CHAPTER 4: THE CENTRAL NERVOUS SYSTEM AND CHAPTER 5: THE PERIPHERAL NERVOUS SYSTEM

CHAPTER 4: THE CENTRAL NERVOUS SYSTEM AND

CHAPTER 5: THE PERIPHERAL NERVOUS SYSTEM

THE NERVOUS SYSTEM

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THE CENTRAL NERVOUS SYSTEM The central nervous system (CNS) is one

of the two major branches of the human nervous system.

The CNS is comprised of the brain and spinal cord.

The spinal cord connects the brain to the second major branch – the peripheral nervous system (PNS).

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THE SPINAL CORD The spinal cord is the cable-

like column of nerves that extends from the bas of the brain to the lower back

Its major role is to receive sensory information from the body and transmit it to the brain, and receive information from the brain and relay it to the body to control muscles, glands and organs

The spinal cord consists of two major components, white matter and grey matter

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THE SPINAL CORD White matter is made up solely of axons which run the length of

the spinal cord

An axon is the part of a neuron that sends information away from the soma (cell body) to other neurons or to cells in muscles or glands

Axons are covered in a white protective coating known as myelin and are located mainly in the other layers of the spinal cord

The grey matter contains cell bodies, together with their axons and dendrites, and is mainly located in the centre of the spinal cord

Dendrites are thin extensions of a neuron that receive information from other neurons and transmit it to the cell body

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NEURONS Neurons are cells that are specialised to

receive, process and/or transmit information to other cells within the body

Neurons can be broadly categorised into 3 categories depending on the primary function they perform

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SENSORY NEURONS Sensory Neurons are nerve cells that carry messages

from sensory organs and receptors through nerves in the PNS, up the tracts in the spinal cord and to the brain.

Sensory neurons generally respond only to a particular type of stimulus.

Pressure Receptors – touch, pressure, pain Temperature Receptors – hot and cold Pain Receptors – linked to pressure & temp receptors Taste Receptors Smell Receptors

Sensory neurons are also known as afferent neurons because they transmit sensory information along afferent tracts in the CNS

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SENSORY NEURONS

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MOTOR NEURONS Motor Neurons are nerve cells that carry

messages away from the brain and spinal cord towards muscles, glands and organs that enable bodily movements or glandular secretions.

Motor neurons are also known as efferent neurons as they transmit motor activity along the afferent tracts of the spinal cord

The main distinction between motor neurons and sensory neurons is the direction of the nerve impulses and what occurs at their respective destinations.

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MOTOR NEURONS

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SENSORY AND MOTOR NEURONS

S – Sensory

A – Afferent

M – Motor

E – Efferent

So sensory/afferent neurons carry information from sensory receptors in the PNS towards the CNS while motor/efferent neurons carry information away from the CNS to the muscles and glands

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INTERNEURONS Interneurons perform the important function of

making the connection between motor and sensory neurons.

Interneurons relay messages from one group of neurons to another group of neurons.

They exist only in the CNS (brain & spinal cord).

Interneurons have a single axon leaving the cell body and a single dendrite coming into the cell body from a receptor cell.

Interneurons play an important role in reflexes12

THE PERIPHERAL NERVOUS SYSTEM The Peripheral Nervous System (PNS) is

the complete network of neurons located outside the CNS.

The PNS extends from the top of the head to the rest of the body.

The PNS has two main functions: To carry information from the sensory organs to the

CNS. To convey information from the CNS to the muscles,

organs and glands. (up to here)13

THE PERIPHERAL NERVOUS SYSTEM The PNS can be subdivided into two quite

distinct nervous systems, each of which have different functions:

The Somatic Nervous System The Autonomic Nervous System

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THE SOMATIC NERVOUS SYSTEM The Somatic Nervous System has both a

sensory function and a motor function.

The Somatic Nervous System, also called the Skeletal Nervous System, is the network of neurons that transmits messages from the sensory receptors to the CNS and controls voluntary movement of skeletal muscles through messages sent from the CNS.

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THE SOMATIC NERVOUS SYSTEM

Therefore the Somatic Nervous System allows you to – Feel any sensations from your external

environment. Make any voluntary movements or actions to

respond to your external environment.

Paraplegics and Quadriplegics have had their spinal cord severed at a certain point allowing no neural information to pass through the somatic nervous system below the severed cord.

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THE AUTONOMIC NERVOUS SYSTEM The Autonomic Nervous System (ANS) is

a network of neurons that connects the CNS to all the body’s internal muscles, organs and glands.

When you are suddenly frightened – your heart rate and breathing rate increases, your pupils dilate and goosebumbs appear. The Autonomic Nervous System is responsible for all of these events occurring.

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THE AUTONOMIC NERVOUS SYSTEM The term autonomic means automatic,

independent or self governing.

Regardless of our awareness or alertness, the ANS keeps the vital organs and systems of our body functioning, thus maintaining our survival.

This occurs automatically without our conscious control.

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THE AUTONOMIC NERVOUS SYSTEM The ANS modulates (modifies or changes)

the activity of visceral muscles, organs and glands.

The ANS receives constant neural activity from the CNS by either increasing or decreasing messages to bring about changes in the body.

While most of the actions controlled by the ANS are not within our control, there are a few responses of the ANS that we can voluntarily control – blinking, heart rate and breathing rate.

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THE AUTONOMIC NERVOUS SYSTEM In India, it has been reported that

Yogis (Hindu holy men) can change their heart rate from 75 bpm, to 300 bpm or 50 bpm. People who aren’t Yogis can also learn to control various autonomic responses using a technique called

Biofeedback Training.

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DIVISIONS OF THE ANS Most muscles, organs and glands receive

messages from two sets of neurons from two distinct divisions of the ANS.

The Sympathetic division is responsible for increasing the activity of muscles, glands and organs in times of vigorous activity, stress or threat.

The Parasympathetic branch is responsible for decreasing the activity of muscles, glands and organs and keeping the body functioning at a normal rate.

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DIVISIONS OF THE ANS When you play a game or sport, the

sympathetic nervous system speeds up your heart rate, increases metabolism and induces sweating in order for you to perform to your maximum.

When you stop playing, the parasympathetic nervous system slows your heart and breathing rate, to help the body return to its normal state.

While the sympathetic and parasympathetic nervous systems are both active at the same time, one system usually predominates over the other at any given time.

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THE SYMPATHETIC NERVOUS SYSTEM The sympathetic nervous system

originates in the spinal cord, and enhances survival by providing an immediate response, in a split second, to any kind of emergency.

In an emergency situation, the sympathetic nervous system sends a message to the adrenal glands to secrete the hormones adrenalin and noradrenalin.

These hormones are released into the bloodstream and activate muscles organs and glands for preparation of the potential emergency. 23

THE SYMPATHETIC NERVOUS SYSTEM The results of these hormones include:

Increased heart rate and blood pressure, Increased breathing rate, Increased pupil size, Increased sweat production, Decreased digestion, Heightened senses (ie: goosebumps, see p136)

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THE PARASYMPATHETIC NERVOUS SYSTEM The parasympathetic nervous system has the

effect of counterbalancing the activities of the sympathetic nervous system.

The parasympathetic nervous system has three main functions:

It keeps the systems of the body functioning efficiently.

It helps maintain a constant internal environment. It restores the body to a state of calm after

vigorous or strenuous activity. 25

THE AUTONOMIC NERVOUS SYSTEM

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THE BRAIN The human brain is encased in a hard

protective skull and weighs about 1.5kg.

It has the consistency of firm jelly and is covered by a strong plastic like membrane.

The brain contains over one billion neurons and over one trillion synaptic connections.

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THE BRAIN Neuropsychologists

often describe the brain as having three main regions:

The Hindbrain, The Midbrain, The Forebrain

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THE CEREBRAL CORTEX The cerebral cortex is one of the most easily recognised

parts of the brain

It is recognisable as the folded outer layer or covering of the cerebral hemispheres of the brain

The cerebral cortex is largely involved with:

information-processing activities, language, speech, learning, memory, thinking, problem solving the control of sensory and motor abilities.

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THE CEREBRAL CORTEX It is believed that the size of a species’

cerebral cortex is linked to intellectual ability.

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THE CEREBRAL CORTEX

In terms of function, the cerebral cortex has three main parts:

The various sensory areas (which receives information about vision, smell and sound)

The motor cortex (which transmits information about bodily movements)

The association cortex (which integrates sensory and motor information)

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CEREBRAL HEMISPHERES The cerebral cortex is described as

having two halves.

The cerebral hemispheres are two almost symmetrical brain structures that appear to be separated by a deep groove (known as the longitudinal fissure).

These hemispheres appear to be separated completely, but they are joined in the midbrain by a structure called the corpus callosum.

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CEREBRAL HEMISPHERES

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CEREBRAL HEMISPHERES The cerebral hemispheres is described as

having inverted functions.

This means that sensory and motor information in the right side of the body is controlled by the left cerebral hemisphere.

Sensory and motor information in the left side of the body is controlled by the right cerebral hemisphere.

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CORPUS CALLOSUM The corpus callosum is a strand, or

‘bridge’, of nerve tissue that connects the left and right cerebral hemispheres and serves as the main communication pathway between them.

Its function is one of a ‘cross-over’ station for neural messages between the two cerebral hemispheres.

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CORPUS CALLOSUM

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FOUR LOBES OF THE CEREBRAL CORTEX The cerebral cortex covering each

hemisphere can be divided into four anatomical regions.

These cortical lobes are called:

The frontal lobes The parietal lobes The occipital lobes The temporal lobes

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FOUR LOBES OF THE CEREBRAL CORTEX

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Each cortical lobe has areas that specialise in receiving and processing sensory or motor information.

There are also association areas that integrate sensory, motor and other information for complex mental processes.

FOUR LOBES OF THE CEREBRAL CORTEX

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FOUR LOBES OF THE CEREBRAL CORTEX

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THE FRONTAL LOBE

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THE FRONTAL LOBE The frontal lobe is the largest of all lobes and

occupies the front half of the brain.

The primary motor cortex is located at the rear of the frontal lobes. The PMC controls voluntary movements.

Stimulation in the left hemisphere of the PMC will trigger movements of body parts in the right side of the body and vice versa.

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THE PRIMARY MOTOR CORTEX

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THE PRIMARY MOTOR CORTEX

The primary motor cortex is mapped so that a specific area of the cortex controls a specific part of parts of the body

The amount of area in the cortex devoted to a specific part demonstrates the amount of finesse or control given to movement in that part of the body

Areas with great precision such as fingers are allocated a much greater cortex area than parts which have less control such as toes

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THE FRONTAL LOBE In the forward section of the frontal

lobes is the association area responsible for higher mental functions such as judging, planning and initiative.

The frontal lobes are also involved with expression of characteristics such as personality and emotional behviour.

Possibly the best case to support this, is that of the case of Phineas Gage.

Box 4.1 – Case study: damage to the frontal lobes of Phineas Gage

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BROCA’S AREA Broca’s area is located in the frontal lobe of the left

hemisphere and is thought to be responsible for articulate speech.

In particular, Broca’s area is involved with the movement of the muscles required to speak (ie coordination of lips, jaw, tongue and vocal cords).

Broca’s area is also concerned with the meaning of words, the structure of sentences and specific parts of speech.

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BROCA’S AREA

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THE PARIETAL LOBE

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THE PARIETAL LOBE The parietal lobes receive information about touch,

pressure, temperature, muscle movement and position. These are known as somatosensory functions. The

somatosensory cortex is located in the parietal lobe behind the PMC.

The parietal love also contains association areas which integrate information from within the lobe and other structures and areas of the brain – for example one of these functions enables us to sense our position in space, to do this we need to integrate information from our body’s limbs with information from our visual and auditory receptors from the temporal and occipital lobes

Other functions of the parietal lobe include attention and spatial reasoning – determining where an object is located in the environment

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THE SOMATOSENSORY CORTEX

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THE PARIETAL LOBE This man is called

the ‘homunculus’. What does he tell us?

There is a sensory and a motor homunculus. How are they different? How are they similar?

Box 4.2 - Phantom Limbs pg. 189

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THE TEMPORAL LOBES

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THE TEMPORAL LOBES The temporal lobes are primarily associated with

hearing , but also play an important role in memory, particularly the recognition of faces

The primary auditory cortex is found in each hemisphere and receives information from the ears and inner ears

The association cortex of the temporal lobes allow us to remember and perceive features of our environment

A severe blow or damage to the temporal lobe would result in loss of memory (amnesia) or decreased mental function

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THE PRIMARY AUDITORY CORTEX Both cortices of the PAC are linked

together with a slight delay so as direction of sound can be determined. Otherwise you would hear a sound on the left and recognise it coming from the right

Each PAC has areas which receive and process different features of sound

The two main features of sound are the frequency (pitch) and the amplitude (loudness)

Sounds with different frequencies and amplitudes are perceived in different areas of the cortex 54

THE PRIMARY AUDITORY CORTEX Each cortex is also specialised to process

different types of sound

The left primary auditory cortex processes verbal sounds (such as words) while the right primary auditory cortex processes non-verbal sounds (such as music)

This does not mean however, that verbal and non-verbal information is processed exclusively in each hemisphere – there is integration and overlap 55

WERNICKE’S AREA

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WERNICKE’S AREA Wernicke’s area is located in the temporal lobe of

the left hemisphere next to the PAC.

Wernicke’s area is involved with interpreting sounds – especially human speech and language.

Wernicke’s area is vital not just for understanding words, but also for locating appropriate words from memory to express intending meanings when we speak and write.

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THE OCCIPITAL LOBES

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THE OCCIPITAL LOBES The occipital lobes are regions in which visual

information is received and processed.

The primary visual cortex is located at the base of each lobe and is where information arrives from the eyes.

The visual association areas receive information from the PVC and other regions of the brain to form visual perceptions, to think visually and to remember visual images.

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THE PRIMARY VISUAL CORTEX The primary visual cortex is located at the base of each

occipital lobe

Information from visual sensory receptors (photoreceptors) in the retinas is sent to the PVC

Each hemisphere receives and processes half of the visual information –the left half of each eye (which receives information from the right half of the visual field) sends info only to the PVC in the left occipital lobe, the right half of each eye (which receives information from the left half of the visual field) sends info only to the PVC in the right occipital lobe

Neurons in the PVC are specialised to respond to different types of visual information such as orientation, shape, motion, colour etc. 60

THE OCCIPITAL LOBES

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HEMISPHERIC SPECIALISATION In general the right and left hemispheres appear to

be identical – but they in fact have some specialised functions that are not duplicated in the other.

Hemispheric Specialisation refers to the specialisation and dominance of certain functions by each hemisphere of the brain.

These differences are most apparent in stroke victims and people that had suffered damage to one side of the brain only.

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HEMISPHERIC SPECIALISATION

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LEFT HEMISPHERE FUNCTIONS Verbal functions that require the use or

recognition of words (reading, writing and speaking) are primarily the responsibilities of the left hemisphere.

Analytical Functions such as logical thinking and the ability to analyse, organise and interpret data are also dominated by the left hemisphere.

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RIGHT HEMISPHERE FUNCTIONS Non-verbal Functions are those which do

not require the use or recognition of words and are primarily the responsibility of the right hemisphere.

Non-verbal tasks include: producing artwork, recognising faces and tunes, solving puzzles and daydreaming.

http://mtsu32.mtsu.edu:11406/hemispheric_dominance.as

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RESEARCH ON HEMISPHERIC SPECIALISATION Evidence of hemispheric specialisation

comes from three main approaches:

1. Research involving people with a damaged brain.

2. Studying those with split brain surgery.

3. Research with people whose brain is intact and undamaged

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THE RETICULAR ACTIVATING SYSTEM Running up through the centre of the brain stem is a structure

called the reticular formation

In an early experiment, Moruzzi & Magoun (1949) electronically stimulated the reticular formation of cats which caused the animals to awaken suddenly and remain alert

When they severed the connections between the reticular formation and the rest of the brain, it caused the cats to fall into a prolonged coma and remain that way until they died

As a result they concluded that the function of the reticular formation was to control sleeping and waking

The reticular formation thus became known as the reticular activating system (RAS)

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THE RETICULAR ACTIVATING SYSTEM The RAS is a network of neurons that extends to many

directions from the reticular formation to the brain and spinal cord

Its ascending tracts extend to parts of the brain while its descending tracts descend to the spinal cord

The general functions of the RAS are to regulate cortical arousal, increasing or decreasing depending on feedback from the brain and spinal cord

It also influences whether we are awake, asleep or drowsy

The RAS also appears to influence attention, and more specifically our selective attention, by highlighting neural information which is of importance

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THE RETICULAR ACTIVATING SYSTEM

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THE THALAMUS The thalamus is a brain structure located in the middle of the

brain, right on top of the brain stem

Its role is to filter information from all of the senses (except smell) and transmit or relay that information to the cerebral cortex

The thalamus appears to also play an important role in attention and actively filters information from the senses based on importance

Evidence from individuals with thalamus damage shows that they have difficulty filtering information; that is, attending to one input and ignoring others

The thalamus also has a role in regulating arousal through the RAS

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THE THALAMUS

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