The body has two major message networks – the nervous system and the endocrine system – which provide the main links between the brain and body. Understanding how these systems work is key to understanding how the mind affects the health of the body.
The nervous system, with its control center, the brain, produces all our conscious acts and thoughts, as well as maintaining our unconscious body operations. It keeps the heart beating and the digestion functioning; it prompts us to breathe, sleep, wake, and eat; it enables us to walk; and it brings us sights, sounds, and sensations. The endocrine system distributes the body’s hormones, controlled by the hypothalamus and the pituitary gland at the base of the brain, with far-reaching effects on our health.
A two-part structure
The nervous system consists of two interconnected parts: the central nervous system (C N S), made up of the brain and spinal cord, and the peripheral nervous system (P N S), a network of sensory and motor nerves that stretches throughout the body. Within the CNS, the brain has a dense network of some 100 billion nerve cells or neurons. Most of these nerve cells communicate only with each other, carrying out the activity required to process thoughts, sensations, perceptions, and the unconscious functions that underlie them. The nerve cells form two-way communication pathways that keep each part of the brain informed about what is happening in the other parts.
The Developing Brain- From Conception to Childhood
Up to Birth
- Seven weeks after Conception, the brain’s main structures- the hindbrain, cerebellum, midbrain, and forebrain – are clearly visible. The primitive structures at the base of the human brain develop first, reflecting the order in which different parts of the brain evolved.
- After three months, the spinal cord has formed, and the cerebellum and cerebral cortex are well-developed but still smooth.
- By six months, the cerebral cortex has outgrown the lower regions of the brain and starts to form the characteristic wrinkles on its surface. These ridges and furrows are known as gyri and sulci.
- At birth, a baby’s brain contains as many nerve cells as it will have as an adult but has relatively few connections between them. It looks similar to an adult’s brain, but the cortex is still smoother.
BIRTH ONWARDS
- From birth to three years of age, the brain develops neural connections at extraordinary speed.
- By the age of six, there are more connections between cells than there are in adulthood; unused neural connections then begin to die back.
How information gets to the brain
A great deal of sensory information enters the brain via 12 pairs of cranial nerves in the head. These carry messages directly to and from the eyes, ears, taste buds, and nose. The cranial nerves also control muscles in the face, neck, and shoulders. Information to and from all the other parts of the body is carried by the peripheral nerves, which enter and leave the brain via the spinal cord.
Nerve signals travel along several major ‘ up’ and ‘ down ‘ pathways in the spinal cord. Some of the up pathways carry sensory messages on body position and posture to the brain, while others carry information about pain, basic touch, and temperature. Down pathways include those that carry the brain’s instructions for different types of movement, from fine movements such as delicate manipulations of the finger to large movements like turning or bending. The brain gathers information continuously from sensory receptors at the end of each nerve fiber.
The thickness of the peripheral nerve fibers dictates how fast they can conduct information. The thickest and fastest est (motor fibers) connect to muscles and tendons, while the thinnest and slowest convey information about digestion and external temperature. Pain-sensing nerve fibers are thinner and slower than motor fibers, which explains why you pull your hand away from something hot before you are consciously aware of feeling pain.
The autonomic nervous system
Included in the peripheral nervous system is the autonomic nervous system (ANS), whose main role is to keep internal organs, glands, and muscles working appropriately; for example, the ANS ensures that the heart rate is low when the body is at rest and speeds up during exertion. Most of the time, we are unaware of the continuous, subtle changes produced by the ANS. However, if we are suddenly plunged into a new or challenging situation, we immediately become conscious of dramatic bodily changes, which register as feelings of fear, anger, or anxiety.
The most familiar example of this is the ‘fight or flight response to fear. Fear stimulates the amygdala in the brain’s limbic system, which triggers the neighboring hypothalamus to send signals to endocrine glands all over the body. The glands release chemicals that activate the nervous system, which in turn speeds up the heartbeat, opens sweat glands, and constricts some small blood vessels, typically draining the face of color. Information about these events is then fed back to the brain, producing a cycle that may continue, with the sensation of fear building each time. The situation is usually resolved either by action or by the thinking parts of the brain sending signals to the amygdala to quieten its activity.
Neurons, axons, and dendrites
All the activity in the nervous system relies on neurons. There are dozens of different types of neurons, but all have extensions – long, finger-like nerve fibers called axons and short, branching projections from the cell body called dendrites – that enable them to make contact with other neurons.
Contact between nerve cells takes place at ‘docking points on the surface of the dendrites, where axons from other cells connect with them. Most nerve cells have one axon, but often many dendrites and thousands of docking points. Axons actively seek out dendrites to connect with, growing towards their goal. Axons send out signals, and dendrites receive them. A signal consists of a wave of electrical charge that starts at the cell body and travels along the axon. So that the charge does not dissipate along the way, axons are encased in an insulating covering called the myelin sheath. If this covering breaks down – as occurs in multiple sclerosis – the signals are interrupted, and the nerves cannot convey correct instructions to the body. Each neuron works like a tiny computer, taking in and comparing signals from many other neurons before deciding whether and at what strength to ‘fire’ and send the signal on. Usually, they give priority to signals coming in from neurons with a history of sending ‘ reliable’ information.
STRUCTURE OF A NEURON
There are many different types of neurons (nerve cells), but they all have the same elements: a cell body, an axon, and several dendrites.
- Synapse: Docking point where an axon from another cell meets a dendrite
- Cell body: Contains the cell’s nucleus
- Myelin sheath: Surrounds the axon, speeding up the transmission of signals
- Axon: Carries information from the cell body to other neurons
- Dendrite: Extension of nerve cell body; receives signals from other nerve cells
- Neurotransmitters: Convey signals across the synaptic gap to another nerve cell
- Tip of axon: Contains ‘packets’ of neurotransmitters
Chemical messengers – neurotransmitters and hormones
An axon meets a dendrite at the synaptic gap. In most cases, signals are transmitted across the gap by chemical messengers called neurotransmitters contained in the tip of the axon. When a cell fires, neurotransmitter molecules are released into the synaptic gap and attach to receptors on the neighboring dendrite. If there are enough of them, they trigger a signal in the body of the receiving cell that travels along its axon to repeat the process. In this way, a signal can travel through a huge network of cells, forming a neural firing pattern that may represent a thought, a feeling, or a perception. Some neurotransmitters, however, have a ‘closing-down effect on neurons: GABA (gamma-aminobutyric acid), for example, pre vents neighboring cells from firing and stops other areas of the brain from becoming active, so it produces a quietening effect on the body. Tranquilizers and sleeping pills work by stimulating the neurons that produce CABA.
Neurotransmitters are not the only chemical messengers in the nervous system. The hypothalamus regulates the ebb and flow of hormones that stimulate growth, sexual development, and egg or sperm production, and this important area of the brain is also a link between the nervous system and the endocrine system. Hormones released by the hypothalamus trigger the pituitary gland to release its hormones, which travel to the ovaries and prompt them to produce estrogen or to the testes to prompt testosterone production. Most hormonal systems are circular: what happens in a gland is transmitted back to the brain, where it has a further effect on the hypothalamus, which in turn regulates the gland. So, although the endocrine system is often described in textbooks as separate from the nervous system, in reality, the two systems are interdependent.
NEUROTRANSMITTERS
The brain’s main neurotransmitters are:
- Acetylcholine controls activity in areas concerned with attention, learning, and memory.
- Dopamine activates cells involved in motivation and pleasure. In Parkinson’s disease, there is a loss of dopamine cells in the motor area of the brain.
- Encephalins and endorphins: natural opioids that reduce pain and stress.
- Gamma-aminobutyric acid (GABA) inhibits brain activity and has a sedating effect.
- Glutamate: the ‘workhorse’ chemical that keeps the brain ticking over.
- Noradrenaline induces physical and mental arousal and heightens mood.
- Serotonin: the ‘feel-good’ chemical. It produces feelings of well-being and regulates sleep, appetite, and blood pressure.
