Chapter 12

Anatomy & Physiology

Fifth Edition

Seeley/Stephens/Tate

(c) The McGraw-Hill Companies, Inc.

NERVOUS TISSUE• The nervous system, which consist of all three neural tissues and

be divided into neurons and neuroglia.

• The nervous system receives signals from the environment or within the body and then process them and make the body respond.

• Neurons transmit information and neuroglia are for support and protection.

• Fig. 12.1 – 12.3 summarizes the functional overview of the nervous system. The students should be familiar with such words as: the central and peripheral nervous systems (CNS and PNS), sensory receptors, afferent and efferent divisions, and the somatic and autonomic nervous system and how they are related functionally.

• Cells of the Nervous System

• Cytologically it has been seen that within the same region, usually smaller neuroglial cells outnumber the neurons.

• Neurons

– Functionally neurons are divided into sensory, motor and inter-neurons.

– Sensory neurons are part of the afferent division. Ten million sensory receptors and neurons transmit signals from the sensory receptors to the CNS. The sensory receptors are further divided into:

• Exteroceptors which respond to stimuli from the exterior i.e. touch, vision, smell, heat etc…

• Proprioceptors respond to stimuli from the skeletal muscles and joints.

• Interoceptors respond from stimuli from interior organs.

• Motor neurons of the efferent division of about a million carry instructions from the CNS to the effectors. There are two types of motor neurons, i.e. the somatic motor neurons for skeletal muscles and the automatic motor neurons (visceral motor neurons) for cardiac and smooth muscles and glands.

• Interneurons found exclusively in the brain and spinal cord amount to 20 billion cells and connect to other neurons. They are the coordinators of incoming and outgoing information.

• The structure each neuron consist of a: cell body (soma), dendrites, axons and the terminal boutons (synaptic knobs).

• The soma has a large nucleus containing a nucleolus, but no centrioles. Since centrioles are needed to from spindle fibers, without them the neurons cannot divide, i.e. no mitosis or replacement of neurons.

• Regeneration of a cut axon (clinical focus) when an axon in a peripheral nerve is cut, the portion of axon not attached to the stroma will regenerate. The schwann cells, which include myelin sheath, form a regeneration tube and the part of axon attached to the schwann cell body begins to stretch, possibly stimulated by the chemical released from the schwann cell. A severed major nerve can successfully be surgically connected if this is done before degeneration of the neuron. The axons of CNS regeneration is, however, rare.

• The cytoplasm of the soma is capable of producing large quantity of energy. It contains mitochondria, golgi app., ribosomes, and RER. The gray clusters of RER and free ribosomes are called Nissl bodies.

• Extensions from the soma become variable lengths of dendrites and a long axon. These protrusions are sensitive to stimuli of various kinds and tend to form an action potential at the axon hillock, which will be transmitted down the axon.

• The axon may branch and form collaterals.

• At end of the axon there is a bouton or synaptic knob which forms a synapses with other cells.

• Classification – at least three types of neurons have been identified based on their structures and their primary places of locations.

• Multipolar neuron – which has been described in the above, is common in the CNS and the motor neurons for the skeletal muscles.

• A unipolar neuron found in the sensory neurons of the PNS transmits the action potential from the elongated dendrite to the axon. The soma is placed at the side of this continuing dendrite and axon.

• A bipolar neuron found in more specialized sense organs, eyes, ears, has one extended dendrite and axon, placing the soma between. Flow of the action potential is from the dendrite to the axon.

Neuroglia

• Glial cells are found in both CNS and PNS, but at least four types of glial cells are identified in CNS.

• Astrocytes and the blood-brain barrier - they are the most abundant of the neuroglia in the CNS and could constitute up to 90% of the nervous tissue.

• Astrocytes interact with neurons in two ways:– Scavenge K+ for the extracellular fluid released by active neurons.– Scavenge specific neurotransmitters released from the knobs.

Glutamic acid, gamma-aminobutyric (GABA) will be picked up and released as glutamine for recycling.

– The endothelial cells of blood capillaries in the brain are held together with tight junctions, which might be brought about by astrocytes.

– Thus, exchange between the plasma and tissues outside the brain capillaries is exclusively with specific diffusion, active transport, endocytosis and exocytosis. Thus, blood-brain barrier.

• Oligodendrites – from myelin sheath around the neural axon of CNS. Synchronous formation creates periodic gaps (~ 1 mm intervals), the nodes of Ranvier, on the axon. (saltatory conduction)

• In the CNS, the regions of myelinated axons appear white, while those dominated with the soma are gray.

• Schwann Cells (satellite cells)– are found in the peripheral nervous system and they wrap around the axon forming sheath of schwann.

• Microglia – are phagocytotic small and rare glial cells found in the CNS.

• Ependymal cells – sit around the central canal of the spinal cord and chambers of the brain and form the choroid plexuses, which secrete the cerebrospinal fluid.

• Organization of nervous tissue

Grey (soma) and white (axons) matters

PNS --- ganglia --- CNS

nerve track consisting of axon bundle embedded in C.T.

• Basic structure of synapses– Recall the basic structure and function of a synapses.

• Excitatory and Inhibitory postsynaptic potentials• At the synapses, a chemical neurotransmitter is

released and its effects either depolarizes or hyperpolarizes the postsynaptic membrane, thereby exciting or inhibiting the post synaptic neuron.

• When the depolarization of post synaptic membrane occurs, the response is stimulatory, and the local depolarization is an excitatory post synaptic potential (EPSP).

• The neuron that releases a neurotransmitter causing EPSP is an excitatory neuron.

• Recall that EPSP is the result of increasing permeability to Na+.

• Glutamine in CNS and Ach in PNS are some examples.

• Contrary to the above, when a neurotransmitter causes hyperpolarization of the post synaptic membrane, the result will be inhibitory and an inhibitory post synaptic potential (IPSP) is observed.

• The neuron is an inhibitory neuron.

• The IPSP is the result of an increased permeability to Cl- or K+.

• In the spinal cord glycine binds to the post synaptic membrane to increase permeability to Cl-, the ion will flow inward to the cell according to the concentration gradient. Thus, increases negative membrane potential (hyperpolarization).

• In the cardiac muscles, acetylcholine binds to its receptors causing G-protein-mediated opening of K+ ion channels. Thus, K+ ions flow outward from the the cells leading to hyperpolarization.

• Review Table 12.1

• Presynaptic Inhibition an Facilitation

• Many of the synapses of the CNS are axoaxonic synapses.

• In other words, the axon of one neuron synapses with presynaptic terminal of another.

• The axonaxonic synapses can control the amounts of neurotransmitter release from the presynaptic membrane to the cleft.

• Thus, it could become either a presynaptic inhibitor or facilitator.

– Examples: enkephalin and endorphins – inhibitory

Glutamate and nitric oxide - facilitatory

• Summation of postsynaptic local potentials• Action potential is all-or-none.• But, within the CNS and in many PNS a single

presynaptic action potential may not cause enough local depolarization of the post synaptic membrane reach to the threshold.

• Multiples of presynaptic actions must be summed to provided enough depolarization on the soma of the membrane to have an action potential at the axon hillock.

• The summation could be spatial, i.e. stimulation from more than one bouton.

• Temporal summation is the result of two close successive stimuli from one bouton.

• An introduction to reflexes

• Reflexes are involuntary automatic motor responses.

• Examples are the control the heart rate, blood pressure, swallowing, sneezing, etc……

• The responses reproduce the same response.

• Two types of responses: spinal reflexes are processed within the spinal cord and cranial reflexes are processed in the brain.

• There are five components:

– A sensory receptor

– an afferent or sensory neuron

– Association neuron

– An efferent or motor neuron

– An effector organ

• The response produced by the reflex arc is called a reflex.

• Pathways:• A neuronal pool consist of a number of inputs and

outputs. Within it, excitatory and inhibitory neurons may be found.

• Neuronal pools may communicate each other either excitatory or inhibitory.’

• Neuronal communication nets work may be classified in several ways. At least three are possible.– Divergence, convergence and oscillating circuits.

The End.