Nervous Tissue

1. Nervous Tissue Dr. Michael P. Gillespie

2. Nervous System • The nervous system is an intricate, highly organized network of billions of neurons and even more neuroglia. • The nervous system has a mass of only 2 kg (4.5 lb), which comprises approximately 3% of total body weight.

3. Structures of the Nervous System (CNS) • Brain (100 billion neurons) • Spinal cord (100 million neurons)

4. Structures of the Nervous System (PNS) • Spinal nerves (31 pairs) • Cranial nerves (12 pairs) • Ganglia (Masses of primarily neuron cell bodies) • Enteric plexuses (networks of neurons in the GI tract) • Sensory receptors (dendrites of sensory neurons)

6. Functions of the Nervous System • Sensory function – afferent neurons • Sensory receptors detect internal and external stimuli • Integrative function – interneurons • The nervous system processes sensory information and coordinates responses. It perceives stimuli. • Motor function – efferent neurons • The cells contacted by these neurons are called effectors (muscles and glands)

7. Organization of the Nervous System • Central nervous system • Brain • Spinal cord

8. Organization of the Nervous System • Peripheral nervous system • Cranial nerves and their branches • Spinal nerves and their branches • Ganglia • Sensory receptors • Somatic nervous system • Autonomic nervous system • Enteric nervous system

11. Somatic Nervous System (SNS) • Sensory neurons. • Motor neurons located in skeletal muscles. • The motor responses can be voluntarily controlled; therefore this part of the PNS is voluntary.

12. Autonomic Nervous System (ANS) • Sensory neurons from the autonomic sensory receptors in the viscera. • Motor neurons located in smooth muscle, cardiac muscle and glands. • These motor responses are NOT under conscious control; Therefore this part of the PNS is involuntary.

13. ANS Continued… • The motor portion of the ANS consists of sympathetic and parasympathetic divisions. • Both divisions typically have opposing actions.

15. Enteric Nervous System (ENS) • “The brain of the gut”. • Functions independently of the ANS and CNS, but communicates with it as well. • Enteric motor units govern contraction of the GI tract. • Involuntary.

17. Types of Nervous Tissue Cells • Neurons. • Sensing. • Thinking. • Remembering. • Controlling muscular activity. • Regulating glandular secretions. • Neuroglia. • Support, nourish, and protect neurons.

18. Neurons • Have the ability to produce action potentials or impulses (electrical excitability) in response to a stimulus. • An action potential is an electrical signal that propagates from one point to the next along the plasma membrane of a neuron. • A stimulus is any change in the environment that is strong enough to initiate an action potential.

19. Parts of a Neuron • Cell Body • Dendrites • Axon

20. Parts of a Neuron (Cell Body) • Cell body (perikaryon or soma). • Contains the nucleus surrounded by cytoplasm which contains the organelles. • Clusters of rough ER called Nissl bodies (produce proteins to grow and repair damaged nerves)

21. Parts of a Neuron (Nerve Fiber) • Nerve fiber – any neuronal process that emerges from the cell body of a neuron. • Dendrites • Axon

22. Parts of a Neuron (Dendrites) • Dendrites (= little trees). • The receiving (input) portion of a neuron. • Short, tapering, and highly branched.

23. Parts of a Neuron (Axon) • Axon (= axis). • Each nerve contains a single axon. • The axon propagates nerve impulses toward another neuron, muscle fiber, or gland cell. • Long, thin, cylindrical projection that often joins the cell body at a cone-shaped elevation called the axon hillock (= small hill). • The part of the axon closest to the hillock is the initial segment. • The junction between the axon hillock and the initial segment is the trigger zone (nerve impulses arise here). • The cytoplasm of the axon is the axoplasm and is surrounded by a plasma membrane known as the axolemma (lemma = sheath).

25. Synapse • The synapse is the site of communication between two neurons or between a neuron and an effector cell. • Synaptic end bulbs and varicosities contain synaptic vesicles that store a chemical neurotransmitter.

26. Axonal Transport • Slow axonal transport. • 1-5 mm per day. • Travels in one direction only – from cell body toward axon terminals. • Fast axonal transport. • 200 – 400 mm per day. • Uses proteins to move materials. • Travels in both directions.

27. Structural Diversity of Neurons • The cell body diameter can range in size from 5 micrometers (μm) (slightly smaller than a RBC) up to 135 μm (barely visible to the naked eye). • Dendritic branching patterns vary. • Axon length varies greatly as well. Some neurons have no axon, some are very short, and some run all the way from the toes to the lowest part of the brain.

28. Classification of Neurons • Both Structural and Functional features are used to classify neurons.

29. Structural Classifications of Neurons • Structurally, neurons are classified according to the number of processes extending from the cell body. • 3 Structural Classes • Multipolar neurons • Bipolar neurons • Unipolar neurons

30. Multipolar Neurons • One axon and several dendrites. • Most neurons of the brain and spinal cord are of this type.

31. Bipolar Neurons • Bipolar neurons. • One axon and one main dendrite. • Retina of the eye, inner ear, and the olfactory areas of the brain.

32. Unipolar Neurons • Unipolar neurons. • The axon and the dendrite fuse into a single process that divides into two branches. • The dendrites monitor a sensory stimulus such as touch, pressure, pain, heat, or stretching. • Called psuedounipolar neurons.

35. Functional Classification of Neurons • Functionally, neurons are classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS. • 3 Functional Classes • Sensory or afferent neurons • Motor of efferent neurons • Interneurons or association neurons

36. Sensory (Afferent) Neurons • Either contain sensory receptors or are located adjacent to sensory receptors that are separate cells. • Conveyed into the CNS through cranial or spinal nerves. • Most are unipolar.

37. Motor (Efferent) Neurons • Away from the CNS to effectors (muscles and glands). • Most are multipolar.

38. Interneurons (Association Neurons) • Mainly located within the CNS between sensory and motor neurons. • They process sensory information and elicit a motor response. • Most are multipolar.

39. Neuroglia • Half the volume of the CNS. • Generally, they are smaller than neurons, but 5 to 50 times more numerous. • They can multiply and divide. • Gliomas – brain tumors derived from glia.

40. Types of Neuroglia • CNS • Astrocytes • Oligodendrocytes • Microglia • Ependymal cells • PNS • Schwann cells • Satellite cells

41. Astrocytes • Star shaped cells with many processes. • Largest and most numerous of the neuroglia.

42. Astrocytes • Functions • Support neurons. • Processes wrap around capillaries to create a blood-brain barrier. • Regulate growth, migration and interconnection among neurons in the embryo. • Maintain chemical environment for impulse transmission • Influence formation of neural synapses.

43. Astrocytes

44. Astrocytes

45. Astrocytes

46. Oligodendrocytes • Similar to astrocytes, but smaller with fewer processes. • Function • Form and maintain the myelin sheath around the CNS axons.

47. Oligodendrocytes

48. Microglia • Small cells with slender processes giving off numerous spine like projections. • Function • Phagocytes.

49. Microglia

50. Ependymal Cells • Cuboidal to columnar cells. • Possess microvilli and cilia. • Functions • Produce cerebrospinal fluid (CSF) • Assist in circulation of CSF • Possibly monitor CSF