Nervous System: Functions and Divisions

• The nervous system's functions include sensory input, integration, and motor output.

Sensory Input

• Sensory receptors monitor changes inside and outside the body, and the information is gathered.
• A stimulus is any change in the environment or body, such as touching a hot surface.
• Sensory receptors monitor changes throughout the body.
• Afferent pathways carry sensory input toward the CNS.

Integration

• Integration involves processing and interpreting sensory input to make decisions.

Motor Output

• Motor output is the response to sensory input, activating glands or muscles.
• Efferent pathways carry motor output away from the CNS.
• Effector organs are glands and muscles, which produce a response.

Central Nervous System (CNS)

• The CNS is the integration and command center, receiving, interpreting sensory signals, and dictating motor responses.
• The brain and spinal cord comprise the CNS.

Peripheral Nervous System (PNS)

• The PNS is outside the CNS, with peripheral nerves linking all body regions to the CNS.
• Cranial and spinal nerves, along with ganglia, are components of the PNS.
• Ganglia are clusters of neuron cell bodies in the PNS.

Somatic and Visceral Regions

• The somatic region includes structures external to the ventral body cavity, such as skin, skeletal muscles, and bones.
• The visceral region includes the viscera within the ventral body cavity, such as the digestive tract, lungs, heart, and bladder.

Sensory and Motor Divisions

• The sensory division carries signals from sensory receptors throughout the body via PNS nerves to the CNS.
• The motor division carries signals from the CNS via PNS nerves to muscles and glands.

Somatic Sensory

• Somatic sensory innervation includes the outer body tube, skin, body wall, and limbs.
• General somatic include senses with receptors spread widely, such as touch, pain, pressure, vibration, and temperature.
• Special somatic senses have receptors in small areas, including hearing, equilibrium, and vision.
• Proprioception detects stretch in muscles, tendons, and joints, providing information on body position and movement.

Visceral Sensory

• Visceral sensory innervation involves the viscera.
• General visceral are senses felt widely in digestive, urinary, and reproductive tracts, including stretch, pain, temperature, hunger, and nausea.
• Special visceral senses include taste and smell, with receptors localized to the tongue and nasal cavity.

Somatic Motor

• Somatic motor stimulation causes contraction of skeletal muscles and is voluntary.

Visceral Motor

• Visceral motor function regulates smooth and cardiac muscle contraction and gland secretion.
• General visceral motor neurons make up the autonomic nervous system.

General vs. Specific Senses

• General senses are widespread throughout the body.
• Specific senses are more localized.

Autonomic Nervous System (ANS)

• The ANS controls visceral organ function and is also known as the visceral motor division of the PNS.
• The sympathetic and parasympathetic divisions comprise the ANS.
• The sympathetic division prepares the body for activity, known as "fight or flight".
• The parasympathetic division conserves energy and promotes digestion, known as "rest and digest".

Neurons

• Neurons are basic structural units of the nervous system, conducting electrical signals.
• Neuroglial cells are nonexcitable cells that support, protect, and insulate neurons.
• Neurons conduct electricity, have extreme longevity, do not divide, and have a high metabolic rate.
• Electrical signals transmit along the plasma membrane, or neurilemma, as nerve impulses.
• Neurons can function for over 100 years.
• Fetal neurons lose their ability to divide (amniotic) as they become communication links.
• Neurons require continuous oxygen and glucose.
• The soma (cell body) varies in size, contains a single nucleus, and is surrounded by cytoplasm.
• Nissl bodies (chromatophilic substance) are rough ER and free ribosomes that renew cell membranes and protein components.
• Neurofibrils are intermediate filaments (neurofilaments) that run between Nissl bodies and prevent the cell from being pulled apart.
• Dendrites are processes that branch from the cell body and function as receptive sites.
• Dendrites conduct electrical signals toward the cell body.
• The axon is a thin process extending from the cell body, generating impulses and transmitting them away from the cell body.
• The axon hillock is the cone-shaped region where the axon extends from the cell body.
• Axon collaterals are occasional branches along the axon's length.
• Telodendria are terminal branches of the axon.
• Axon terminals (end knobs, bulbs, boutons) are the end knobs of the terminal branches.
• Synaptic vesicles are membrane-bound sacs filled with neurotransmitters in axon terminals.
• The myelin sheath is an insulating layer around nerves that allows electrical impulses to transmit quickly and efficiently.
• A ganglion is a cluster of cell bodies along nerves in the PNS.
• A nucleus is a collection of cell bodies in the CNS.
• Graded potentials are changes in membrane potential that vary in size and occur in dendrites.
• A nerve fiber is any long axon.
• Action potentials are fast, transitory changes in resting membrane potential, occurring in axons.
• Dendrites are afferent in relation to the cell body.
• Axons are efferent in relation to the cell body.
• Axon action potentials target muscles, glands, and other neurons.
• A synapse is the site where neurons communicate using chemical messengers or electrical signals.

Synapses

• Axon terminals contain synaptic vesicles.
• Synaptic vesicles contain neurotransmitters.
• Neurotransmitters transmit signals across the synapse.
• The synaptic cleft is the space between the plasma membranes of two neurons.
• The presynaptic neuron conducts signals toward the synapse.
• The postsynaptic neuron transmits signals away from the synapse.
• An axodendritic synapse occurs between the terminal boutons of one neuron and the dendrites of another.
• An axosomatic synapse occurs between axons and neuron cell bodies.
• An axoaxonic synapse occurs between the axons of two neurons.

Neuron Structure

• Neurons are structurally grouped by the number of processes extending from the cell body.
• Multipolar neurons have more than two processes, usually numerous dendrites and one axon (99% of neurons).
• Bipolar neurons have two processes extending from opposite sides of the cell body, occurring in the retina, nasal olfactory epithelium, and inner ear.
• Unipolar neurons have a short, single process that emerges from the cell body and divides like an inverted T into two long branches, found in sensory ganglia in the PNS.
• The central process is a branch of the unipolar neuron that runs into the CNS (efferent).
• The peripheral process is a branch of the unipolar neuron that extends to the receptors (afferent).

Neuron Function

• Sensory neurons (afferent neurons) make up the sensory division of the PNS and transmit impulses toward the CNS from sensory receptors in the PNS; most are pseudounipolar.
• Pseudounipolar neurons occur when a bipolar neuron's two processes fuse together near the cell body.
• Motor neurons (efferent neurons) make up the motor division of the PNS and carry impulses away from the CNS to effector organs; they are multipolar.
• Interneurons lie between motor and sensory neurons, linking them together in chains that form complex neuronal pathways; they are multipolar, confined to the CNS, and make up 99.98% of neurons.

CNS Neuroglial Cells

• CNS neuroglial cells include astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
• Astrocytes are the most abundant glial cells, with radiating processes that cling to neurons or capillaries.
• Astrocytes regulate neurotransmitter levels, signal increased blood flow in active brain regions, control the ionic environment around neurons, propagate signals involved in memory, help synapses form in developing neural tissue, and produce molecules necessary for neural growth.
• Microglial cells are the smallest and least abundant neuroglia, with elongated cell bodies and processes with many pointed projections.
• Microglial cells migrate to and engulf microorganisms and injured or dead neurons; they are phagocytes and macrophages.
• Ependymal cells form a simple epithelium that lines the central cavity of the spinal cord and brain.
• Ependymal cells provide a permeable layer between cerebrospinal fluid and tissue fluid and have cilia that circulate cerebrospinal fluid.
• Oligodendrocytes have fewer branches than astrocytes and wrap their cell processes around the thicker axons in the CNS.
• Oligodendrocytes produce myelin sheaths.

PNS Neuroglia

• PNS neuroglia include satellite cells and Schwann cells.
• Satellite cells surround neuron cell bodies within ganglia.
• Schwann cells surround all axons in the PNS and form myelin sheaths around many of them.
• Myelin sheaths are segmented structures composed of the lipoprotein myelin, surrounding thicker axons and forming an insulating layer that prevents leakage of electrical current.
• In myelinated axons, nerve impulses jump from one myelin sheath gap to the next (faster).
• Nonmyelinated axons lack a myelin sheath and are thin, slowly conducting axons.
• Schwann cells envelop an axon, rotate around it, wrap their plasma membrane loosely around it in successive layers, and force cytoplasm from between the membranes to form the myelin sheath (formation of myelinated axon in PNS).
• Oligodendrocytes produce the myelin sheath in the CNS.
• The neurilemma is the plasma membrane of neurons.
• An internode is a stretch of nerve cell axon sheathed in myelin, between two nodes of Ranvier
• Nodes of Ranvier are gaps in the myelin sheath.

Nerves

• A nerve is a cable-like organ in the peripheral nervous system with many axons arranged in parallel bundles and enclosed by connective tissue.
• Nerve fibers consist of long axons that make up nerves.
• Fascicles are groups of axons bound into bundles, each surrounded by Schwann cells.
• The endoneurium is a delicate layer of loose connective tissue covering the Schwann cells or axons.
• The perineurium is a layer of connective tissue surrounding each fascicle.
• The epineurium is a tough fibrous sheath surrounding the whole nerve.

Cranial and Spinal Nerves

• Cranial nerves originate from the brain (12 pairs) and are part of the PNS.
• Spinal nerves originate from the spinal cord (31 pairs).
• Sensory nerves transmit only sensory input.
• Motor nerves transmit only motor output.
• Mixed nerves transmit both sensory input and motor output; all spinal nerves and most cranial nerves are mixed.

Cranial Nerves - Type

• Olfactory and optic nerves are sensory cranial nerves.
• Oculomotor, trochlear, abducens, accessory, and hypoglossal nerves are motor cranial nerves.
• Trigeminal, facial, vestibulocochlear, glossopharyngeal, and vagus nerves are mixed cranial nerves.

Cranial Nerves - Names

• The 12 cranial nerves are: olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal.
• Olfactory (I) is sensory and responsible for smell.
• Optic (II) is sensory and responsible for vision.
• Oculomotor (III) is motor and innervates four of the extrinsic eye muscles.
• Trochlear (IV) is motor and innervates an extrinsic eye muscle that hooks through a pulley-shaped ligament in the orbit.
• Trigeminal (V) is mixed and provides general sensory innervation to the face and motor innervation to the chewing muscles.
• Abducens (VI) is motor and innervates the muscle that abducts the eyeball.
• Facial (VII) is mixed and innervates the muscles of facial expression and senses taste.
• Vestibulocochlear (VIII) is mixed and responsible for hearing and equilibrium.
• Glossopharyngeal (IX) is mixed and innervates the tongue and pharynx.
• Vagus (X) is mixed and wanders beyond the head into the thorax and abdomen.
• Accessory (XI) is motor and carries motor innervation to the trapezius and sternocleidomastoid muscles.
• Hypoglossal (XII) is motor and innervates the tongue muscles.

Tracts

• Tracts are axons in the white matter that communicate between two regions of the CNS.
• Tracts are in the CNS, while nerves are in the PNS.

Gray vs White Matter

• Gray matter is where neuron cell bodies are clustered, consisting of cell bodies, dendrites, nonmyelinated neurons, and neuroglia.
• White matter is external to gray matter and contains myelinated axons and neuroglia; the color comes from the myelin sheaths.

Reflexes

• A reflex is a rapid, automatic motor response to stimuli; it is unlearned, unpremeditated, and involuntary.
• Somatic reflexes result in contraction of skeletal muscles.
• Visceral reflexes activate smooth muscle, cardiac muscle, or glands.
• A reflex arc is a chain of neurons that causes simple, reflexive behaviors.
• The receptor is the site where the stimulus acts, located at the terminal end of the peripheral process of a sensory neuron.
• The integration center consists of one or more synapses in the gray matter of the CNS.
• A simple integration center has a single synapse between a sensory neuron and a motor neuron.
• A complex integration center has multiple synapses that send signals through long chains of interneurons to other portions of the CNS.
• The effector is the muscle or gland cell that responds to the efferent impulses by contracting or secreting.
• The reflex arc pathway includes the receptor, sensory neuron, integration center, motor neuron, and effector.

Reflexes Cont.

• Monosynaptic reflexes have no interneuron between the sensory neuron and the motor neuron; there is only one synapse.
• The stretch reflex is a monosynaptic reflex that helps maintain equilibrium and upright posture and is the fastest of all reflexes; an example is the knee jerk reflex.
• Polysynaptic reflexes have one or more interneurons in the reflex pathway between the sensory and motor neurons.
• The withdrawal reflex is a polysynaptic reflex that occurs when we pull away from danger and contains 3 neurons; an example is pricking a finger.

Neuronal Circuits

• A neuronal circuit is a population of neurons and interneurons interconnected by synapses to carry out a specific function.
• In a diverging circuit, one presynaptic neuron synapses with several other neurons, and information is distributed through multiple neuronal pathways.
• In a converging circuit, many neurons synapse on a single postsynaptic neuron, with a single motor neuron receiving both exitatory and inhibitory impulses from many other neurons.
• In a reverberating circuit, one neuron in the circuit receives feedback from another neuron in the same circuit; a branch off the axon of one neuron circles back and synapses with a previous neuron.

Serial vs Parallel Processing

• Serial processing involves neurons passing their signal along a single pathway from one neuron to the next, like links in a chain.
• Parallel processing involves information from a single neuron being sent along two or more parallel pathways; it occurs when a single sensory stimulus results in multiple perceptions and allows the brain to evaluate stimuli with incredible speed.

Multiple Sclerosis

• Multiple sclerosis is a progressive disease that destroys patches of myelin in the brain and spinal cord, disrupting neuronal signals in the CNS and leading to sensory disorders and weakened musculature. The result of an autoimmune disease in which the immune system attacks the myelin around axons in the CNS.

Stages of Neuronal Regeneration

• The axon becomes fragmented at the injury site.
• Macrophages clean out the dead axon distal to the injury.
• Axon filaments grow through a regeneration tube formed by Schwann cells.
• The axon regenerates, and a new myelin sheath forms.

Development of Nervous System

• The ectoderm is the primary germ layer that forms the nervous system.
• The alar plate is dorsal, and its neuroblasts become interneurons that stay in the CNS.
• The basal plate is ventral, and its neuroblasts become motor neurons and sprout axons that grow out to the effector organs.
• Sensory neurons arise from the neural crest, which is why they lie outside the CNS.
• Cells from the neural tube (pseudostratified neuroepithelial cells) migrate externally to become neuroblasts, which cluster to form the basal and alar plates.