Neurophysiology Primer : part 2

Structure of the brain

The Central nervous System (CNS) is generally divided into 3 different sections. The periphery, the spinal cord and the brain itself.

The periphery

The periphery is made up of all the nerve endings and sensory neves that are where information begins and ends in the body. If information is gathered here or this is the destination of a message to do something, that is likely to be a part of the periphery. The many sections of the periphery are complex and widely varied and beyond the scope of this primer.

The spinal cord

The spinal cord communicates between the periphery and the brain. Sensory nerves enter it, bringing information to the brain about our world. Motor nerves exit carrying messages to the muscles and organs informing them how to operate.

In cross section, the central part of the spinal cord is vaguely h-shaped. The central h is known as grey matter and is composed mainly of un-myelinated inter-neurons, cell bodies and dendrites all tightly packed. The surrounding area is white matter that is composed mostly of myelinated axons (myelin is white). This is where information travels up to the brain. Each segment of spinal cord communicates with a particuluar section of the body and with the segments directly above and below it.

An important part of the spinal cord is what is known as the reflex arc. Sensory nerves enter the spinal cord and synapse with small interneurons within. These synapse with more interneurons and with exiting motor neurons. It is here that reflexes are stored, creating a short loop direct from sensation to action completely bypassing the brain.

The brain

The brain itself consists of 3 major subdivisions: the hindbrain, the midbrain and the forebrain. I`ll briefly go through the basic functions associated with each minor structure (the full amount is huge and really deserves further looking into if you are interested).

The hindbrain

The hindbrain (underneath and behind the larger parts of the brain) consists of the medulla, pons and cerubellum. The medulla, pons, midbrain and certainm structures of the forebrain are also collectively known as the brain stem.

Medulla: Controls basic reflexes such as breathing and heart rate so damage to this area is invariably fatal. Because of the types of functions of the medulla, large doses of drugs that effect this area can also be extremely harmful. This is what happens when a person overdoses

Pons: French for bridge, this structure serves as the gateway for sensory nerves that cross from the left side of the body to the right side of the brain (and vice versa). It also contains nuclei that are centres for the integration of sensory information and often regulate motor output.

Cerebellum: This is best known for control of learned movements or classically conditioned responses (when you learn to ride a bike, the processes are stored here). All your programmed stuff goes here, so when you drive your car `on-autopilot` this is the part you`ve put in charge

The midbrain

The midbrain mainly contains the superior and inferior colliculus. These play an important part of the routes for sensory information. The superior colliculus is active in vision and visuo-motor coordination. The inferior colliculus deals with auditory information.

The forebrain

The major part of the forebrain is the cortex. This is the biggest section of the brain and is what most people think of when you say the word `brain`. Hidden underneath it lie other forbrain structures including the thalamus, the basal ganglia, the limbic system. The cortex is quite complex and deserves its own subsection below.

Thalamus: This is main source of sensory input to the cerebral cortex. It acts as a waystation for sensation, but also does a significant amount of processing on that information before it hands it on to the appropriate lobes of the cortex for final evaluation.

Basal ganglia: This contributes information to the cortex regarding movement, including speech and other complex behaviours.

Limbic system

This is a heavily linked set of structures controlling motivated and emotional behaviours (eg eating, drinking, sexual behaviour and aggression). The list below is a partial list of the limbic systems main structures.

Olfactory bulb: This is where we process the sense of smell. As we know, research has been carried out on the effect of smell on sexual behaviour through the use of pheromones. It occurs in the animal knigdom so we wonder if it affects us also.

Hypothalamus: Deals with regulation of motivational behaviours. It also regulates hormones by regulating the pituitary through both nerves and through hormones that it releases.

Pituitary gland: This is an endocrine gland (hormone producing) attached to the hypothalamus. It recieves messages from the hypthalamus then releases hormones into the blood stream. It controls timing and amount of hormone secretion for the rest of the body.

Hippocampus: Plays a vital role in learning and memory.

The cerebral cortex.

This is the biggest structure of the brain. It controls a wide variety of complex functions and behaviours, from sensation to actions to personality. The brain is split in two down the middle, each half known as a hemisphere. Generally, the right hemisphere deals with sensations and motor instructions for the left side of the body and vice versa.

The cortex is further split into 4 main lobes, each hemisphere has one of the lobes but usually deals with that type of information from the opposite side of the body. There are notable exceptions, for example, the language centres seem to be split into a section dealing with the words and a section that deals mainly with the emotion put in via intonation.

The frontal lobe

This is generally thought to be the seat of personality. Movements are planned here and behaviours are modified.

A large strip right at the back of the frontal lobe reaches around the head like a headband. This is known as the precentral gyrus and also as the primary motor cortex. It is the centre for control of fine movements of the body (for example precisely moving ones fingers). The precentral gyrus contains a detailed map of the body stretched over the surface of the lobe (see diagram below).

The Parietal Lobe

Located just behind the Frontal lobe, this area specialises in body information. Its expertise includes touch and muscle and joint receptors.

At the very front of this lobe is the post-central gyrus. This sits directly behind the precentral gyrus and contains a very similar body map. The function of the post-central gyrus, however, is to recieve sensations from each of the body areas.

The occipital lobe

Located right at the back of the head, the main function of this lobe is to process the information from our eyes. This is the largest area devoted to one small sense as we devote much of our attention to our vision - we rely on it more than any other sense.

An interesting thing about our occipital lobe is that if we are damaged here, we can no longer see. Even if our eyes are still fully functional. An interesting pheonomenon that shows this is called blindsight. If you shine a spot of light on a dark wall, a person with blindsight cannot see it. BUT, if you ask them to guess where they think it might be, they will point to it with disturbing accuracy. Obviouls, information is still coming into the brain from the eyes, it just isn`t being processed any more as a visual stimulus.

The temporal lobe

The temporal lobe is located around the sides of the brain, near the temples. It is the main centre for auditory information but seems to add something to reconition of complex visual images as well (such as recognition of faces). The temporal lobe also houses the two areas that deal with the understanding of language.

References:

Kalat, JW Biological psychology [5th Ed]
Brookes/Cole publishing Co., Pacific Grove California, USA, 1995