesencephalon (Midbrain)
Has the Tectum which comprises of the Inferior (for auditory systems 👂) and the Superior colliculi (for visual system 👁️), Tegmentum ⚙️, cerebral peduncles 🔌.
🔙 Metencephalon
Comprises of the Cerebellum 🏀 which lies dorsally over the Pons 🌉, and the Pons.
🦴 Myelencephalon
Comprises of the Medulla Oblongata 🔌.
Brain stem: includes the Mesencephalon, Metencephalon and the Myelencephalon i.e., technically everything between spinal cord and cerebral cortex 🧠🦴.
🦴 Myelencephalon: Medulla Oblongata
Medulla Oblongata (Oblong and narrow structure 📏) is continuation of spinal cord and the caudal part of the brain stem 🔙. Another important structure which is located in the Brain stem are is the reticular formation 🕸️.
Medulla contains all the ascending ⬆️ and descending ⬇️ fiber pathways connecting the brain 🧠 and the spinal cord 🦴. It also has nuclei for the cranial nerves especially related to the control of respiration 💨; heart ❤️ and digestive activity 🍽️ enter the brain at the level of medulla.
Cranial nerves for tongue 👅, larynx 🗣️, pharynx, ear 👂, vestibular ⚖️, involved in control of breathing while sleeping (sleep apnea) 😴, sneezing 🤧, swallowing, vomiting 🤢 are also found here.
🧬 Cranial Nerves
| Nerve | Function | Cranial Number |
|---|---|---|
| Hypoglossal nerve 👅 | Tongue | Cranial Nerve XII |
| Accessory 🗣️ | Larynx, voice, mouth mastoid | Cranial nerve XI |
| Vagus 👂 | Taste, larynx, pharynx, ear | Cranial Nerve X |
| Glossopharyngeal 😋 | Taste, posterior part of the Tongue | Cranial IX |
| Vestibulocochlear ⚖️ | Ear and vestibular | Cranial VIII |
🤧 Sneezing Center
When nasal mucosa is irritated 😤. The sneezing center is located in the medial part of medulla (it also involves the trigeminal nerve).
🤢 Vomiting Center
Vomiting is found in the Medullar Reticular Formation.
🕸️ Reticular Formation
This is a Criss cross of nerve fibers 🔀, nuclei and cell bodies (like a net 🕸️ (reticulum means: the net). It extends from the spinal cord to the Thalamus 🦴🧠. It is a phylogenetically older structure 🦴. It is a well-organized area with both efferent and afferent ascending ⬆️ and descending ⬇️ fibers.
⬆️ Ascending Fibers
Go to thalamus 🎯, cortex 🧠, and are involved in sleep 😴 and awakening ⏰. Raphe Nuclei produces serotonin 🧪 and extends from the lower pons, and medulla. Analgesia (pain relief 💊) is produced by the electrical stimulation of the Periaqueductal gray (PAG) ⚡, is through the Raphe nuclei. It gets input from cerebral cortex 🧠, the cerebellum ⚖️ and output to cerebellum and the spinal cord 🦴.
🔋 The Reticular Activating System (RAS)
RAS which is important in arousal ⏰. Sleep 😴, arousal ⏰, muscle tone 💪, alertness ⚡ was found to be located here by experiments of Moruzzi and Magoun (1949) on cats 🐱. They stimulated the raphe Nuclei of a sleeping cat which led to an arousal EEG response (low voltage, high frequency theta response) 📊, it woke up ⏰, and when the awake cat was stimulated and it became more alert ⚡. However, when this area was lesioned animals went into a comatose stuporous state 😴💀. Destruction of the Raphe Nuclei led to Insomniac cats 😵 thus it was found that the RAS is important in sleep and attention 💡.
🌉 Metencephalon
Has many tracts going up and down ⬆️⬇️, and contains two important brain areas the Cerebellum 🏀 and the Pons 🌉.
🏀 Cerebellum
Lies over Pons 🌉, it is a phylogenetically older structure 🦴, for sensory motor coordination 🤸, movement 🏃 and balance ⚖️. It has two lobes like the CC with a large number of lobules separated by fissures (vermis, smaller grooves) with a large number of convolutions 🌀, but these are similar all over (unlike the cerebral cortex in which there is a difference). The nerve cell layer is 2mm thick below which lies the white matter ⚪ under which lies the cerebellar nuclei 🧬.
Cerebellum receives inputs from the vestibular system ⚖️, auditory 👂 and visual system 👁️, reticular formation 🕸️, and various regions of the cerebral cortex 🧠. It sends out fibers to the reticular formation, thalamus and the vestibular system 📡.
⚠️ Damage to the Cerebellum
Damage to the cerebellum leads to disturbances of gait 🚶 in all regions. If one lobe is damaged then there is postural imbalance ⚖️❌. If both cerebellar poles are damaged there is tendency to fall backwards ⬅️ because of the inability to maintain upright postures and gait disturbance with damage to the posterior region.
🚨 Ataxia in Humans
In Humans, Ataxia, characterized by unsteadiness of gait is seen 🚶❌, this is with anterior lobe syndrome: with the following symptoms:
- Abnormalities in force, rate, and direction of rapid movements ⚡❌ a) overshooting while reaching targets 🎯❌ b) jerky not smooth movements 🌊❌. Damage to the cerebellum impairs standing 🧍, walking 🚶 and performance of coordinated movements 🤸
- Birds 🦅 and reptiles 🦎 have large cerebellums? Why?? To maintain balance during flight 🕊️ and since sensory–motor fibers are coming and going out to various areas, coordination of smooth movements takes place in this area 🎯.
👁️ Oculomotor Signs of Cerebellar Damage
Nystagmus (rhythmic and involuntary oscillatory movements of the eyes 👁️🔄)
Cerebellectomy (removal of the cerebellum ✂️🏀) lead to inability to maintain gaze 👁️❌, defective smooth pursuit movements 🔄❌, difficulty of fixation 📍❌.
🧬 Cerebellar Neurons
- Purkinje cells (long dendritic trees 🌳)
- Basket cells (short axons, dendrites cover Purkinje cells 🧺)
- Stellate Cells (axons terminate on Purkinje cells ⭐)
- Granule cells (smallest, go vertical for communication ⬆️)
🌉 Pons
Pons: Bridge 🌉 between the medulla, midbrain, and the cerebellum. The medulla is caudally located 🔙 with reference to Pons and the midbrain is rostrally located 🔝. The Dorsal surface of PONS is covered by the cerebellum 🏀.
Large rounded pontine nuclei contain ascending ⬆️ and descending ⬇️ fiber bundles. One such bundle connects the brain stem and the cerebellum and contains the Pyramidal fibers from Cerebral cortex to the spinal cord (part of the cortico-spinal tract) 🔌. The Trigeminal cranial nerve enters and leaves the brain at the level of Pons. Further the Cranial nerves for feeding 🍽️, facial expression 😊, respiratory nuclei 💨, relay nerves for auditory systems 👂 are also located here.
The cortico-ponto-cerebellar tract is the largest group of fibers which originate from a wide area of the cerebral cortex (has over 19 million fibers on each side) 🧠➡️. These rises from the primary cortical areas are involved in rapid correction of movements ⚡🏃.
😴 Pontine Nuclei and Sleep
PONTINE NUCLEI, THE GIANT PONTINE CELLS play an important role in rapid eye movement sleep (REM sleep) 😴👁️ by inhibiting the movement of body muscles to prevent damage which could occur due to activation of the body (as the brain is active 🧠⚡).
🔄 Mesencephalon (Midbrain)
Mesencephalon is a small portion between the hind brain and the diencephalons 🔙🔝- the anterior section of the brain stem, it is tubular in form 📏. It has three main areas: tectum, tegmentum, and basis pedunculi.
🎯 Tectum
Tectum comprises of two pairs of relay nuclei 🔁, which look like 4 little lumps on the surface of the brain stem 🔴🔴. These are the Superior Colliculi and the Inferior Colliculi.
👁️ Superior Colliculi
Superior Colliculi: laminated grey and white matter important in visual reflexes 👁️⚡ and eye movement 👀, well defined and organized in terms of receptive fields and maps of visual space 🗺️. It is part of the pathway coming from the optic tract to the visual cortex for eye movement and gross spatial localization 📍. This is important for vertical gaze ⬆️⬇️ and pupillary reflexes 👁️. It receives inputs from retina of eyes 👁️, thalamus 🎯, and inferior colliculus 👂 and sends outputs to thalamus and frontal and visual cortices 🧠.
👂 Inferior Colliculi
Inferior colliculi: it is an oval mass of small and medium sized neurons and is major relay nuclei for the auditory pathway 👂🔊. Fibers come in from the Thalamus (Medial Geniculate Nuclei 🎯), auditory cortex 🧠 and cerebellar cortex 🏀, and fibers from Inferior colliculi project to the MGN, superior Colliculi 👁️ and the cerebellum 🏀… turn to look at sound reflex 👂🔄 and localizing the source of sound 📍🔊. Therefore, both the inferior and superior colliculi work together as part of the reflex system which take care of attending and turning towards the direction of a sound 🎯🔊 (This is why we see extensive connection with the thalamus, the cerebellum and the relevant cortices 🔗).
We would be continuing discussion on location of various brain areas 📍 and their connections 🔗 and functions ⚙️ in the next class.
📝 Practical Work
Draw five diagrams of major areas of brain 🎨🧠
📚 References
- Kalat, J.W. (1998). Biological Psychology. Brooks/Cole Publishing Company.
- Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
- Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
- Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
- Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
- Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York