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

2. 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.

3. 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)

4. Resources available…
Anatomy and Physiology Laboratory Study Pages; submenu: Brain

5. Lecture

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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

12. 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…

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

14. 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

15. Myelinated Axon

16. 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)

17. 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

18. 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

19. 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

20. 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.

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

22. 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

23. 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

24. 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

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

26. 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)

27. 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)

28. 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

29. 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…)

30. 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

31. 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)

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

33. 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

34. 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

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

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

37. 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

38. Two Types of Neuroglia in the PNS
Satellite Cells
Schwann Cells

39. 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

40. Schwann Cells and Peripheral Axons

41. Schwann Cells and Peripheral Axons

42. 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

43. 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

44. 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 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