BASIC FEATURES OF NERVOUS SYSTEM
AN OVERVIEW
The nervous system has two main parts:
- The central nervous system is made up of the brain and spinal cord.
- The peripheral nervous system is made up of nerves that branch off from the spinal cord and extend to all parts of the body.
The nervous system transmits signals between the brain and the rest of the body, including internal organs. In this way, the nervous system’s activity controls the ability to move, breathe, see, think, and more.
MENINGES
Three layers of membranes known as meninges protect the brain and spinal cord.
- The delicate inner layer is the pia mater. T
- he middle layer is the arachnoid, a web-like structure filled with fluid that cushions the brain.
- The tough outer layer is called the dura mater.
Together, the arachnoid mater and pia mater are called leptomeninges.
There are three spaces within the meninges:
- The epidural space is a space between your skull and dura mater and the dura mater of your spinal cord and the bones of your vertebral column. Analgesics (pain medicine) and anesthesia are sometimes injected into this space along your spine. The spinal cord ends between the first and second lumbar vertebra in the middle of your back, at which point, only cerebrospinal fluid is present. This is the site where a lumbar puncture (“spinal tap”) is performed.
- The subdural space is a space between your dura mater and your arachnoid mater. Under normal conditions, this space isn’t a space, but can be opened if there’s trauma to your brain (such as a brain bleed) or other medical condition.
- The subarachnoid space is a space between your arachnoid mater and pia mater. It’s filled with cerebrospinal fluid. Cerebrospinal fluid cushions and protects your brain and spinal cord.
What do the meninges do?
- Protect your CNS (central nervous system) from trauma injury to your brain, such as a blow to your head by acting as a shock absorber. They anchor your CNS and keep your brain from moving around within your skull.
- Provide a support system for blood vessels (including your middle meningeal artery) that deliver blood to your CNS tissues, nerves (including your trigeminal and vagus nerves), lymphatics (drainage system) and cerebrospinal fluid.
features of dura mater
- dura mater is the outer, thick, strong membrane layer located directly under your skull and vertebral column.
- In Latin, dura mater means “hard mother.” It consists of two layers of connective tissue.
- One side of your dura attaches to your skull and the other adheres to your middle membrane layer (arachnoid mater).
- dura mater contains a drainage system, called the dural venous sinuses, which allows blood to leave your brain and allows cerebrospinal fluid to re-enter the circulation.
- dura mater receives its blood supply from your middle meningeal artery and vein, and your trigeminal nerve runs through it.
features of arachnoid mater
- arachnoid mater, the middle layer of your meninges, lies directly below your dura mater.
- It’s a thin layer that lays between your dura mater and pia mater.
- It doesn’t contain blood vessels or nerves. It has a spiderweb-like appearance (“arachnoid” means spider) because it has connective tissue projections that attach to your pia mater.
- Between your arachnoid mater and pia mater is the subarachnoid space, which contains cerebrospinal fluid that helps cushion your brain.
features of the pia mater
- pia mater, the innermost layer, is a thin layer that’s held tightly — like shrink wrap — to the surface of your brain and spinal cord. Many blood vessels pass through this layer to supply your brain tissue with blood. It also helps contain cerebrospinal fluid. In your spinal cord, pia mater helps maintain the stiffness of the cord.
Ventricular system and production of cerebrospinal fluid
VENTRICULAR SYSTEM AND CEREBROSPINAL FLUID
The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF) and located within the brain parenchyma. The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle (see the images below). The choroid plexuses are located in the ventricles produce CSF, which fills the ventricles and subarachnoid space, following a cycle of constant production and reabsorption.
Cerebrospinal Fluid (CSF) flows through the four ventricles and then flows between the meninges in an area called the subarachnoid space. CSF cushions the brain and spinal cord against forceful blows distributes important substances and carries away waste products.
Under normal conditions, a delicate balance exists between the amount of CSF produced and the rate at which it is absorbed. Our bodies produce approximately one pint of CSF every day, continuously replacing it as it is absorbed.
Hydrocephalus develops when this balance is altered and is characterized by an abnormal accumulation of CSF within the ventricles. This accumulation of CSF increases the pressure in the brain causing the ventricles to enlarge and the brain to be pressed against the skull.
Cerebrospinal Fluid is primarily produced within the lateral third ventricles by delicate tufts of specialized tissue called the choroid plexus. In some cases, hydrocephalus can develop when the choroid plexus produces too much CSF. This can happen when there is a tumor on the choroid plexus, for example.
CSF flows from the lateral ventricles through two narrow passageways into the third ventricle. From the third ventricle, it flows down another long passageway known as the aqueduct of Sylvius into the fourth ventricle. From the fourth ventricle, it passes through three small openings called foramina and into the subarachnoid space surrounding the brain and the spinal cord.
If the flow of CSF at any of these points is blocked, hydrocephalus can develop. This is often referred to as non-communicating hydrocephalus.
Three Main Functions of CSF
Cerebrospinal fluid has three main functions:
- Protect brain and spinal cord from trauma.
- Supply nutrients to nervous system tissue.
- Remove waste products from cerebral metabolism.
CELLS OF THE NERVOUS SYSTEM
NEURON
- The basic unit of the nervous system is a nerve cell, or neuron.
- The human brain contains about 100 billion neurons. A neuron has a cell body, which includes the cell nucleus, and special extensions called axons and dendrites .
- Bundles of axons, called nerves, are found throughout the body. Axons and dendrites allow neurons to communicate, even across long distances.
Different types of neurons control or perform different activities.
motor neurons transmit messages from the brain to the muscles to generate movement.
Sensory neurons detect light, sound, odor, taste, pressure, and heat and send messages about those things to the brain. Other parts of the nervous system control involuntary processes. These include keeping a regular heartbeat, releasing hormones like adrenaline, opening the pupil in response to light, and regulating the digestive system.
When a neuron sends a message to another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal changes to a chemical signal. The axon then releases the chemical signal with chemical messengers called neurotransmitters into the synapse —the space between the end of an axon and the tip of a dendrite from another neuron. The neurotransmitters move the signal through the synapse to the neighboring dendrite, which converts the chemical signal back into an electrical signal. The electrical signal then travels through the neuron and goes through the same conversion processes as it moves to neighboring neurons.
Neurons are the central building blocks of the nervous system, 86 billion strong at birth.
Like all cells, neurons consist of several different parts, each serving a specialized function.
- A neuron’s outer surface is made up of a semipermeable membrane. This membrane allows smaller molecules and molecules without an electrical charge to pass through it, while stopping larger or highly charged molecules.
- The nucleus of the neuron is located in the soma, or cell body. The soma has branching extensions known as dendrites.
- The neuron is a small information processor, and dendrites serve as input sites where signals are received from other neurons. These signals are transmitted electrically across the soma and down a major extension from the soma known as the axon, which ends at multiple terminal buttons.
- The terminal buttons contain synaptic vesicles that house neurotransmitters, the chemical messengers of the nervous system.
- Axons range in length from a fraction of an inch to several feet. In some axons, glial cells form a fatty substance known as the myelin sheath, which coats the axon and acts as an insulator, increasing the speed at which the signal travels.
- The myelin sheath is not continuous and there are small gaps that occur down the length of the axon. These gaps in the myelin sheath are known as the Nodes of Ranvier.
- The myelin sheath is crucial for the normal operation of the neurons within the nervous system: the loss of the insulation it provides can be detrimental to normal function.
- In healthy individuals, the neuronal signal moves rapidly down the axon to the terminal buttons, where synaptic vesicles release neurotransmitters into the synaptic cleft.
- The synaptic cleft is a very small space between two neurons and is an important site where communication between neurons occurs. Once neurotransmitters are released into the synaptic cleft, they travel across the small space and bind with corresponding receptors on the dendrite of an adjacent neuron.
- Receptors, proteins on the cell surface where neurotransmitters attach, vary in shape, with different shapes “matching” different neurotransmitters.
GLIAL CELLS
The nervous system also includes non-neuron cells, called glia . Glia perform many important functions that keep the nervous system working properly.
For example, glia:
- Help support and hold neurons in place
- Protect neurons
- Create insulation called myelin, which helps move nerve impulses
- Repair neurons and help restore neuron function
- Trim out dead neurons
- Regulate neurotransmitters
The brain is made up of many networks of communicating neurons and glia. These networks allow different parts of the brain to “talk” to each other and work together to control body functions, emotions, thinking, behavior, and other activities.
Neural supporting cells
Supporting cells in the central nervous system.
In the central nervous system, there are four types of supporting cells.
1. Oligodendrocytes.
The axons of many neurons are insulated by a myelin sheath, which increases the rate at which an axon can conduct an action potential.
In multiple sclerosis, the myelin sheaths in the CNS are destroyed, and action potentials are slowed.
Myelin is formed by oligodendrocytes in the CNS, (and by Schwann cells. in the PNS). Oligodendrocytes myelinate several axons from different nerves (up to around 50).
2. Microglia
These types of cell are less common. They have a role in immune defence and become phagocytic in response to infections or tissue damage
3. Astrocyte
These cells are the most common type of supporting cell. They are involved in metabolic exchange between neurons and blood.
4. Ependymal cells. These cells line the vetricles and spinal canal.
They have cilia on their luminal surface.
The blood-brain barrier – Neural Communication
AN OVERVIEW
- The brain is precious, and evolution has gone to great lengths to protect it from damage. The most obvious is our 7mm thick skull, but the brain is also surrounded by protective fluid (cerebrospinal – of the brain and spine) and a protective membrane called the meninges. Both provide further defence against physical injury.
Another protective element is the blood–brain barrier.
As the name suggests, this is a barrier between the brain’s blood vessels (capillaries) and the cells and other components that make up brain tissue. Whereas the skull, meninges and cerebrospinal fluid protect against physical damage, the blood–brain barrier provides a defence against disease-causing pathogens and toxins that may be present in our blood.
The blood–brain barrier was discovered in the late 19th century, when the German physician Paul Ehrlich injected a dye into the bloodstream of a mouse. To his surprise, the dye infiltrated all tissues except the brain and spinal cord. While this showed that a barrier existed between brain and blood, it wasn’t until the 1960s researchers could use microscopes powerful enough to determine the physical layer of the blood–brain barrier.
We now know the key structure of the blood–brain barrier that offers a barrier is the “endothelial tight junction”. Endothelial cells line the interior of all blood vessels. In the capillaries that form the blood–brain barrier, endothelial cells are wedged extremely close to each other, forming so-called tight junctions.
The tight gap allows only small molecules, fat-soluble molecules, and some gases to pass freely through the capillary wall and into brain tissue. Some larger molecules, such as glucose, can gain entry through transporter proteins, which act like special doors that open only for particular molecules.
Surrounding the endothelial cells of the blood vessel are other components of the blood–brain barrier that aren’t strictly involved in stopping things getting from blood to brain, but which communicate with the cells that form the barrier to change how selective the blood–brain barrier is.
Why do we need it?
- The purpose of the blood–brain barrier is to protect against circulating toxins or pathogens that could cause brain infections, while at the same time allowing vital nutrients to reach the brain.
- Its other function is to help maintain relatively constant levels of hormones, nutrients and water in the brain – fluctuations in which could disrupt the finely tuned environment.
So what happens if the blood–brain barrier is damaged or somehow compromised?
One common way this occurs is through bacterial infection, as in meningococcal disease. Meningococcal bacteria can bind to the endothelial wall, causing tight junctions to open slightly.
As a result, the blood–brain barrier becomes more porous, allowing bacteria and other toxins to infect the brain tissue, which can lead to inflammation and sometimes death.
It’s also thought the blood–brain barrier’s function can decrease in other conditions. In multiple sclerosis, for example, a defective blood–brain barrier allows white blood cells to infiltrate the brain and attack the functions that send messages from one brain cell (neuron) to another. This causes problems with how neurons signal to each other.
The blood–brain barrier is generally very effective at preventing unwanted substances from accessing the brain, which has a downside. The vast majority of potential drug treatments do not readily cross the barrier, posing a huge impediment to treating mental and neurological disorders.