NERVOUS TISSUE

CHAPTER 12

NERVOUS SYSTEM

• Structures that make up the nervous system are the brain, 12 pairs of cranial nerves and their branches, the spinal cord, 31 pairs of spinal nerves and their branches, ganglia, enteric plexuses and sensory receptors.

• Functions: sensory (changes w/in body and outside); integrative: interprets changes; motor: responds by initiating action -muscle contraction or glandular secretion.

Nervous System

• The three functions maintain homeostasis.

• Split second reactions->nerve impulses->adjustments for body functioning.

• Homeostasis-due to nervous and endocrine systems (hormones are slower than neurotransmitters)

• Science: Neurology

NERVOUS SYSTEM

• Two types of cells-neurons and neuroglia

• Neurons are either sensory, interneurons or motor neurons.

• Divided into CNS (consists of brain and spinal cord); PNS (all nervous tissue outside the CNS.

CNS

• Brain and spinal cord.

• All body sensations relayed from receptors to CNS (afferent N)-responses (efferent).

Peripheral Nervous System

• Afferent-sensory. All nerve processes in form of nerves -> connect brain and spinal cord w/receptors, muscles and glands (afferent) neurons.

• Efferent-motor neurons. From centers to effectors (muscles and glands). Divided into somatic nervous system and autonomic nervous system.

PERIPHERAL NERVOUS SYSTEM

• The PNS is divided into somatic nervous system (SNS), autonomic nervous system (ANS), and enteric nervous system (ENS).

– SNS:consists of neurons that conduct impulses from somatic and special sense receptors to the CNS. Also motor neurons from the CNS to skeletal muscles. Conscious action.

– ANS: sensory neurons from visceral organs. Motor neurons that convey impulses from the CNS to smooth, cardiac muscle tissue & glands. Sympathetic and parasympathetic.

PERIPHERAL NERVOUS SYSTEM

• Sympathetic= increases organ’s activities, excitatory.

• Parasympathetic=decreases organ’s activities, inhibitory.

• The ENS consists of neurons in two enteric plexuses that extend the length of the GI tract and function independently of the ANS and CNS.

HISTOLOGY OF NERVOUS TISSUE

• Consists of two principal cells-neurons and neuroglia.

• Neurons: have function of electrical excitability, sensing, thinking, remembering, controlling muscle activity, regulating glandular activity.

• Neuroglia:smaller than neurons, support for neurons, intertwine with neurons, line certain structures in brain and spinal cord, bind nervous system to blood vessels, insulate and cover neuron axons with myelin, increase speed of transmission, phagocytic.

HISTOLOGY OF NERVOUS TISSUE

• Neuroglia include astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells (neurolemmocytes), and satellite cells.

• The oligodendrocytes (axons of CNS) and Schwann cells (axons in the PNS) produce myelin sheaths.

HISTOLOGY OF NERVOUS TISSUE

• Most neurons consist of many dendrites, which are the main receiving or input region; a cell body that has organelles; and usually a single axon that propagates nerve impulses towards another neuron, a muscle fiber or gland cell.

• Membrane:axolemma. Cytoplasm:axoplasm. No mitotic apparatus, can have centrioles (unknown function), endoplasmic reticulum-protein synthesis.

• Conduction:dendr->body->axon hillock->axon

NEURONS

• Cell body essential for synthesis of substance-neuron life.

• Axoplasma flow 1 mm/day (slow)-from body to • Axomal transport 300 mm/day-conveys materials

in both directions(proximal and distal) along tracts of microtubules and filaments. Moves organelles & materials from axolemma, end bulbs etc.

NERVE FIBERS

• Any process projecting from cell body. Commonly refers to axon and its sheaths.

• Myelinated-neurofibril nodes (Ranvier) central=oligodendrocytes (do not regenerate); peripheral=neurolemmocytes (do regenerate).

• Unmyelinated-contain only neurolemmas w/o multiple wrappings.

CLASSIFICATION

• I-based on # of processes extending from cell body: multipolar, bnipolar or unipolar.

• Multipolar: several dendrites, 1 axon, most neurons in brain and spinal cord.

• Bipolar: 1 dendrite and 1 axon. Retina of eye.

• Unipolar: posterior root ganglia or sensory root of spinal nerves and ganglia of cranial nerves.

CLASSIFICATION

• Based on function:– Sensory (afferent) (unipolar). Receptors in skin,

sensory organs, joints, muscle and viscera to brain and spinal cord from lower to higher centers of CNS. (general somatic afferent;general visceral afferent)

– Motor(efferent). Impulses from brain and spinal cord to effectors (muscles or glands).

– Interneurons: carry impulses form sensory to motor neurons. Located in brain or spinal cord.

HISTOLOGY OF NERVOUS TISSUE

• White matter consists of aggregations of myelinated processes, whereas gray matter contains neuron cell bodies, dendrites, and axon terminals or bundles of unmyelinated axons and neuroglia.

• In the spinal cord, gray matter forms an H-shaped inner core that is surrounded by white matter. In the brain, a thin superficial shell of gray matter covers the cerebrum.

ELECTRICAL SIGNALS IN NEURONS

• Two types of ion channels are:leakage and gated.

• Three types of gated: voltage-gated, ligand-gated and mechanically-gated.

• The membrane of a resting neuron is positive outside and negative inside. This is owing to the distribution of different ions and the relative greater permeability of the membrane to K+ than to Na +.

ION CHANNELS

• When these are open, they allow specific ions to diffuse across the plasma membrane, down their electrochemical gradient.

• Similarly, cations will move toward a negatively charged area and anions toward a positively charged area.

• In all cases, the result is a flow of current that can cause a change in the membrane potential.

ION CHANNELS

• Leakage Channels- these channels are always open, like a garden hose. A plasma membrane, typically has many more potassium ion leakage channels than sodium ion leakage channels. So the membrane’s permeability to potassium is higher than sodium.

GATED CHANNELS

• Voltage-gated ion channel: this opens in response to a change in the membrane potential. This is used in the generation and conduction of action potentials.

• Ligand-gated ion channel: this opens and closes in response to a specific chemical stimulus. (neurotransmitters, hormones, ions) these channels operate in two basic ways. The ligand molecule itself may open or close the channel or the ligand may act indirectly via a protein called G protein that activates another molecule (second messenger) that operates the gate.

• Mechanically gated ion channel: opens or closes in response to mechanical stimulation-vibration, pressure, tissue stretching.

RESTING MEMBRANE POTENTIAL

• The resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the membrane and an equal buildup of positive ions in the ECF alon the outside surface of the membrane.

• In neurons, the resting membrane potential ranges from -40 mV to -90 mV. The minus sign indicates that the inside is negative relative to the outside. A cell that exhibits membrane potential is said to be polarized.

Resting membrane potential

• This is maintained by two factors:– Unequal distribution of ions across the plasma

membrane. ECF is rich in Na+ and chloride ions (Cl-). In cytosol the main cation is K+ and the two dominant anions are organic phosphates and amino acids in proteins.

– Relative permeability of the plasma membrane to Na+and K+ . In a resting neuron, the permeability of the plasma membrane is 50-100 times greater to K+ than to Na+.

Graded Potential

• When a stimulus causes ligand-gated or mechanically-gated ion channels to open or close in an excitable cell’s plasma membrane, that cell produces a graded potential.

• When the response is a more negative polarization, it is termed hyperpolarizing graded potential. When the response is less negative-depolarizing graded potential.

Graded potential

• Graded potentials occur most often in the dendrites and cell body of a neuron and less often in the axon.

• The term graded means that they very in amplitude depending on the strength of the stimulus.

• The opening or closing of ion channels produces a flow of current that is localized.

• Graded potentials are useful only for short-distance communication.

Action Potentials

• An action potential or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential and then restore it to the resting state.

• Two types of channels open: the first channels allow for Na+ to rush into the cell and then K+ channels open to allow K+ to flow out.

Depolarizing Phase

• If a depolarizing graded potential or some other stimulus causes the membrane to depolarize to a critical level called threshold, then voltage-gated Na channels open. Inward diffusion of Na+ occurs and this results in the depolarizing phase of the action potential. The membrane potential changes from -55mV to 0mV to +30mV.

• Each Na+ channel has 2 gates-activation and inactivation gate. At threshold both gates open and Na+ rushes in. Further repolarizatin and further movement-positive feedback system.

Repolarizing Phase

• More slowly, depolarization also opens voltage-gated K+ channels which permit outflow of K + . At the same time Na + channel inactivation gates are closing.

• This produces the repolarizing phase in which the resting membrane potential is restored.

• Na + inflow slows, and K + outflow accelarates.• Membrane potential changes from +30 mV to 0

mV to -70 mV.

Refractory Period

• The period of time during which an excitable cell cannot generate another action potential is called refractory period.

• During an absolute refractory period, a second action potential cannot be generated even after a strong stimulus.

• The relative refractory period is the period of time during which a second action potential can be initiated, but only by a suprathreshold stimulus.

Continuous and Saltatory Conduction

• An action potential propagates from point to point and hence is useful in long-distance communication. This occurs in muscle fibers and unmyelinated axons and is called continuous conduction.

• A nerve impulse in which the impulse “leaps” from one node of Ranvier to the next is called Saltatory conduction.

Speed of Nerve Impulse Propagation

• Factors that determine nerve impulse propagation are presesnce or absence of myelin and the diameter of the axons.

• Localized cooling can retard nerve impulse conduction.

• Saltatory conduction travels faster and is more energy eficient.

• Larger diameter axons conduct impulses faster than smaller ones.

Nerve Fibers

• A fibers: largest diameter axons(5-20um), all are myelinated, brief absolute refractory period. The axons of sensory neurons that conduct impulses associated with touch, pressure, thermal sensations, motor neurons.

• B fibers: have axon diameters of 2-3 um and a somewhat longer refractory period than the A fibers. Also myelinated. These conduct sensory nerve impulses from the viscera to the brain and spinal cord.axons of ANS.

• C fibers:smallest diameter,longest refreactory period. Unmyelinated axons. Pain, touch, pressure, heat, fibers to stimulate heart, smooth muscle, and glands.

SYNAPSES

• Synapse are the site of functional contact between two excitable cells.

• Synapses are electrical or chemical.• A chemical synapse produces only one-way information

transfer-from a presynaptic neuron to a postsynaptic neuron.

• Axon terminals contain synaptic vesicles filled with neurotransmitter molecules.

• An excitatory neurotransmitter depolarizes and an inhibitory hyperpolarizes.

Neurotransmitters

• Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS. A neurotransmitter may be excitatory in some locations and inhibitory in others.

• Two classes:small-molecule (Ach, amino acids, biogenic amines, ATP, purines, gases); neuropeptides-chains of 3-40 amino acids.

Regeneration and Repair of Nervous Tissue

• The nervous system exhibits plasticity.• At the level of neurons the changes that can occur

include sprouting of new dendrites, synthesis of new proteins, and changes in synaptic contacts with other neurons.

• Mammalian neurons have very limited powers for regeneration.

• In the PNS damage to myelinated axons and dendrites may be repaired if the Schwann cells are intact.

• In the CNS, little or no repair occurs.

General Factors That Affect Neural Function

• Environmental factors: changes in pH, ionic composition. Temperature.

• Metabolic Processes: active neurons need ATP for1) synthesis, release and recycling of neurotransmitter molecules 2) movement of materials by axoplasmic flow 3) maintenance of normal resting potential 4) recovery from action potentials.