The Nervous System

1. The Nervous System Chapter 10

2. The human nervous system - overview • Generally, the more complex an animal is, the more complex the organisation and coordination of the nervous system. • Humans are the most complex. • Comprised of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS) which are interconnected.

3. The central and peripheral nervous systems. • The CNS: • Includes the nerve cells in the brain and spinal cord and the nerve cells connecting to these to other parts of the body. • Is the control centre of the nervous system. • The PNS: • All other nerve cells that lie wholly in or partially outside the CNS.

4. The central and peripheral nervous systems.

5. Further divisions in the CNS and PNS

6. Parts and functions of the CNS and PNS Reference: Nature of Biology Book 1; p313

7. The CNS functions The CNS: • Cerebral cortex • Motor activity • Sensory input • Speech • Sight • Hearing Hypothalamus: • Information relating to well-being and functions in maintaining homeostasis.

8. Cerebellum: • Coordination of muscles, posture, balance and movement. Brainstem: • Control of the heart, blood vessels and lung ventilation. • Stores information

9. Sensory and motor areas of the brain

10. The PNS Functions • The motor component can be divided into two sub-systems: • Somatic (voluntary) – controls movement which you can control. (e.g. Movement using skeletal muscles). • Autonomic (involuntary) – controls movements which you cannot control.

11. The autonomic nervous system • The autonomic system is involved in unconscious responses: • Constriction of the pupils • Secretion from glands • Changes in heart rate • Signals to smooth muscle, the heart and glandular tissues • Regulates activities of the digestive, cardiovascular, excretory, respiratory and endocrine systems

12. Sub-dividing the autonomic nervous system • The autonomic system can be further divided into: • Sympathetic: • Increases energy use • Prepares the body for emergency action by increasing heart and metabolic rate (‘fight or flight’) • Parasympathetic: • Involved in activities which conserve energy – digestion and slowing heart rate. • Enteric: • Network of neurons and reflexes within the wall of the stomach.

13. Overview- The nervous system

14. Neuronal structure

15. Neuronal structure • Sensory, motor and interneurons have different structures and shape but all comprise of the same parts. • Cell body – performs normal cell functions (e.g. Making proteins) • Dendrites – projections which carry electrical impulses to the cell body. • Axon – transmits impulses from the cell body to the axon terminals • Myelin sheath – fatty coating which insulates axon.

16. Neurons have numerous dendrites and axon terminals but only one axon. • Nerves – bunches of neurons wrapped in a connective tissue tube.

18. Structure of neuron Nature of Biology Book 1; p314

19. Structure of a sensory and motor neuron

20. Different nerve cells • Nerve cell = neuron • Three types of neurons: • Affector neurons: • = sensory neurons • Receptors detect change in external or internal environment. • Transmit information to the CNS through electrical impulses. • Effector neurons: • = motor neurons • Transmit impulses from the CNS to muscle cells or glands triggering a response.

21. Connecting neurons: • = interneurons • Normally located in CNS • Link sensory and motor neurons

22. Location of neurons in the CNS and PNS Reference: Nature of Biology 1; p314

23. Location and orientation of interneurons, sensory and motor neurons (Refer to the diagram on the previous slide.) • Sensory neurons: • Axon terminals in the CNS • Cell body and receptor end in the PNS • Interneurons: • Normally all in the CNS • Motor neurons: • Dendrites and cell body in the CNS • Axon terminal in the PNS

24. How the nervous system functions – an overview • A stimulus (a condition or signal) is detected by a receptor (a structure which detects a stimulus or signal). • The signal is transmitted by sensory neurones to the CNS. • The CNS sends response signals to effectors via motor neurons. • Effectors are usually muscles and glands which carry out a response.

25. Neuronal functioning

26. Neuronal structure • A single neuron can connect with as many 10 000 other interneurons. • Neurons are bundled together to form nerves – 100’s-1000’s bound together all pointing in the same direction.

27. Neuronal function • Three basic steps: • Generation of electrical impulse (action potential) • Conduction of an impulse along axons • Chemical transmission of a signal to another cell across a synapse.

28. Depolarisation • Sensory neurons respond to stimuli by depolarisation – the inside of the cell becomes less negative than the outside. • If depolarisation is large enough an action potential is produced.

29. The action potential • If depolarisation is sufficient Na+ channels in the membrane open and Na+ flood in along the concentration gradient.  Creates an action potential which moves down the axon.  The neuron becomes more +  K+ ions channels open and diffuse out of the cell along concentration gradient causing the neuron to become briefly + and then –

30. Refractory period – once an action potential is produced, the neuron is inactive for a period of time. • Nerve impulse animation http://www.youtube.com/watch?v=dSkxlpNs3tU

31. Between neurons • Between the presynaptic terminal of one axon and the postsynaptic region of the adjacent cell is a small gap called the synapse. • Electrical signals are passed through the synapse by neurotransmitters. • Neurotransmitters bind to specific postsynaptic receptors. • They provoke or inhibit activity in the adjoining neuron. • Once activated they are either taken up terminal or broken down by enzymes.

32. Synapses and neurotransmitters http://www.youtube.com/watch?v=FR4S1BqdFG4

33. Reflex actions

34. Reflex responses • The simplest type of nervous response (may involve only a few cells) usually to avoid pain, danger or predation. • Reflex responses are unconscious and automatic. • Reflex arc: The pathway of a reflex impulse.

35. The knee-jerk response • Simplest reflex involving only two neurons. (monosynaptic) • A receptor sends a signal via a sensory neuron to the spinal chord which transmits a signal through a motor neuron to the effector muscle.

36. The knee jerk reflex response

37. The withdrawal reflex The withdrawal reflex is triggered by pain receptors in the skin and involves sensory, interneurons and motor neurons.

38. Other reflex responses • Maintaining posture. • Homeostatic regulation of systems such as the circulatory system. • E.g. Baroreceptor-heart rate reflex helps maintain constant blood pressure.

39. Interneurons, reflexes and complexity • When muscles contract, interneurons need to send messages to opposing muscle not to contract. (e.g. Bicep and tricep) • Many reflexes are more complex • Many movements involve complex balances between opposing muscles.

40. Sense organs

41. Sense organs • The five senses • Sense organs – process sensory information • General senses – pressure, pain and joint position • Visceral receptors – detect internal states

42. Types of receptors • Photoreceptors – vision • Chemoreceptors – taste, smell, communication • Mechanoreceptors – hearing, balance, pressure, touch • Thermoreceptors - heat

47. `