“Brain and Spinal Cord” Laboratory Exercise “Brain and Spinal Cord”

The Brain and the Spinal Cord In this exercise, you will become familiar with the gross anatomy and functions of the brain, and identify the microscopic anatomy of neurons. Terms describing should be familiar to you: … This laboratory exercise correlates with Chapter 12, 13 and 14 of your textbook.

What procedures are we doing? Examine the gross structures of preserved sheep brain (analogous to human brain) Examine histological sections of the spinal cord, cerebral cortex, cerebellar cortex and a myelinated axon (neuron)

Resources available… Anatomy and Physiology Laboratory Study Pages; submenu: Brain http://ctle.hccs.edu/biologylabs/AP1/AP1index.html

Lecture

An Introduction to the Neural Tissue The Nervous System includes all neural tissue in the body Neural tissue contains two kinds of cells Neurons, the cells that send and receive signals Neuroglia (glial cells), the cells that support and protect neurons Organs of the Nervous System include: The brain and the spinal cord Sensory receptors of sense organs (eyes, ears, nose, mouth, etc.) Nerves that connect the nervous system with other systems

Divisions of the Nervous System The Nervous System is divided into Anatomical and Functional Divisions Anatomical divisions of the Nervous System include: The Central Nervous System (CNS) The Peripheral Nervous System (PNS) Functional divisions of the Nervous System include: The Afferent Divison The Efferent Division

The Central Nervous System (CNS) Anatomical Divisions The Central Nervous System (CNS) Consists of the spinal cord and the brain It contains neural tissue, connective tissues and blood vessels Functionally, the CNS processes and coordinates: Sensory data from inside and outside the body Controls activities of the peripheral organs through the motor system (e.g., skeletal muscles) Executes higher functions of brain intelligence, memory, learning and emotion

The Peripheral Nervous System (PNS) Includes all neural tissue outside the CNS The Peripheral Nervous System functions to: Deliver sensory information to the CNS (afferent) Carry motor commands to peripheral tissues and systems (efferent) Peripheral Nerves are: Bundles of axons with connective tissues and blood vessels They carry sensory information and motor commands in the PNS Cranial nerves connect to the brain Spinal nerves connect to the spinal cord

Functional Divisions of the PNS include: The Afferent division Which carries sensory information from the PNS sensory receptors to the CNS integrative system (receptors include the sensors in the skin, eyes, ears, mouth, etc…) The Efferent division Which carries motor commands from the CNS integrative system to the PNS muscles and glands (muscles and glands are not part of the nervous system, they are the endpoint)

The efferent division is further subdivided into the: Somatic nervous system (SNS), which controls voluntary and involuntary reflexes, leading to skeletal muscle contraction Autonomic nervous system (ANS), which controls subconscious actions, the contractions of smooth and cardiac muscle, and glandular secretions Sympathetic division has a stimulating effect Parasympathetic division has a relaxing effect

Somatic nervous system (SNS) Parasympathetic division The Organization of the Nervous System Central Nervous System (CNS) (brain and spinal cord) Integrate, process, and coordinate sensory data and motor commands Sensory information within afferent division Motor commands within efferent division Peripheral Nervous System (PNS) (neural tissue outside the CNS) includes Somatic nervous system (SNS) Autonomic nervous system (ANS) Parasympathetic division Sympathetic division “Rest and Digest” “Fight or Flight” Receptors Effectors Smooth muscle Cardiac muscle Glands Adipose tissue Special sensory receptors Visceral sensory receptors Somatic sensory receptors monitor smell, taste, vision, balance, and hearing monitor internal organs monitor skeletal muscles, joints, and skin surface Skeletal muscle Skeletal muscle start with receptors…

Neurons are Specialized Communicators Structure of a Neuron Neurons are the basic functional units of the nervous system

The multipolar neuron is the most common type of neuron in the Central Nervous System (know this structure for the lab) It is composed of a (1) Cell body (soma) with short, branched dendrites, (2) a long, single axon* and (3) the telodendric termini with synaptic termini (synaptic “knobs”) Synaptic termini *-axons can be insulated with myelin

Myelinated Axon

Figure 14-8b The Cerebellum (Part 2 of 2). Dendrites Cell body of Purkinje cell Axons Purkinje cells LM × 320 b A sectional view of the cerebellum, showing a Purkinje cell (a multipolar, cerebellar neuron)

A deep look at the anatomy of a Multipolar Neuron Nissl bodies (RER and free ribosomes) Dendritic branches Mitochondrion Axon hillock Initial segment of axon Axolemma Axon Telodendria Direction of action potential Golgi apparatus Axon terminals Neurofilament Nucleolus Nucleus Dendrite See Figure 12–3 Presynaptic cell b An understanding of neuron function requires knowing its structural components. Postsynaptic cell 17

The Cell Body The cell body has a large nucleus and nucleolus The perikaryon is the cytoplasm The cytoskeleton is composed of transporting neurotubules and structural neurofilaments (bundled as neurofibrils) Mitochondria produce energy, ribosomes and RER produce neurotransmitters. The RER are compartmentalized as Nissl bodies, which stain gray (thus, gray matter) There are no centrioles, thus, no mitosis (neurons can not reproduce themselves) The axon hillock is a thickened region that attaches to the axon. The action potential is generated here and propagated down the axolemma (plasma membrane) of the axon

Dendrites and Axons Dendrites are highly branched, fine processes that receive information from other neurons and conduct impulses towards the cell body. 80–90% of the neuron surface area is dendritic The axon is a long cellular extension that carries the electrical signal (action potential) to the target cell The axon conducts impulses away from the cell body The axon has structures designed for its function

Structures of the Axon The axoplasm is the cytoplasm of the axon. It contains neurofibrils, neurotubules, enzymes and organelles The axolemma is a specialized cell membrane that covers the long extension of the axon; frequently, the axolemma is insulated by a myelin “sheath” (this is analogous to the copper wiring in an electric cord being insulated with rubber around it) The initial segment of the axon attaches to the axon hillock of the cell body Collaterals are side branches. The telodendria (axon arborization, terminal branches) are fine extensions of the collaterals and at the distal axon The synaptic terminals (synaptic knobs/bulbs/boutons) are at the tips of the telodendria. They contain vesicles filled with neurotransmitters, which conduct the nervous impulse from cell- to-cell.

Synapse The synapse is the area where a neuron communicates with another cell It involves a: Presynaptic cell Synaptic cleft Postsynaptic cell

Presynaptic Cell The presynaptic cell will always be a neuron that sends a message It contains a synaptic terminus (knob/bulb). This is an expanded area of the axon with many synaptic vesicles, each containing a lot of neurotransmitter Neurotransmitters are chemical messengers released at the presynaptic membrane They affect neurotransmitter receptors of the postsynaptic membrane When their function is completed, they are broken down by enzymes or recycled back to the presynaptic cell

Synaptic Cleft The synaptic cleft is a small space in between the presynaptic and postsynaptic cells It is filled with extracellular fluid through which the neurotransmitters can diffuse It also contains control elements which degrade or recycle the neurotransmitter, limiting the synaptic response

Postsynaptic Cell The postsynaptic cell receives the message from the presynaptic cell; its plasma membrane has receptors for specific neurotransmitters The postsynaptic cell can be a neuron or another type of cell At a neuromuscular junction, the synapse is between a neuron and a muscle At a neuroglandular junction, the synapse is between a neuron and a gland

A Synapse Telodendrion Synaptic terminal Endoplasmic reticulum Mitochondrion Synaptic vesicles Presynaptic membrane Postsynaptic membrane Synaptic cleft 25

Functional Classification of Neurons There are three functional classifications of neurons Sensory neurons are the affectors (10 million) Interneurons are the integrators (20 billion!) Motor neurons are the effectors (500,000)

Located in sensory ganglion The Structure of the Neuron is related to its function Anaxonic neuron Bipolar neuron Unipolar neuron Multipolar neuron Dendrites Dendrites Initial segment Cell body Dendritic branches Axon Dendrite Cell body Cell body Located in sensory ganglion Axon Axon very long! Cell body Axon Axon terminals Axon terminals Axon terminals a Anaxonic neurons have more than two processes, and they are all dendrites. b Bipolar neurons have two processes separated by the cell body. c Unipolar neurons have a single elongated process, with the cell body located off to the side. d Multipolar neurons have more than two processes; there is a single axon and multiple dendrites. Unknown function Special Sensory (sight, smell, etc…) Sensory Afferents (PNS) Motor Efferents and Interneurons (PNS and CNS)

Sensory Neurons Sensory receptors monitor the internal environment (visceral sensory neurons) and the external environment (somatic sensory neurons) There are three types of sensory receptors Interoceptors monitor internal systems (i.e. digestive, respiratory, cardiovascular, urinary, reproductive). Senses include taste, deep pressure, pain, etc… Exteroceptors monitor external senses (touch, temperature, pressure). Senses include sight, smell, hearing, etc… Proprioceptors monitor position and movement (skeletal muscles and joints). This is a complex concept; it helps us balance ourselves This information is passed on by sensory neurons (afferent fibers) of the PNS to the integrative circuit (interneuron fibers) of the CNS

Interneurons Interneurons are located in between sensory and motor neurons; these are the most common type of neuron They are mostly located in the brain and spinal cord (CNS) They are responsible for: The distribution of sensory information The coordination of motor activity Higher functions (memory, planning, learning, etc…)

Motor Neurons Motor neurons are efferent fibers of the PNS; they carry instructions from the integrative circuit of the CNS to the efferent systems of the body There are two major efferent systems: The Somatic Nervous System (SNS) includes all somatic motor neurons that innervate the skeletal muscles The Autonomic Nervous System (ANS) includes all visceral motor neurons that innervate other peripheral effectors. This includes smooth muscle, cardiac muscle, glands and adipose tissue

Neuroglia Support and Protect Neurons Neuroglia compose roughly half the volume of the central and peripheral nervous system; they support, isolate, nourish and protect neurons Unlike neurons, they actively divide (this can result in types of brain cancer – glioma) There are different types of glia in the Central Nervous System (four types) and the Peripheral Nervous System (two types)

Four Types of Neuroglia in the CNS Ependymal cells Astrocytes Oligodendrocytes Microglia

Ependymal Cells form an epithelium (called ependyma), which lines the central canal of the spinal cord and the ventricles of the brain The cells secrete cerebrospinal fluid (CSF) and have cilia or microvilli that circulate the fluid, protecting the nervous tissue from shock and circulating material a Light micrograph showing the ependyma of the central canal of the spinal cord   Ependymal cells Central canal Gray matter White matter Gray matter Central canal of spinal cord

Astrocytes line CNS capillaries, maintaining the blood– brain barrier (keeping the CNS isolated from toxins) They also create a three-dimensional framework for the CNS, repair damaged neural tissue, guide neuron development and control the interstitial environment

Oligodendrocytes myelinate neurons, increasing the speed of the action potentials Myelination occurs at internodal segments of the axon

Microglia migrate through neural tissue and clean up cellular debris, waste products and pathogens (housekeepers)

A Summary… Central Nervous System Neuroglia are found in contains Ependymal cells Astrocytes Oligodendrocytes Microglia Line the ventricles (brain) and central canal (spinal cord); they assist in the production, circulation, and monitoring of the cerebrospinal fluid Maintain the blood–brain barrier; provide structural support; regulate ion, nutrient, and dissolved gas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury Myelinate the CNS axons and provide a structural framework Are very migratory. They remove cell debris, waste, and pathogens by phagocytosis

Two Types of Neuroglia in the PNS Satellite Cells Schwann Cells

Ganglia are masses of neuron cell bodies that are located outside of the spinal cord; they are surrounded by PNS neuroglia. Satellite cells (amphicytes) surround the ganglia and regulate the environment around the neurons enclosed within the ganglia Schwann cells (neurilemma cells) form a myelin sheath (neurilemma) around peripheral axons One Schwann cell sheaths one segment of an axon; thus, many Schwann cells sheath an entire axon

Schwann Cells and Peripheral Axons

Schwann Cells and Peripheral Axons

A Summary… Peripheral Nervous System Neuroglia are found in contains Satellite cells Schwann cells Surround neuron cell bodies in ganglia; regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury

Neural Responses to PNS Injuries Schwann cells play an important role in repairing damaged neurons in the PNS In the process of Wallerian degeneration, axon and myelin degenerate distal to the site of injury If the axon makes normal synaptic contacts, normal function may be regained If the axon stops growing or wanders off, normal function may not return This process is stimulated by nerve growth factor (NGF) and inhibited by astrocyte factors

The process of repair of damaged PNS nerves, or Wallerian degeneration Site of injury Step 1: Distal to the injury site, the axon and myelin degenerate and fragment. Axon Myelin Proximal stump Distal stump distal end Step 2: The Schwann cells do not degenerate; instead, they proliferate along the path of the original axon. Over this period, macrophages move into the area and remove the degenerating debris distal to the injury site. Macrophage Cord of proliferating Schwann cells Step 3: As the neuron recovers, its axon grows into the site of injury and then distally, along the path created by the Schwann cells. Figure 11.4.4 Schwann cells and satellite cells are the neuroglia of the PNS Step 4: As the axon elongates, the Schwann cells wrap around it. If the axon reestablishes its normal synaptic contacts, normal function may be regained. However, if it stops growing or wanders off in some new direction, normal function will not return. 44