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E.5 The Human Brain

 

E.5

The human brain (HL only)

E.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.

 
 
 
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E 5.1 You must be able to label on a diagram of the brain, the medulla oblongata, cerebellum, hypothalamus, pituitary gland and cerebral hemispheres.
 
 
E5.2  Outline the funcyions of the medulla oblongata, cerebellum, hypothalamus, pituitary gland and cerebral hemispheres.
 
 
 

 

 

 

 

 

E.5.2 Functions of brain regions

 

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.

 

FMRI

 

 

 

Functional magnetic resonance imaging: a form of magnetic resonance imaging of the brain that registers blood flow to functioning areas of the brain. It is a technique that helps to pinpoint functional areas of the brain. While the MRI is scanning, the patient is asked to perform a series of activities and movements, such as reading a list or tapping fingers. The areas of the brain that correlate to these movements and activities "light up" on the scan and create an image. This information is used by surgical navigation computers in the planning of incisions, skull openings and tumor removal to minimize neurological deficits.

Benefits are that it is a non invasive proceedure and the exact part of the brain used can be easily seen.

 

ANIMAL EXPERIMENTATION
 
Many experiments have been done on animals, usually primates where the skull is removed to get to the brain.
The brain must be kept alive so that it functions correctly
The animal must then be given tasks and the responses observed
 
There are many ethical objections to this, such as animal sacrifice for human gain
 
 

 

 

 LESIONS
Accidents to the brain, stroke and tumours can damage specific parts of the brain, the damaged area is a lesion. From the position of the lesion, we can dertemine functions of that part of the brain by observing what the patient can no longer do.
 
This proceedure is as a reult of an accident and can'y be used to test for brain function otherwise.
 

Tumour on the brain

 

 

E5.4 Explain sympathetic and parasympathetic control of the heart rate, movements of the iris and flow of blood to the gut.
 
 
 
 

Target

Sympathetic Nervous System

FLIGHT!!!

Parasympathetic Nervous System

Heart rate

Increases heart rate.

Decrease in Heart rate 

Blood vessels

Decreases diameter of major arteries therefore increasing blood pressure 

Increases diameter of major arteries therefore decreasing blood pressure.

Flow of blood to intestines

Decreased flow to intestines 

Increased flow to intestines 

 

Iris movement

Iris muscles cause pupil to dilate 

Iris muscles cause pupil to constrict. 

 
E5.5 Explain the pupil reflex
  • If light is shone into ONE eye, BOTH pupils will constrict.
  • PHOTORECEPTORS in the retina detect the light stimulus.
  • Impulses are sent down the SENSORY neurone (in the optic nerve)to the brain.
  • The MEDULLA OBLONGATA (in the brain stem) processes the impulses.
  • Messages are sent to CIRCULAR muscles of IRIS and these CONTRACT. (Radial muscles Relax)
  • The pupil is constricted.
 

E 5.6 Discuss the concept of brain death and the use of the pupil reflex in testing for this.

 

  • The pupil reflex is used to test for brain death
  • The absence of a reflex indicates that the brain stem is no longer functioning
  • Brain death is stated as " no purposeful movement or response as the brain no longer responds to stimuli"
  • Cardiac death is where is no heart beat or ventilation but in this case machines can carry out these basic life processes.
  • It is important to define "death" as organs can be harvested and legally wills can be implemented.
 Reasons to use pupils reflex to idicate brain death                                            Reasons to not use pupils reflex to idicate brain death
 
 
 1. The pupil is a cranial reflex/ ANS reflex controlled by                              1.  Some drugs eg Barbiturates interfere with the pupils reflex
the brain stem. If this is lost then they are likely to be                                    2. Nerve damage/ retinal damage may interfere with the pupil reflex
brain dead.
 
 
 
E 5.7 Outline how pain is perceived and how endorphins can act as painkillers.
 
  1. A sensory neuron, relaying impulses from a pain receptor, forms a synapse with an associate neuron. Near this synapse, another synapse is found, coming from the pain control centre in the brain.
  2. The pain control centre in the brain sends out an impulse
  3. A synapse is stimulated to release endorphins.
  4. Endorphins are the body’s natural form of painkiller.
  5. The endorphins will interfere with the transmission of the impulse by attaching to receptors on the post-synaptic neuron.
  6. One example of endorphins are encephalins.
  7. Encephalins will block calcium channels, which stop calcium from flowing into the pre-synaptic knob.
  8. Without calcium, the vesicles containing neurotransmitters will not move to the pre-synaptic membrane, so the impulse will fade out.
  9. The synaptic transmission is blocked and prevents the pain from being felt.
 
 
Opioids bind to receptors on interneurons in the pain pathways in the central nervous system. The natural ligands for these receptors are two enkephalins

 

The two enkephalins are released at synapses on neurons involved in transmitting pain signals back to the brain. Instead of synapsing with a dendrite or cell body, the enkephalin synapse occurs close to the terminal of a pain-signaling neuron. The enkephalins hyperpolarize the postsynaptic membrane thus inhibiting it from transmitting these pain signals.

The drawing shows how this mechanism might work. The activation of enkephalin synapses suppresses the release of the neurotransmitter (substance P) used by the sensory neurons involved in the perception of chronic and/or intense pain.

The ability to perceive pain is vital. However, faced with massive, chronic, intractable pain, it makes sense to have a system that decreases its own sensitivity . Enkephalin synapses provide this intrinsic pain suppressing system.

Morphine and the other opioids bind these same receptors. This makes them excellent pain killers.

 
 
 
 
 
 

1. Sensation of painful stimuli

  •      Sensory neurons send action potential    These are:       
 
> nociceptors > mechanoreceptors > thermoreceptors
 
 
  •  There is a synapse between sensory neuron and ascending spinal neuron
     - ascending fibers transmit message of pain to various brain locations
     - neurotransmitter = substance P
     - excitatory neurotransmitter released by pre-synaptic neuron
     - increases Na+ permeability in post-synaptic membrane

2. Inhibition of pain

  • Endorphins (enkephalins) are natural pain-inhibiting neurotransmitters

 

  • They are produced by reticular formation in the brain ( a complex neural network in the central core of the brainstem)

 

  • Descending fibers synapse  and release endorphins into synapse between sensory neurons and ascending pain neurons

 

  • Endorphins have specific receptor sites on post-synaptic neurons

 

  • Inhibitory action > open K+ channels > close Ca+2 channels

 

  • This causes hyperpolarizion of the  post-synaptic membrane

 

  • They act as pain killers by inhibiting pain signals along ascending pain neurons

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