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

	Structure	Function
Forebrain (Proencephalon)	Diencephalon - Thalamus, Hypothalamus, Pituitary Telencephalon - Cerebral Cortex Limbic System Olfactory bulb	Thalamus - Organising sensory information Hypothalamus - Endocrine System, Thermoregulation Cerebral Cortex - Conciousness, language etc Limbic System - Memory, motivation, emotion Olfactory bulb - Smell

Thalamus

The thalamus has many functions:

• Processes and relays sensory information selectively to various parts of the cerebral cortex.

• Translates signals to the cerebral cortex from lower centres including auditory, somatic, visceral, gustatory and visual systems.

• Regulates states of sleep and wakefulness.

• The thalamus plays a major role in regulating arousal, the level of conciousness, and activity.

Hypothalamus

The function of the hypothalamus is mainly endocrine.

See Hypothalamus.

Pituitary

The function of the hypothalamus is mainly endocrine.

See Pituitary.

Cerebral Cortex

• Essential for memory, attention, awareness, thought, language, and consciousness.

• Outer layer of the cerebrum is grey matter.

• Grey matter is formed by neurons and their unmyelinated fibres, (white matter below the grey matter of the cortex is formed predominantly by myelinated axons as 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 three parts: sensory, motor, and association areas:

Sensory areas

• The sensory areas are the areas that receive and process information from the senses; INSERT HERE.

• 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 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.

Association areas

• Association areas function is to produce a perception of the world, so an animal can interact effectively.

• The parietal, temporal, and occipital lobe - all located in the posterior part of the cortex - organize sensory information.

• The frontal lobe or prefrontal association complex is involved in planning actions and movement.

Motor areas

• The motor areas are located in both hemispheres of the cortex.

• They are shaped like a pair of headphones stretching from ear to ear.

• The motor areas are related to the controlling voluntary movements, especially fine fragmented movements.

Species Differences

• Other than size the brain of most species is fairly consistant.