Introduction Remember studying Nervous Tissue as a Basic Tissue? 1. Epithelial tissue 2. Connective tissues 3. Muscle tissue 4. Nervous tissue Well, during this lab you will review these tissues as observed in both the: i. Central Nervous System (CNS)- the brain and spinal cord. ii. Peripheral Nervous System (PNS) - peripheral nerves/ganglia outside the CNS. Learning objectives and activities Using the Virtual Slidebox: A Recognize nervous tissue in histological sections and discuss the organization/function of the cells present. B Examine the features of peripheral nerves in both longitudinal and cross section that allow you to identify them in tissue sections. C Differentiate between sensory and autonomic (sympathetic/parasympathetic) ganglia. D Identify the different functional regions and cells of the spinal cord. E Identify the major layers and cells of the cerebellum ad the cerebral cortex. F Complete the self-quiz to test your understanding and master your learning. Despite its complexities, the nervous system is composed entirely of two cell types: a network of nerve cells (neurons) that transmit signals throughout the nervous system and the glial cells that support them. Neurons Examine Slide 1 (16) to investigate the basic structure of a neuron ii i. Cell body (perikaryon). Most cell bodies are located in the central nervous system (brain and spinal cord) or in ganglia of the peripheral nervous i system. They contain darkly stained granules called Nissl bodies which are the ribosomes used to manufacture iii neurotransmitter in the cell. iv ii. Dendrites Dendrites extend from the cell body. These are short, highly branched structures that receive nerve impulses from other neurons. Nissl bodies can be seen extending into the dendrites. iii. Nucleus and nucleolus. Neurons have a large nucleus with prominent nucleolus. The ribosomes seen in the cell body are assembled in the nucleolus. iv. Axon hillock and axon The axon hillock is where the axon branches from the cell body. It contains the Golgi apparatus and is therefore devoid of Nissl bodies. The axon is a single 'fiber' that transmits impulses. Most neurons have an axon surrounded by fat – the myelin sheath – which assists propagation of electrical impulses along the axon. There are three basic classes of neuron i. Multipolar neurons Located in CNS Located in PNS Have multiple dendrites and a long axonal process. They convey either somatic motor or visceral/autonomic Skeletal muscle motor information from the CNS. Somatic motor Examine the following slides to see examples of multipolar neurons: Pre-ganglionic Slide 1 (16) : somatic motor Autonomic ganglion Slide 2a (19) : somatic motor Smooth muscle (e.g. vessels or Slide 3a (21) : visceral motor digestive tract) Post-ganglionic Visceral motor ii. Pseudounipolar neurons Have one axon that divides close to the cell body to form two long axonal branches. Dorsal root ganglion They convey sensory information to the CNS and their cell bodies are located within the dorsal root ganglion (or cranial nerve ganglia). Examine Slide 4a (20) to see pseudounipolar (sensory) neurons in a dorsal root ganglion. iii. Bipolar neurons Have one axon and one dendrite. They convey special sensation to the CNS. Examine Slide 5 (82) to see bipolar neurons in the retina. Glial cells Glial cells support and protect the neurons. i. In the CNS There are many different types of glial cell: - oligodendrocytes - the equivalent of Schwann cells in the CNS - microglia - specialized macrophages that protect CNS neurons - astrocytes - regulate chemical environment in CNS (blood-brain barrier) - ependymal cells - line ventricles of brain and central canal of spinal cord Together they form the ‘connective tissue’ of the CNS known as ‘neuropil’. With the exception of ependymal cells it is very difficult to specifically identify the different types of glial cell within the central nervous system without specific stains. Therefore you will not be asked to do this. Explore Slide 2b (19) to see glial cells, neuropil and ependymal cells in the CNS. ii. In the PNS, There are two types of glial cell: - Schwann cells - form the myelin sheath surrounding the axon of the neuron - satellite cells - surround and support neuron cell bodies located in the PNS. Explore Slide 3b (21) and Slide 4b (20) to see Schwaan cells and satellite cells in the PNS (ganglia). In a typical peripheral nerve individual nerve axons are bundled together to form a series of layers v. surrounded by connective tissue. These are best seen vi. in cross sections through nerves. iv. Identify the features of a peripheral nerve in both the iii. longitudinal and cross section shown in Slide 6 (17) i. Epineurium External coat of dense irregular connective tissue. ii. Perineurium ii. Connective tissue surrounding individual axon bundles that are called fascicles. iii. Endoneurium Thin connective tissue layer surrounding each individual axon and its myelin sheath. iv. Myelin sheath The fatty sheath formed by the Schwann cell surrounding the axon. v. Axon i. The long process of an individual neuron that extends from the cell body. vi. Node of Ranvier A Na+ channel rich ‘gap’ between adjacent Schwann cells that allows rapid propagation of impulses along an axon. These are only visible in longitudinal sections of nerves. Ganglia are nodular masses of connective tissue and neuron cell bodies surrounded by supporting glial cells (satellite cells). There are two types of ganglia in the PNS: i. Sensory ganglia Located in CNS Located in PNS Collections of pseudounipolar neuron cell Dorsal root ganglion bodies that: Satellite cells - convey afferent impulses towards the CNS - are associated with cranial nerves (cranial ganglia) and the spinal cord (dorsal root ganglia) - are tightly clustered and bounded by a CT capsule Examine Slide 4a (20) again to identify the key features - have large nuclei with a central of a sensory ganglion and ensure you can differentiate located nucleolus. this from an autonomic ganglion. - surrounded by supportive satellite cells (glial cells). - may contain lipofuscin (brown granules containing residuals of oxidation and lysosomal digestion) Located in CNS Located in PNS ii. Autonomic ganglia Sympathetic ganglion Collections of multipolar, post ganglionic (in sympathetic chain) neuron cell bodies that: - control activity of smooth muscle, glands and cardiac rhythm Satellite cells - can be located close to the spinal cord = sympathetic (see Slide 3a) Parasympathetic - can be located close to/within an ganglion organ = parasympathetic (see (in wall of organ) Slide 7). - are scattered and bounded by a CT capsule - have small nuclei with an Satellite cells eccentric nucleolus. - are each surrounded by Examine Slide 3a (21) of the sympathetic chain and supportive satellite cells (glial Slide 7 (54) which contains a parasympathetic ganglion cells) to identify the key features of autonomic ganglia. Ensure - may contain lipofuscin granules you can differentiate these from sensory ganglia. Different sections through the spinal cord exhibit different functional zones (tracts and ganglia) that have been linked to specific functions around the body. All spinal cord sections have an outer region of white matter composed mainly of neuron axons and an inner area of grey matter where neuron cell bodies can be found. In all areas you can recognize a variety of glial cells. Review the organization and cells of the sacral spinal cord segment shown in Slide 8 Connective tissue - the spinal cord is surrounded by three layers of connective tissue: dura, arachnoid and pia mater. - the dura is often removed during histological preparation a. Pia mater - a thin visceral layer of connective tissue that adheres to the spinal cord Identify the pia mater - its inner surface is a simple squamous epithelium in - it forms a double fold as it reflects in and out of the ventral median Slide 8a fissure - in some areas it may become artificially separated from the spinal cord during preparation b. Arachnoid mater Identify arachnoid - abuts the inner surface of the dura (not seen histologically) mater and arachnoid - has delicate projection called arachnoid trabeculae that extend trabeculae in through the subarachnoid space to the surface of the pia Slide 8b c. Subarachnoid space - the space between the arachnoid and pia that is bridged by the arachnoid trabeculae Identify subarachnoid - it contains CSF and blood vessels space in - the arachnoid trabeculae collapse during preparation due to loss of Slide 8c CSF making the space look like a layer of loose irregular connective tissue White matter - in the spinal cord this is the outer layer of nervous tissue surrounding the grey matter - contains axons of neurons running from one part of the nervous system to another. - contains glial cells and blood vessels accompanying theses axons. - axons that are going to/coming from specific locations are grouped into functionally related bundles called tracts (although these tracts are indistinguishable from surrounding white matter that is not a component of the tract) d. Corticospinal tract Identify the - aggregations of the axons of upper motor neurons that are corticospinal tract of traveling from the cerebral cortex within the spinal cord they are white matter in involved in control of motor functions of the body. Slide 8d e Spinothalamic tract - contains axons of sensory neurons traveling from skin to thalamus. Identify the - it has two components: anterior and lateral. spinothalamic tract of - the anterior spinothalamic tract carries info about crude touch. white matter in - the lateral spinothalamic tract carries info about pain/temperature Slide 8e f. Gracile fasciculus Identify the gracile - it contains the axons of sensory neurons that convey information fasciculus of white regarding proprioception from the lower limbs and trunk to the matter in Slide 8f brainstem. Grey matter - in the spinal cord this is the inner layer of nervous tissue deep to the white matter - it is organized into dorsal and ventral horns - it contains the cell bodies of neurons, their dendritic processes and axons and supportive glial cells and blood vessels.