Forebrain - Anatomy & Physiology

Introduction

The forebrain (proencephalon) is the largest part of the brain, most of which is cerebrum. Other important structures found in the forebrain include the thalamus , the hypothalamus and the limbic system. The cerebrum is divided into two cerebral hemispheres connected by a mass of white matter known as the corpus callosum. Each hemisphere is split into four lobes; the frontal, parietal, occipital and temporal lobes. The surface of each hemisphere is made up of grey matter known as the cerebral cortex and is folded to increase the surface area available within the skull. The cortex has roles within perception, memory and all higher thought processes. Inside the cortex is the white matter, within which are a number of nuclei (grey matter), known as the basal nuclei. The basal nuclei receive information from the cortex to regulate skeletal movement and other higher motor functions.

The thalamus functions to relay sensory information to the cerebral cortex and the hypothalamus, regulating visceral functions including temperature, reproductive functions, eating, sleeping and the display of emotion. The limbic system describes a collection of structures within the forebrain, including the amygdala hippocampus, also known as the 'emotional brain', the limbic system is important in the formation of memories and in making decisions and learning.

Forebrain Structure and Function

Brain Region	Structure	Function
Diencephalon	Thalamus	Organising sensory information
Diencephalon	Hypothalamus	Endocrine System, Thermoregulation
Diencephalon	Pituitary	Endocrine System
Telencephalon	Cerebral Cortex	Conciousness, language etc
Telencephalon	Limbic System	Memory, motivation, emotion
Telencephalon	Olfactory bulb	Smell

Thalamus

The thalamus has many functions including; processes and relaying sensory information selectively to various parts of the cerebral cortex, translating signals to the cerebral cortex from lower centres including auditory, somatic, visceral, gustatory and visual systems and also regulating states of sleep and wakefulness. The thalamus plays a major role in regulating arousal, levels of conciousness and levels of activity.

Hypothalamus

The function of the hypothalamus is mainly related to the overall regulation of the Endocrine System. The hypothalamus is closely related to the pituitary gland, controlling a large proportion of the activity go the pituitary gland. For a more detailed analysis of the function of this part of the brain, please use the link: Hypothalamus Anatomy and Physiology.

Pituitary

The function of the pituitary is mainly related to the production of hormones as part of the Endocrine System. For further information on the pituitary gland please use this link: Pituitary Gland Anatomy and Physiology.

Cerebral Cortex

The cerebral cortex is essential for memory, attention, awareness, thought, language and consciousness. There outer layers of the cerebrum are made up of grey matter. Grey matter is formed by neurons and their unmyelinated fibres. The white matter below the grey matter of the cortex is formed predominantly by myelinated axons (myelin is white in appearance). The surface of the cerebral cortex is folded in mammals; more than two thirds of the surface is within the grooves or "sulci". The cerebral cortex is connected to structures such as the thalamus and the basal ganglia, sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. The cortex is commonly described as comprising of three parts; sensory, motor and association areas.

Sensory areas

Homonculus (Human), btarski, 2006, WikiMedia Commons

The sensory areas are the areas that receive and process information from the senses. Inputs from the thalamus are called primary sensory areas. Vision, hearing, and touch are processed by the primary visual cortex, primary auditory cortex and primary somatosensory cortex. The two hemispheres of the cerebral cortex receive information from the opposite (contralateral) side of the body. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation. The association areas of the brain function to produce a perception of the world enabling an animal to interact with their environment effectively.

There are a number of anatomical areas of the brain responsible for organising this sensory information. The parietal lobe is located within the dorsocaudal aspect of the cortex. The temporal lobes are located laterally and the occipital lobes are located in the caudal most aspect of the cortex. The frontal lobe or prefrontal association complex is involved in planning actions and movement.

Motor Cortex

The motor cortex areas of the brain are located in both hemispheres of the cortex and are shaped like a pair of headphones stretching from ear to ear. The motor areas are related to controlling voluntary movements, especially fine movements. There are two main types of connection between the motor cortex and motor neurones found in the ventral horn of the spinal cord; the Pyramidal tracts and the Extrapyramidal tracts.

Pyramidal tract connections are direct with no synapses in the brain stem. Axons pass through the ventral aspect of the medulla oblongata. The extrapyramidal tracts pass through the medulla oblongata outside the ventral pyramidal tracts and have synapses within the brain stem nuclei. These synapses make it possible for signals travelling down the extrapyramidal horns to be influenced by other areas of the brain including the cerebrum.

The pyramidal tracts are responsible for aspects of fine motor skills that require a degree of conscious thought and concentration. The extrapyramidal tracts are generally responsible for activation of larger muscle groups and often work in a coordinated manner to achieve smooth synchronous movements.

Limbic System

The Limbic system is made up of parts of the brain bordering the corpus collosum. The Limbic system contains areas of cerebral cortex, the cingulate gyrus (dorsally), the parahippocampus gyrus (ventrally), the amygdala, parts of the hypothalamus (mamillary body) and the hippocampus. The Limbic system is principally responsible for emotions and the various types of emotion can affect the activity of the Autonomic Nervous System, facilitated by the hypothalamus. For example, anger can lead to increased heart rate and blood pressure.

Olfactory Bulb

Ventral Brain with Olfactory bulbs rostral (Human), Grays Anatomy, 1918

The olfactory bulb is responsible for olfaction and the bulb itself is located within the rostral forebrain area, supported by the cribiform plate and the ethmoid bone. The olfactory nerves are connected to directly to the limbic system which is unique amoing mammalian sensory organs. As a result, olfaction plays a central role and is particularly important in regulating/stimmulating sexual behaviour in many species.