The human brain

THE HUMAN BRAIN

Option E.5

ASSESSMENT STATEMENTS E.5.1 Label, on a diagram of the human brain, the medulla

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

E.5.2 Outline the function of each of the parts of the brain listed above.

E.5.3 Explain how animal experiments, lesion 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 the 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.

Cerebral hemispheres act as the integrating center for high complex functions such as learning, memory and emotions.

Hypothalamus maintains homeostasis, coordinating the nervous and the endocrine systems, secreting hormones of the posterior pituitary, and releasing factors regulating the anterior pituitary.

Pituitary gland has two lobes. The posterior lobe stores and releases hormones produced by the hypothalamus and the anterior lobe. It also produces and secretes hormones regulating many body functions.

Medulla oblongata controls automatic and homeostatic activities, such as swallowing, digestion, vomiting, breathing, and heart activity.

Cerebellum has two hemispheres and a highly folded surface. It coordinates unconscious functions, such as movement and balance.

IDENTIFICATION OF BRAIN PARTS INVOLVED IN

SPECIFIC FUNCTIONS

BRAIN LESIONSarea of tissue that has been damaged through injury or disease

RIGHT AND LEFT HEMISPHERES Brain divided into right and left hemispheres Connected by a thick band of axons called the

corpus callosum Left hemisphere

Contains areas important for communication If damaged, person may have difficulty speaking

or doing complicated movements Right hemisphere

Specializes in receiving and analyzing information which comes in through all of our senses

If damaged, person may have difficulty identifying faces and locating an object correctly in space or even identifying melodies

EARLY EXPERIMENTS Mid 1800s: Neurologists observed that

people who had injuries on the left side had speech and language problems

People who had injuries in the same areas but on the right side of the brain had no language problems

Two areas of brain important for language are named for those neurologists: Injury to Brocca’s interferes with the ability to

volcalize words Injury to Wernicke’s area affects the ability to

put words into sentences

1960s: group of scientists interested in patients who had undergone surgery to sever their corpus callosum to relieve symptoms of epilepsy (the optic chiasma remains intact)

The Split Brain Experiment

FUNCTIONAL MAGNETIC RESONANCE IMAGING fMRI

fMRI uses radio waves and a strong magnetic field, not X-rays

Enables scientists to see the blood flow in the brain as it is occurring

Makes movies of what is going on in the brain as the subject performs tasks or is exposed to various stimuli

Can determine with some precision when regions of the brain become active and how long they remain active

fMRI used by doctors to determine:A plan for surgeryTreatment for a strokePlacement of radiation therapy for a brain

tumorEffects of degenerative brain disease such

as Alzheimer’sDiagnosing how a diseased or injured brain

is working

Animal Experiments Expose animals to addictive substances

in controlled situations Respond similarly to human:

Want more and more of the substanceSpend lots of time and energy getting itKeep taking it despite adverse conditionsHave withdrawal symptoms on withdrawal

of substanceGo back to the substance when stressedGo back to the substance with another

exposure to that substance

Animal model for addiction? Animal is trained to press a lever to get

a reward Animal is given an injection of the

addictive substance as it pushes the lever

Two levers available: one gives substance, one does not

If substance is reinforcing, animal will seek to repeat the experience by pushing that lever much more frequently and therefore, support the hypothesis that substance is addictive

Animal experiments can help us to determine way in which drugs promote abuse

Animal experiments cannot replicate the complete interaction of humans and drugs

Social factors can play a role Addiction studies

Sympathetic and parasympathetic control Peripheral nervous system considered in

two parts, somatic system and autonomic system

Somatic system takes sensory information from sensory receptors to the CNS and then sends back motor commands from the CNS to the muscles

Autonomic system is involuntary and regulates activities of the glands, smooth muscle, and the heart.Sympathetic systemParasympathetic system

Comparison chartSympathetic system Parasympathetic system

Important in emergency Important in returning to normal

Response is “fight or flight”

Response is to relax

Neurotransmitter is noradrenaline

Neurotransmitter is acetylcholine

Excitatory inhibitory

Antagonistic systems Sympathetic system associated with

fight or flightYou need quick energySystem increases heart rate, stroke volume

to supply more glucose and oxygenDilates bronchi to give more oxygenDilates pupil by contracting radial muscles

surrounding irisBlood to gut is restricted by contraction

smooth muscle of blood vessels carrying blood there

Parasympathetic takes over in a relaxed stateNerves return the system to normalPupil of eye constrictsHeart rate slows, stroke volume is reducedBlood returns to the digestive systemSmooth muscle of the blood vessels relax

Pupil reflex Close your eyes and then suddenly open

them Pupil will close in response to the

sudden input of light as the eyes open Cranial reflex Iris contains two sets of smooth muscle

to open and close the pupil Response caused by acetylcholine Atropine stops the action of

acetylcholine

PATHWAY OF THE PUPIL REFLEX Optic nerve receives the messages from

the retina in the back of the eye Optic nerve connects with the pretectal

nucleus of the brain stem From the pretectal nucleus, a message

is sent to the Edinger-Westphal nucleus whose axons run along the oculomotor nerves back to the eye

Oculomotor nerves synapses on the ciliary ganglion

Axons of the ganglion stimulate the circular muscle of the iris so it contracts

BRAIN DEATH Def: that time when a physician has

determined that the brain and brain stem have irreversibly lost all neurological function

Patients in a coma have neurological signs that can be measured

EXAMINATIONS FOR BRAIN DEATH INCLUDES CHECKING: Movement of extremities – if arms and legs are

raised and let fall, there must be no other movement or hesitation in the fall

Eye movement – eyes must remain fixed showing lack of brain-to-motor-nerve reflex (as the head is turned there is no rolling motion of the eyes)

Corneal reflex – this must be absent (when a cotton swab is dragged over the cornea, the eye does not blink)

Pupil reflex – this must be absent (pupils do not constrict in response to a very bright light shone into both eyes)

Gag reflex – this must be absent (insertion of a small tube into the throat of a comatose patient will cause a gag reflex)

Respiration(breathing) response – this must be absent (if the patient is removed from a ventilator, the dead brain gives no response)

AFTER BEING DECLARED BRAIN DEAD Can still have spinal reflexes such as the

knee jerk reflex Spinal reflexes do not involve the brain A short reflex motion can still be

exhibited if the hand or foot is touched in a certain manner

Further tests:Electroencephalogram (EEG)Cerebral blood flow (CBF)

EEG Measures brain activity in microvolts Very sensitive test Some electrical activity is shown on the

EEG if a patient is in a deep coma Life after death

CBF Radioactive isotope is injected into the

bloodstream Radioactive counter is then placed over

the head for about 30 minutes If no activity is detected, this is

conclusive evidence of brain death

PERCEPTION OF PAIN Pain signals are carried by peripheral nerve

fibers from all over the body to the spinal cord and relayed to the sensory area of the brain

Peripheral fibers connect with pain receptors called nocioreceptors

Nocioreceptors are capable of sensing excess heat, pressure or chemicals from injured tissues

Nocioreceptors are located in the skin and also in the muscle, bones, joints and membranes around your organs

Nerve impulses of pain travel to the spinal cord

Ascending tracts in the spinal cord send the messages up to the brain

RESPONSE OF PAIN BY CEREBRAL CORTEX Can tell the muscles to stop the action

which is causing the pain stimulus Can alert the autonomic nervous system

if the pain requires change in heart rate or breathing

Can direct other brain cells to release pain-suppressing endorphins

ENDORPHINS First discovered by scientists studying

opium addiction Found receptors for the opiates,

morphine and heroin in brain cells Scientists found that the molecules

made by plants were mimicking endorphins

Endorphins are CNS neurotransmitters with pain-relieving properties

Small peptides which bind to opiate receptors and block the transmission of impulses at synapses involved in pain perception