Neurology

Chapter 9: The nervous system

The central computer• Technology has managed to create computers

that can, individually:• Walk• Play chess • Create facial expressions• Answer basic questions• Yet, despite all of our great technological

advances, we have yet to create a computer that is capable of doing all of those things at the same time

Our brain

• The human brain is one of the most complex computers in existence

• Hundreds of thousands of years of evolution have created an organ that is capable of doing all of those things at once – and process abstract concepts such as emotion

Good communication

• The main purpose of the nervous system is to process information from our environment, and use this information to respond to the changes in the environment

• In order to do that, the nervous system must be able to SENSE what is going on, communicate that to the brain, and then elicit a RESPONSE by activating organs or muscles in our body

Communication lines

• NERVES in our body are like telephone lines• They branch all over our body and connect

various parts to our brain• The meet up together at the spinal cord, and

all run into the brain• If you can imagine all the telephone lines in

Toronto that meet up together and connect to one central system – this is similar to how the nerves in the nervous system is organized

Neurons• Neurons are the

• They possess all the basic organelles inside a normal cell but their shape is unique to their function

• In the nervous system they are

• They receive and transmit

• Neurons are attached to a long “telephone line” – a group of neurons form nerves that extends outwards to communicate with other cells

Nerves

• There are two basic types of nerves or nerve pathways in our body

• SENSORY/AFFERENT PATHWAYS:

• MOTOR/EFFERENT PATHWAYS

The system

• Our nervous system has evolved to help us process information to produce responses

• Early organisms required mainly physical responses – move your muscles to escape or to pounce, maintain homeostasis in the body

• However, as organisms became more complex, emotional and psychological responses became necessary in order to interact more and more with other organisms

The whole system

• As usual, when looking at the organization of the nervous system, we need to start by looking at the basic gross anatomy

• It is divided into two main parts:• CENTRAL NERVOUS SYSTEM (CNS

• PERIPHERAL NERVOUS SYSTEM (PNS):

CNS

• The CNS is the main processing center

• The spinal cord is the

• The brain is the processing unit –

PNS

• The PNS are the series of sensory and motor nerves that lie outside the spinal cord

• These nerves branch out all over the body and are divided further into a few more branches

• SOMATIC/VOLUNTARY NERVOUS SYSTEM:

• AUTONOMIC/INVOLUNTARY NERVOUS SYSTEM: these are the nerves that we are not aware of and cannot control consciously;

Organization • In the CNS and PNS,

• For example, the amygdala in the brain is a group of neurons that are dedicated to processing memory and emotion

• The cranial nerves are a series of nerves that travel from the brain and then branch out to innervate various muscles located in the face

• They are named slightly differently in the PNS vs. the CNS

Structure CNS PNS

Clusters of neurons Ganglion

Group of axons (note that these groups can be both afferent and efferent)

Nerves

Voluntary/Somatic NS

• The SNS

• Pain, feeling cold or hot – these afferent pathways carry information to your brain that you will notice

• When you decide to move your arm, or walk or run – these efferent pathways are activated consciously – you think about it, and it happens

Involuntary/Autonomic NS• The ANS is responsible for

• Think about the muscle contractions in your gastrointestinal system, or your heart beating – you don’t think about it, but they happen regularly

• The efferent pathways in the ANS monitor and send information to your brain about changes in your internal systems

• The afferent pathways in the ANS activates systems in your body – it tells your heart to beat, your breathing is controlled by your ANS, and it also activates changes to these systems in order to maintain homeostasis

Sympathetic/Parasympathetic NS

• The sympathetic and parasympathetic branches of the ANS is a classical division of the ANS where the sympathetic branches

• Whereas the parasympathetic branches help to “shut down”

NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM

PERIPHERAL NERVOUS SYSTEM

BRAIN SPINAL CORD

SOMATIC NERVOUS SYSTEM

AUTONOMIC NERVOUS SYSTEM

SYMPATHETIC NERVOUS SYSTEM

PARASYMPATHETIC NERVOUS SYSTEM

Note: all branches consist of afferent and efferent pathways

THE CENTRAL NERVOUS SYSTEMAnatomy and Basic Physiology

The Spinal Cord

• The spinal cord is attached to the brain stem, and exits the skull through the

• The spinal cord is composed of tracts of nerves that surround

• It is physically divided into to sections:

Sensory root ganglion

Why massage works• Note that sensory information is carried to the

brain on the dorsal side of the spinal cord whereas motor information is carried on the ventral side

• That means that all sensory information, including pain, is carried along tracts that are in close proximity to each other

• The basis of massage as a form of pain therapy is based on this; by activation touch sensors, the activity of these tracts are thought to inhibit the activity of tracts that are carrying pain information to the brain

The brain

• The brain has 3 major divisions:• HINDBRAIN:

• MID BRAIN:

• FOREBRAIN:

Basic structure• Brain tissue is highly folded• This increases the amount of surface area

needed to house neurons

• This makes sense since the brain needs to organize all the nerve tracts to run downwards towards the spinal cord, whereas the spinal cord needs to run tracts away from itself, so its white matter sits exterior to its nuclei

External structure• The brain is divided into LOBES – major regions

responsible for different functions; this is a basic outline of these functions

• FRONTAL LOBE:

• PARIETAL LOBE:

• OCCIPITAL LOBE:

• TEMPORAL LOBE:

External structure cont.

• PONS:

• CEREBELLUM:

The human brain: our greatest evolutionary tool

• The human brain differs

• Our cereberum – the part of our brain that controls abstract reasoning,

• Also, our cerebral cortex

Still don’t know that much• Though we have an understanding of what parts of our

brain do what we are still very much in the dark of what to expect when things go wrong in the brain

• Certain injuries in the frontal lobe may not kill a person, but can cause other unexpected changes in behaviour or thought (see Case Study: Phineas Gage p. 432)

• Most of our knowledge of the human brain comes from mapping out the human brain via open brain surgery – pioneered by Wilder Penfield in Montreal – where he spent a lot of time working with epileptic patients

Inside the brain

• The brain is divided into two hemispheres• They control opposing areas of the body;

interestingly enough one side of the body is controlled by the opposite side of the brain; sensory information also crosses over as well

• The two hemispheres are connected

Protection• The CNS is bathed in

• This fluid circulates around the brain and down through the spinal cord via the central canal

• This helps to protect the brain along with the skull and the MENINGES

• Imagine an egg suspended in water in a glass jar that is just a bit bigger than the egg itself

NEURONAnatomy and physiology

Our brain, the computer...• The interesting thing is that a basic computer

follows the core principles of how the human brain functions

• Binary code is based on a system of “ones” and “zeroes”

• Putting many of these together creates a code that is read by the computer

• http://upload.wikimedia.org/wikipedia/commons/7/77/Wikipedia_in_binary.gif

Firing neurons

• The human nervous system acts on the same general principle

• If the neuron is turned on, it sends a message to the brain

• The sum total of the neuronal activity is used to collect information about our environment and to process it

• However the way the brain is connected still remains a mystery – and this complexity is what allows it to do the things it can do

Let’s start with the basics...

• The neuron is a very unique and complex cell• It is composed of three main parts:• CELL BODY:

• AXON:

• DENDRITE:

Spread out• Neurons are “STELLATE” or star shaped cells• This characteristic allows them to branch out and

connect to many other cells• There are a few different types of neuron shapes,

and their shapes are based on how their function• Some neurons act as intermediate

communication centers, linking many neurons

• TO INNERVATE =

Electrical wires

• Nervous signals are communicated using electrical impulses

• Therefore, the design of an axon is not too much different from an electrical wire

• Electrical wires are designed to maximize the conduction of electricity – hence the rubber that surrounds electrical wires

• This layer of insulating material helps prevent anything from interfering with the current

Schwann cells• A specialized cell called a Schwann cell wraps

around axons in the PNS• Schwann cells are GLIAL CELLS –

• Schwann cells wrap many times around axons creating multiple layers (imagine a wire running through a cinnamon bun)

• This covering that the Schwann cells create

Saltatory conduction

• Schwann cells are interspersed along the length of the axon and are not connected to each other, leaving small gaps in the myelin sheath

• These gaps are known as• The electrical conduction in myelinated axons

“jump” from one node to another –

PNS vs. CNS nerves• Though the structure of neurons in the PNS and CNS

are similar, there are some slight differences in the neurons

• – think about what happens when you cut yourself- you can regain sensation in your skin after the damaged area heals

• – it is thought that the much more complex connections in the CNS prevent the nerves from reconnecting so easily

• Spinal cord and brain injuries are therefore permanent for this reason

Conducting messages• Not all messages are processed by the brain

• Reflexes are used to generate fast responses to stimuli that signifies possible injury, like touching a hot surface

• They can also be used to regulate muscular changes during movement

• are specialized receptors in tendons that detect the degree of stretch in tendon

• If tendons stretch too fast – the reflex initiates a contraction in the related muscle to counteract the stretch

Sensory ganglia

ACTION POTENTIAL: SENDING MESSAGES THROUGH NEURONS

Section 9.2: Electrochemical impulse

Biological wires

• Most people understand that nervous signals are sent by electrical signals

• We must first understand that the mechanism of electrical signalling in the body is not the same as electrical signals sent through technological devices

• This difference obviously occurs because organisms do not contain large tracts of copper wire in their bodies

What we do have are ions• However, how our body generates electrical signals

is not too much different in how a voltaic cell such as a battery, uses chemicals to produce electric current

• An electric current can be generalized as a group of moving charges – in electricity, this is specifically a set of electrons moving through wires

• But electricity can be simplified to charges moving from point A to point B, creating a difference in electric charge between those two areas

• The human body doesn’t have a large number of free electrons available to move from point A to B, but it does have a large number of charged metal and non-metal ions

A quick physics lesson...• POTENTIAL ENERGY in electricity refers to the

energy that exists when there is a DIFFERENCE in charge between two areas

• A positive and negative charge separated by a distance creates two areas of different charge

• Since opposite charges are attracted to each other, they will move towards each other

Neurons• Neurons, like all cells, are filled with cytoplasm

• The movement of these ions in and out of the cell create the electrical charge that travels down the length of an axon

• Interesting note: some of these channels are mediated via active transport – the nervous system uses about 40% of the body’s energy to power its function – giving you an idea of how important your nervous system is

Membrane potential• Neurons maintain specific concentrations of

Na+ and K- • This concentration is different from the

concentrations outside of the cell• This creates a charge difference between the

interior and exterior of the neuron

Resting membrane potential (RMP)• The neuronal membrane is more permeable to K+ than it is

to Na+

• The situation is reversed for K+: it tends to diffuse out

• Therefore, the net movement of charge is positive out – leaving the RMP of most neuronal cells to be -70mV inside the cell

• The cell is POLARIZED at this point since there is a charge difference between the interior and the exterior

Action potential

• An ACTION POTENTIAL is the movement of an electrical impulse down the length of the axon

• This leads to a change in the RMP: a sudden inward flood of positive charge changes the negative interior of the axon

Propagation of action potential

• An action potential is activated when an electrical impulse, usually started at the cell body, activates the sodium channels

• This happens because the neuron is activated by another cell, or because a receptor that picks up stimuli from the environment is activates the cell

Voltage gated sodium channels

• http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__voltage-gated_channels_and_the_action_potential__quiz_2_.html

Good neighbours

don’t depolarize

again

• From the mechanism studied so far, you should wonder how an action potential doesn’t reverse itself

• What prevents the neighbouring area that just depolarized from polarizing again, and reversing the current?

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__action_potential_propagation_in_an_unmyelinated_axon__quiz_1_.html

Repolarization takes time• This lies in the OVERSHOOT

• This lag period for repolarization ensure that action potentials only travel one way – since areas that were just depolarized cannot repolarize and reverse the current

Let’s see it all at once

• Note that this animation shows action potentials in an unmyelinated axons; in a myelinated axon the same thing does occur – but the channels are located at the nodes of ranvier

• http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__the_nerve_impulse.html

What turns on one doesn’t turn on another...

• Nervous control is achieved by different methods

• You should notice, for example that some muscles in your body are capable of finer movements than others (fingers vs. quadriceps) or that some areas in your body are more sensitive to touch than others (fingertips vs. arm)

Threshold

• Depending on the neuron, different levels of stimulation are required to activate an action potential

• So if a neuron fires off every time we touch something that is hot, how do we differentiate between hot vs. warm?

Binary code• The processing of this information comes back

down to the brain

• For example:

• Touching a hot substance would illicit a higher amount of both activation density and frequency vs. a touching a warmer substance

THE SYNAPTIC CLEFT

Cell to cell transmission

• The action potential explains how one neuron transmits a nervous signal, but what if it needs to communicate with another cell?

• This includes the nerves that innervate muscles or hormonal glands

Synapse• The small gap that exists between nervous

connections is known as a SYNAPSE• The synapse is small, but must be bridged in order

for a cell to communicate with another• This is achieved, as with most systems in the body, by

the use of chemicals known as NEUROTRANSMITTERS

• The PRE-SYNAPTIC CELL

• The POST-SYNAPTIC CELL

The “bouton” (button)

• At the end of axons and cell bodies are DENDRITES: the finer branching portions of the cell

• Dendrites are anatomical structure that facilitates cell to cell communication

Send and receive

• As the action potential reaches the bouton, it stimulates the release of neurotransmitters from the bouton

• Vesicles containing the neurotransmitters exocytose, releasing the neurotransmitter across the synapse

All together

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__chemical_synapse__quiz_1_.html

After the fact• Once neurotransmitters have crossed the cleft, they are

usually metabolized by appropriate enzymes to prevent constant depolarization of the post-synaptic cell

• Bo-tox, for example, is processed from the Botulinum toxin created by Clostridium botulinium

Slow and steady wins the race

• Diffusion takes time; so synapses are slower than action potential

• Whereas more complex problem solving requires more synapses, but you also have the luxury of more time associated with them

Multiple neuron connections• Not all neurotransmitters are excitatory – some

are inhibitory, and act to shut off synapses• By using both excitatory and inhibitory synapses,

a neuron’s activity can be tightly controlled• For example, the clothes on your skin are

constantly activating mechanoreceptors and causing neurons to fire – but your brain my decide that this information is not important, so inhibitory neurons will fire and shut off this message when you are sitting in biology class, trying to absorb 70 powerpoint slides worth of material

• This is known as SUMMATION

+

-

+

+

-

Different mechanisms• Neurotransmitters are not always excitatory or inhibitory• Depending on the connections, some can cause

excitation, but the same neurotransmitter can be inhibitory in a different connection

• Acetylcholine is an example of this – it is excitatory and inhibitory depending on the post-synaptic cell

• Inhibitory neurotransmitters cause HYPERPOLARIZATION

• This increases the emigration of K+ out of the cell,

Chemical imbalances

• The global effect of neurotransmitters on the brain is complex

• Imbalances of these chemicals or decreased sensitivity to them can cause neurological or psychiatric disorders such as depression

• Drugs used to treat these disorders can have many mechanisms:

Addiction

• Addiction is an example of a complex mechanism that involves neurotransmitters

• Addiction to anything is not universal to all humans – some individuals are more prone to developing addiction than others

• The neurotransmitter, dopamine, is associated with reward in the brain

Dopamine pathways

• Dopamine is released in conjunction with reward• It illicits an overall sensation of pleasure – and is

closely related to the anticipation of positive reward• It therefore drives positive, reward seeking

behaviour

What is fun for one becomes a lifetime of misery for others

• Certain drugs like cocaine and amphetamines inhibit the uptake of dopamine, prolonging the euphoria produced by dopamine

• The addiction to narcotics, in the end, is a neurological dependence

• Many narcotics produce other effects: cardiovascular, etc. But these effects are not necessarily positive – and do not benefit the person

• What keeps someone coming back to narcotics is the euphoria associated with dopamine – how these drugs affect the brain is what starts this addiction

You never know...• Because every human being is different in their

range of responses to chemicals, (including neurotransmitters) it is very difficult to predict how one person will react to a narcotic

• Even legal drugs have this problem; some patients respond well, others don’t

• Therefore, whether or not you have an addictive personality is difficult to ascertain; it is best to stay away from narcotics because it is difficult to figure out how your body will react to it

It’s not just to drugs

• That being said, addiction is not limited to narcotics

• Dopamine release is associated with exercise, chocolate, sexual intercourse, eating

• What a person is addicted to can vary • Note that healthy levels of dopamine are

needed to promote the pursuit of positive activities