E5 The human brain

489 - 498

Assessment StatementsE.5.1 Label, on a diagram of the brain, the medulla

oblongata, cerebellum, hypothalamus, pituitary gland and cerebral hemispheres.

E.5.2 Outline the functions of each of the parts of the brain listed in E.5.1.

E.5.3 Explain how animal experiments, lesions and fMRI (functional magnetic resonance imaging) scanning can be used in the identification of the brain part involved in specific functions.

E.5.4 Explain sympathetic and parasympathetic control of the heart rate, movements of the iris and flow of blood to the gut.

E.5.5 Explain the pupil reflex.E.5.6 Discuss the concept of brain death and the use of the

pupil reflex in testing for this.E.5.7 Outline how pain is perceived and how endorphins

can act as painkillers.

On the diagram of the brain, label the following: medulla oblongata; cerebellum; hypothalamus; pituitary gland & cerebral hemispheres

cerebral hemisphere

cerebellum

hypothalamus

pituitary gland

medulla oblongataspinal

cord

Functions of each of the parts of the brain Part of the brain Function(s)

medulla oblongata

controls autonomic functions of the body such as: heart rate; blood pressure; ventilation; swallowing; vomiting; digestion & cranial reflexes

Cerebellum coordinates unconscious functions, such as movement and balance

Hypothalamus

links nervous and endocrine systems; produces hormones secreted by posterior pituitary; controls hormonal secretion by pituitary; maintains homeostasis such as; control of body temperature, hunger, thirst, fatigue, circadian cycles

pituitary glandthe posterior lobe stores and releases hormones produced by the hypothalamus and the anterior lobe, and produces and secretes hormones regulating many body functions

cerebral hemispheres

act as the integrating centre for high complex functions such as learning, memory and emotions

Use of animal experiments to identify the brain part involved in specific functions

experiments involves surgery to remove part of the scull to access the brain

animal must be alive during this procedure

different regions of the brain are stimulated and the response of the animal observed

primates were often used but it raised ethical issues due to their genetic similarities to humans & the pain and suffering the procedure caused

example :- Pierre in 1820s showed that removing thin slice of tissue from the cerebellum of rabbits & birds resulted in animals displaying lack of muscular coordination & poor balance with no other obvious effects

Lesions & their uses in identification of the brain parts involved in specific functions

lesions are any abnormalities in the brain tissue of an organism

lesions could be due to damage to the brain tissue as a result of accidents, stroke, tumour or deliberate injury

lesions indicates effect of loss of a brain tissue by comparing an organism to a normal one

e.g. split brain patients led to understanding different functional roles of left and right hemispheres of the brain

many actions of the body involve different areas of the brain

damage may be to many parts of the brain, thus making it difficult to interpret due to complexity of reactions

fMRI (functional magnetic resonance imaging) scanning

fMRI stands for functional magnetic resonance imaging

fMRI records changes in blood flow to the brain

active parts of the brain have increased blood flow

but not all brain activity is detected by MRI

a subject is given a stimulus which is designed to stimulate brain activity

MRI links stimulus with certain part of the brain

brain activity visualized by coloured images

Use of fMRI scanning in identification of the brain parts involved in specific functions

fMRI gives a more specific knowledge of stimulated area of the brain

e.g. it is used to study (diagnose) ADHD, dyslexia, recovery from strokes, music comprehension etc.

fMRI is non-invasive (no damage to brain), can study healthy subjects

involves recording increased blood flow & supply of oxygen to the active parts of the brain

good spatial but poor temporal resolution

it requires interpretation of coloured images

there could be a problem of statistical interpretations of models

CENTRAL NERVOUS SYSTEM

(CNS)

PERIPHERALNERVOUS SYSTEM

(PNS)

NERVOUS SYSTEM

SOMATIC NERVOUS SYSTEM

(voluntary)

AUTONOMIC NERVOUS SYSTEM

(involuntary)SENSORY AND MOTOR NEURONES TO / FROM

SKELETAL MUSCLE

MOTOR NEURONES TO INTERNAL ORGANS

SYMPATHETIC NERVOUS SYSTEM

(involuntary)

PARASYMPATHETIC NERVOUS SYSTEM

(involuntary)

CONTROLS ORGANS IN TIMES OR STRESS

CONTROLS ORGANS WHEN BODY IS AT REST

The Organisation of the Nervous System

BRAIN AND SPINAL CORD

PERIPHERAL NS

Peripheral Nervous System (PNS)

PUPILS

SALIVARY GLANDS

HEART

BRONCHI

LIVER

STOMACH/ SMALL INTESTINE

ADRENAL GLAND / KIDNEYS

LARGE INTESTINE

BLADDER / GENITALS

The sympathetic nervous system (SNS)

Series of

ganglia

The cell bodies of its motor neurones lie in ganglia outside the spinal cord

It prepares the body for action

The transmitter secreted at these synapses is usually nor adrenaline – which stimulates organ activity

The heart beats faster, eyes get wider i.e. the pupil dilates to improve vision, “sinking” feeling in the stomach due to decreased blood supply to the gut.

SNS functions are FIGHT OR FLIGHT

EYE

SALIVARY GLANDS

BRONCHI

HEART

STOMACH PYLORIC SPHINCTERPANCREAS

LARGE INTESTINE, ANAL SPHINCTER

GENITALS

BLADDER

The Parasympathetic Nervous System (PNS)

All nerve pathways begin in the brain, or at the top or bottom of the spinal cord

The neurones keep going ‘till right inside the organ. Here they synapse with a motor neurone

The transmitter secreted at these synapses is acetylcholine and this has an inhibitory effect on the organHeart rate decreases, increased blood flow to the gut, constricts pupil to protect retina

PNS functions are REST AND DIGEST

Effects of Sympathetic & Parasympathetic NS

Sympathetic NS Parasympathetic NSsecretes noradrenaline

or norepinephrineaccelerates heart ratecauses widening

(dilation) of the pupilsin gut, stomach,

pancreas, intestines & salivary glands inhibits activity by constricting blood flow to arterioles

relaxes (i.e. dilates) bronchi

inhibits emptying of bladder

secretes acetylcholine

slows down heart ratecauses narrowing

(constriction) of the pupilsin gut, stomach, pancreas,

intestines & salivary glands stimulates activity by maintaining normal blood flow to arterioles

constricts bronchipromotes emptying of

bladder

Sympathetic & Parasympathetic control of heart rate

heart muscles contract without nervous stimulation i.e. myogenic contractions

SA node is the pacemaker, it generates heart beat (i.e. initiates each cardiac cycle)

epinephrine (adrenalin) speeds up the heart rate

sympathetic & parasympathetic nervous system control the heart rate

sympathetic NS speeds up heart rate while

Parasympathetic NS slows heart rate (back to normal rate)

Sympathetic & parasympathetic control of movements of the iris

sympathetic and parasympathetic nervous systems are part of the autonomic system

they have antagonistic actionsparasympathetic neurons control

the circular muscle of the iris while sympathetic neurons control the radial muscle of iris

stimulation of radial muscles of the iris by sympathetic NS causes the muscles to contract

dilating (widening) the pupilstimulation of circular muscles of

the iris by parasympathetic NS causes the muscles to contract

constricting (narrowing) the pupil

Sympathetic & Parasympathetic control of blood flow to the gut

sympathetic and parasympathetic nervous systems are part of the autonomic system

they have antagonistic actionssmooth muscle in blood vessels

(arterioles) are controlled by sympathetic & parasympathetic nerves

sympathetic NS release norepinephrine (noradrenaline)

which constricts blood vessels (arterioles) to the gut

decreasing blood flow to gutparasympathetic NS release

acetylcholinewhich dilates blood vessels

(arterioles) to the gut increasing blood flow to gut

Pupil reflex pupil reflex is rapid unconscious response to change in light intensity

it controls amount of light entering eye to prevent damage to retina

it allows sufficient light in for vision

impulses from retina are monitored for intensity by brain stem (part of the brain that include the medulla oblongata)

in bright light, circular muscles in iris contract causing pupil to constrict

in dim, light longitudinal (radial) muscles in iris contract causing pupil dilation

constriction is caused by parasympathetic NS while dilation is caused by action of sympathetic NS

Concept of brain death brain death is a legal medical definition

of death some cases of coma are irreversible while

other cases may recover damage in the medulla oblongata is

generally permanent doctors have to diagnose damage to the

medulla oblongata to decide treatment they use tests of brain stem (part of the

brain that include the medulla oblongata) function to decide whether to preserve patient’s life, without brain stem function life cannot continue

they test pupil reflex by shining light into the eye

if pupils do not constrict with light, this suggests brain death

more than one test used to diagnose brain death including lack of response to pain or cranial reflexes

legal & ethical definition needed for organ donation & long term use of life-support machines may be inappropriate

Use of the pupil reflex in testing of brain death

pupil reflex is a brain stem reflex i.e. shows activity in the medulla oblongata

pupil reflex must be absent if the brain is death

pupil reflex is possible in coma victims where motor function is absent

pupil reflex alone is not enough to diagnose brain death

other criteria of testing brain death include coma, absence of response to pain in all extremities, absence of brain stem reflexes, lack of respiratory movements

some cases of coma are irreversible while some cases may recover

doctors need to diagnose damage to decide treatment, long-term life support or organ donation

How pain is perceived impulses passed from pain

receptors to sensory areas of the cerebral cortex

where pain is perceived (i.e. feelings of pain in the areas of the cerebral cortex)

awareness of pain allows one to avoid acute injury & noxious substances

the pituitary secretes endorphins into the blood stream and the hypothalamus secretes them into the brain to block the receptor molecules at synapses

in so doing, the pain is reduced

How endorphins act as painkillersendorphins released by

pituitary gland during stress, injury or exercise

endorphins block transmission of impulses at synapses involved in pain perception

they bind to receptors in the membrane of neurons involved in sending pain signal

by blocking the release of neurotransmitters

Self Assessment Questions (SAQs)Outline the functions of each

of the following parts of the brain: medulla oblongata; cerebellum; hypothalamus; pituitary gland & cerebral hemispheres.

Explain how lesions & fMRI scanning can be used in the identifying brain part & their functions.

Explain sympathetic and parasympathetic control of the heart rate.

Explain sympathetic and parasympathetic control of movements of the iris .

Explain sympathetic and parasympathetic control of blood flow to the gut.

Outline pupil reflexDiscuss the concept of

brain death.Outline the use of the

pupil reflex in testing brain death.

Outline how pain is perceived.

Explain how endorphins act as painkillers.