Animal Responses

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Why do animals respond to their environment

Animals need to respond to their environment to stay alive. This is done using nerves and hormones to control responses ranging from muscle actions to run away from a predator, to fine control of balance, posture and temperature regulation.

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Organisation of the nervous system

The nervous system is split into the Central Nervous System (CNS), which consists of the brain and the spinal cord, and the Peripheral Nervous System (PNS) which is made up of nerves from sense organs, to muscles and to glands. It is made of sensory and motor neurones and has a role in sending stimuli and controlling effectors. It conducts impulses to and from the CNS. 

The Peripheral Nervous System is further divided into the Somatic and Autonomic nervous systems. The Somatic nervous system controls concious activities, while the Autonomic nervous system controls unconsious activities. 

The Autonomic nervous system is itself subdivided into the sympathetic and parasympathetic nervous systems. 

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Roles of the autonomic nervous system

Sympathetic nervous system: most active in times of stress. The neurones of a pathway are linked at a ganglion just outside of the spinal cord. The pre-ganglion neurones are very short. The post-ganglion neurones secrete noradrenaline at the synapse between the neurone and the effector. The effects of action of the sympathetic nervous system include increased heart rate, pupil dilation, increased ventilation rate.

Parasympathetic nervous system: most active in sleep and relaxation. The neurones of a pathway are linked at a ganglion within the target tissue, so pre-ganglion neurones vary in length. Post-ganglion neurones secrete acetylcholine as the neurotransmitter at the synapse between neurone and effector. The effects of action include decreased heart rate and decreased ventilation rate.

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Structure and function of human brain

Cerebrum: Control of all higher order processes such as memory, language, emotions, thinking and planning (e.g. clapping hands)

Cerebellum: Control and coordination of movement and posture (e.g. automatically correcting balance when riding a bicycle)

Medulla oblongata: Control of breathing, heart rate (rate of contraction of cardiac muscle) and smooth muscle of the gut

Hypothalamus: Control of the autonomic nervous system and some endocrine glands (pituitary gland), regulation of body temperature

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Role of the brain and nervous system in movement

The conscious decision to move voluntarilty is initated in the cerebellum. Neurones from the cerebellum carry impulses to the motor areas so that motor output to the effectors can be adjusted appropriately in these requirements. 

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Skeletal muscles and coordinated movement

Coordinated and appropriate movement requires the controlled action of skeletal muscles about joints. This can be seen in the movement of the elblow joint, the bicep contracts and the tricep relaxes, causing the lower arm to bend upwards at the elbow. The bicep relaxes and the tricep contracts, lowering the arm back down from the elbow.

1. Impulses arriving at the neuromuscular junction cause vesicles to fuse with the presynaptic membrane and to release acetylcholine into the gap by exocytosis.

2. Acetylcholine binds to receptors on the muscle fibre membrane (sarcolemma) causing depolarisation.

3. Waves of depolarisation travel down tubules (T system, T tubules).

4. Depolarisation of the T tubules leads to the release of Ca2+ ions from stores in the sarcoplasmic reticulum.

5. Ca2+ binds to proteins in the muscle, which leads to contraction.

6. Acetylcholinesterase in the gap rapidly breaks down acetylcholine so that contraction only occurs when impulses arrive continuously

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The sliding filament model of muscle contraction

1. The Ca2+ ions bind to troponin molecules, causing them to move, moving the tropomyosin out of the gap in the actin filament, allowing the myosin head groups to attach.

2. Myosin head groups attach to the surrounding actin filaments forming a cross-bridge.

3. The head group then bends, causing the thin filament (actin) to be pulled along and so overlap more with the thick filament (myosin). This is the power stroke. ADP and Pi are released. 

4. The cross bridge is broken as new ATP attaches to the myosin head.

5. The head group moves backwards as ATP is hydrolysed to ADP and Pi. It can form a cross bridge with the thin filament and bend again.

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Role of ATP in muscular contractions

Energy from ATP is required to break the cross bridge connection and re-set the myosin head forwards.

It is maintained by aerobic respiration in mitochondria, anaerobic respiration in the sarcoplasm, and the transfer for a phosphate group from creatine phosphate to ADP in the sarcoplasm.

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Synapses and neuromuscular junctions

Structure - similarity: Both have mitochondria, vesicles and postsynaptic receptors.

Structure - difference: Neuromuscular junction membrane is wavy, the receptors are different shapes, the enzymes are in different plances.

Function - similarity: Both release neurotransmitter which crosses the gap, both changes the potential difference (depolarise) the post-synaptic membrane, both have enzymes which break down the neurotransmitter.

Function - difference: Neurotransmitters may be different (acetylcholine in neuromuscular junction, dopamine in brain), neuromuscular junctions cause muscle contractions, synpases cause nerve impulses, enzymes may be difference (acetylcholinesterase in neuromuscular junction, monoamine oxidase in the brain).

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Voluntary, involuntary and cardiac muscles

Voluntary (skeletal) muscle: Striated (bands of actin and myosin), cylindrical cells, multinucleate. To move bones/skeleton/joints/limbs.

Involuntary (smooth) muscle: Unstriated, spindle-shaped cells, uninucleate. Control diameter of arteries/arterioles/bronchi/bronchioles, peristalsis, contractions of uterus, control pupil size.

Cardiac muscle: Striated, branched cells, uninucleate, intercalated (connected with other cells). To pump blood

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How responses to stimuli are coordinated

Responses to environmental stimuli in mammals are coordinated by nervous and endocrine systems.

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'Fight or flight' response

  • Sensory neurones from the somatic nervous system carry impulses from receptors to the sensory areas of the cerebrum of the brain, giving information about the danger in the environment
  • Sympathetic (motor neurones) nervous system is stimulated causing the neurotransmitter noradrenaline to be released at neuromuscular junctions
  • Adrenaline is secreted and released into the blood from the adrenal medulla
  • Adrenaline and noradrenaline bind to receptors on the target tissue
  • Sinoatrial node increases rate of firing
  • The heart beats faster and more forcefully
  • Bloog flow to the gut and skin is reduced, thus reducing gut secretions and making the skin pale, blood pressure increases
  • Smooth muscle in the gut relaxes and peristalsis slows down
  • Smooth muscle in airways relaxes and widens
  • Pupils dilate
  • Intercostal muscles and diaphragm contract faster, increasing rate and depth of ventilation
  • Bloog flow to the skeletal muscle increases, so they are primed for action

Fight or flight in a dog would cause ears to be laid back and pupils dilated. It would adopt a tensed and lower posture with the hair on its neck standing up. its mouth would be open, showing its teeth and its tail standing up.

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