3.5.3 Responses in the Human Nervous System

1. 3.5.3 Responses in the Human Nervous System

2. Humans use two systems to respond to stimuli: the nervous system for fast action and the hormonal system for slower responses.

3. A stimulus is any change in your environment • e.g. a flash of light, a noise, a fly landing on your nose.

5. Nervous system • Central NS (Brain & Spinal Cord) • Peripheral NS (Cranial & spinal nerves) • Somatic NS (voluntary control of external environment) • Autonomic NS (involuntary control of external environment)

6. Central Nervous System

7. The Nervous System

8. Functions: • Detect changes in external and internal environment. • Interpret these changes and respond in a coordinated manner. • Store information gained by experience.

9. The CNS processes messages and controls our responses. • The PNS carries messages to and from the CNS.

10. Receptors are cells sensitive to a stimulus e.g. pain receptors, eye.

11. An effector responds to a stimulus e.g. a muscle contracts or a gland

12. Nerve • A nerve (nerve fibre) is a bundle of neurons (nerve cells) - both sensory and motor neurons.

13. There are three types of Neuron : • Sensory neuron –carries messages from the sense organ tothe central nervous system (CNS). • Interneuron –connects sensory and motor neurons and socarries messages within the CNS. • Motor neuron –carries impulses from the CNS to muscles andglands.

15. Sensory Neuron

16. Motor neuron

17. Inter Neuron

18. 2012 q13 a b

20. NS ----- Co-ordination & Response • A stimulus is any change in your environment • A receptor is a nerve cell that detects the stimulus • A neuron is a specialised cell that carries electrical messages(impulses) around the body. • An impulse is an electrical message that is carried along aneuron.

21. Movement of impulse • Threshold: This is the minimum stimulus required to start a nervous impulse. • All or Nothing Law: The size of the stimulus (provided it is above the threshold level) has no effect on the size of the impulse. • Either a full message is carried or no message.

22. Refractory period • Is a short timespan after a neuron has carried an impulse during which a stimulus fails to cause a response.

24. Synapse • A region where two neurons come into close contact • Usually found between the axon terminals of one neuron and the dendrites of another

25. Synaptic cleft is the tiny gap between the two neurons at a synapse

28. Electrical impulses cannot simply cross a synapse

30. 1. Activation of Neurotransmitter • Ions stimulate the neurotransmitter swellings to release a chemical substance called neurotransmitters, that diffuse rapidly across the synaptic cleft. • Neurotransmitters are contained in vesicles in the neurotransmitter swellings. • Once released across the synapse, it then combines with receptors on the other neuron and causes the electrical impulse to be regenerated.

31. 2. Inactivation of neurotransmitter • The neurotransmitter is then broken down by enzymes. • They are reabsorbed in the neurotransmitter swellings and allows them to be recycled.

32. Diagram

33. Synaptic cleft—the gap between two neurons, bridged bychemicals (neurotransmitters). • Neurotransmitter—chemical released across a synaptic cleft tocarry a signal from one neuron to another. • The chemical is then destroyed or removed

34. Benefits? • Allows transit of Impulses. • Permit impulses in one direction only – neurotransmitters only present on one side of the synapse. • Allow localisation of a response rather than a total body response (chaos!). • Protect against over-stimulation, as they will slow down if overloaded. • Their complicated interconnections allow for learing and memory. • They ignore low-level stimulations – effectively removing ‘background noise’ from nervous system.

35. Disadvantages • Synapses are relatively slow and their number is often minimised by developing long axons and dendrons. • Allows chemicals to affect N.S. e.g. hallucinatory drugs, painkillers, anaesthetics and certain poisons.

36. Effect of Neurotransmitters • Acetycholine released from motor neurons, triggers muscle contraction. It has an inhibitory effect on cardiac muscle, resulting in a decreased heart rate. • Noradrenaline, serotonin and dopamine affect mood. Their imbalance has been linked to depression, attention deficit disorder (ADD) and psychosis (where behaviour and personality are altered e.g. schizophrenia). Antidepressants and other mood-affecting drugs work by altering the levels of these neurotransmitters in the brain.

37. Drugs - many affect transmission of impulses across synapse by increasing/decreasing the production of the neurotransmitter or by affecting the rate of breakdown of the neurotransmitter

38. Ectasy affects nerve cells that produce serotonin. It causes the nerve cells to release all the stored serotonin at once – this can cause damage to the axons. Serotonin regulates temp. as well as mood. If body temp. reaches 430C (dancing) the blood starts to coagulate, and death can follow. Ectasy affects memory too.

39. Cannabis. Marijuana – a hallucinogen – (from the dried leaves) and hashish (resin from the flowers). In low doses it is a depressant – impairs co-ordination, perception, timing and short-term memory. It slows down motor activity and causes mild euphoria. It also causes disorientation, increased anxiety (panic), delusions (paranoia) and hallucinations. Over time, marijuana can suppress the immune system, impair mental functions and lower sperm and testosterone levels.

40. Cocaine, interferes with the normal breakdown of dopamine. Dopamine is involved with pleasurable feelings. If it is not broken down the synapse keeps on transmitting messages and euphoria follows. The body reduces its production of dopamine which results in addiction as the user has to take more cocaine to produce enough dopamine to feel ‘normal’. Body becomes tolerant to cocaine.

41. The Brain • The brain is composed of over 100 billion neurons, eachreceiving messages simultaneously from thousands of otherneurons. • The brain is protected by the skull bones, meninges (threemembranes) and cerebrospinal fluid.

43. The cerebrum is our conscious brain, with different parts havingdifferent jobs to do.

45. 4) The brain decides to move away the hand 3) Here another sensory neuron carries the signal to the brain 5) This impulse is sent by MOTOR NEURONS to the hand muscles (the effectors) via the spinal chord… 2) The impulse is carried by SENSORY NEURONS to the spinal chord 1) Receptors in your skin detect a stimulus 6) Which then moves the hand away Stimulus Receptor Sensory NeuronCoordinator Motor Neuron Effector Response Conscious actions A CONSCIOUS ACTION IS ONE WHERE THE BRAIN MAKES A CONSIDERED RESPONSE. Here’s what happens:

46. Medulla oblongata • The cerebellum co-ordinates processes that we have learnedto do automatically, such as speaking. cerebellum • The medulla oblongata co-ordinates involuntary, automaticprocesses—such as breathing, heartbeat.

47. The hypothalmus is the centre for the regulation of the internalorgans. • The pituitary ‘[master] gland secretes hormones that stimulateother glands to release their hormones.

48. The spinal cord is well protected by the vertebrae, meninges(three membranes) and cerebrospinal fluid. • It transmits impulses to and from the brain and controls manyreflex actions.

49. A cross-section through the spinal cord shows a small central canal, filled with cerebrospinal fluid, surrounded by an area of grey matter, shaped somewhat like the letter H.

50. Grey matter contains cell bodies and dendrites (regions of a neuron that have no white myelin covering). • Outside the grey matter, the spinal cord consists of whitematter (containing axons only).