Unit 7 nervous system nrs 237

Unit 7: Nervous System

NRS 237 Principles of Anatomy

Dr. Moattar Raza Rizvi

The Nervous system has three major functions:

Nervous System

Major Structures of the Nervous SystemThe nervous system contains two main divisions: the central nervous system (CNS) and the peripheral nervous system(PNS).

The central nervoussystem consists of thebrain and spinal cord.

The peripheral nervoussystem consists of the vastnetwork of nerves throughoutthe body.

Organization of the Nervous System

• Somatic (voluntary) nervous system (SNS)– neurons from cutaneous and special sensory receptors

to the CNS– motor neurons to skeletal muscle tissue

• Autonomic (involuntary) nervous systems– sensory neurons from visceral organs to CNS– motor neurons to smooth & cardiac muscle and glands

• sympathetic division (speeds up heart rate)• parasympathetic division (slow down heart rate)

• Enteric nervous system (ENS)– involuntary sensory & motor neurons control GI tract– neurons function independently of ANS & CNS

Subdivisions of the PNS

Neurons: Functional unit of nervous systemThe cell body (also called the soma) is the control center of the neuron and contains the nucleus.

Dendrites, which look like the bare branches of a tree, receive signals from other neurons and conduct theinformation to the cell body. Some neurons have only one dendrite; others have thousands.

The axon, which carries nerve signals away from the cell body, is longer than the dendrites and contains few branches. Nerve cells have only one axon; however,the length of the fiber can range from a few millimeters to as much as a meter.

The axons of many (but not all) neurons are encased in a myelin sheath. Consisting mostly of lipid, myelin acts to insulate the axon. In the PNS, Schwann cells form the myelin sheath. In the CNS, oligodendrocytes assume this role.

Gaps in the myelin sheath, called nodes of Ranvier, occur at evenly spaced intervals

The end of the axon branches extensively, with each axon terminal ending in a synaptic knob. Within the synaptic knobs are vesicles containing a neurotransmitter.

Functional Classification of Neurons

Structural Classification of Neurons

Neuroglial Cells: support protect and nourish the neurons

Nerve Structure

Nerves are protected by a cloak of connective tissue called epineurium

Myelination in CNS and PNS

In the CNS, the myelin sheath is formed by oligodendrocytes. Unlike Schwann cells—which wrap themselves completely around oneaxon—one oligodendrocyte forms the myelin sheath for several axons.

Specifically, the nucleus of the cell is located away from the myelin sheath and outward projections from the cell wrap around the axons of nearby nerves.

As a result, there is no neurilemma, which prevents injured CNS neurons from regenerating. This explains why paralysis resulting from a severed spinal cord is currently permanent

In the PNS, the myelin sheath is formed when Schwann cells wrap themselves around the axon, laying down multiple layers of cell membrane. It’s these inside layers that form the myelin sheath.

The nucleus and most of the cytoplasm of the Schwann cell are located in the outermost layer.

This outer layer, called the neurilemma, is essential for an injured nerve to regenerate.

SPINAL CORDNerves from the cervical region of the spinal cord innervate the chest, head, neck, shoulders, arms, hands, and diaphragm.

Nerves from the thoracic region extend to the intercostal muscles of the ribcage, the abdominal muscles, and the back muscles.

The lumbar spinal nerves innervate the lower abdominal wall and parts of the thighs and legs.

Nerves from the sacral region extend to the thighs, buttocks, skin of the legs and feet, and anal and genital regions.

Basically a bundle of nerve fibers, the spinal cord extends from the base of thebrain until about the first lumbar vertebra.

Extending from the end of the spinal cord is a bundle of nerve roots called the cauda equina—so named because it looks like ahorse’s tail.

Structure of the Spinal Cord

Meninges of the Spinal Cord

Attachment of Spinal Nerves

Spinal tracts

Spinal NervesSpinal nerves (part of the peripheral nervous system) relay information from the spinal cord to the rest of the body.

Most nerves contain both sensory and motor fibers and are called mixed nerves. These nerves can transmit signals in two directions. A few nerves (such as the optic nerves) are sensory nerves and contain only sensory (afferent) fibers. They carry sensations toward the spinal cord. Others are motor nerves and contain only motor (efferent) fibers and carry messages to muscles and glands.

Categories of Spinal NervesThirty-one pairs of spinal nerves connect to the spinal cord. They include:•8 Cervical nerves (C1-C8)•12 Thoracic nerves (T1-T12)•5 Lumbar nerves (L1-L5)•5 Sacral nerves (S1-S5)•1 Coccygeal nerve (Co)

The four major plexuses are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus.

The first cervical nerve exits the spinal cord between the skull and the axis. The other nerves pass through holes in the vertebra (intervertebral foramina).Once outside the spinal column, each spinal nerve forms several large branches. Some of these branches subdivide further to form nerve networks called plexuses.

Categories of Spinal NervesThe cervical plexus contains nerves that supply the muscles and skin of the neck, tops of the shoulders, and part of the head. The phrenic nerve, which stimulates the diaphragm for breathing, is located here.The brachial plexus innervates the lower part of the shoulder and the arm. Key nerves traveling into the arm from this region include the axillary nerve (which passes close to the armpit, making it susceptible to damage from the use of crutches), the radial nerve, the ulnar nerve, and the median nerve.The lumbar plexus—derived from the fibers of the first four lumbar vertebrae—supplies the thigh and leg. A key nerve in this region is the large femoral nerve.The sacral plexus is formed from fibers from nerves L4, L5, and S1 through S4. (Because of the co-mingling of fibers of the sacral plexus with those of the lumbar plexus, these two plexuses are often referred to as the lumbosacral plexus.) The sciatic nerve, the largest nerve in the body, arises here and runs down the back of thethigh. Irritation of this nerve causes severe pain down the back of the leg, a condition called sciatica.

General Structures of the Brain

– In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter

– In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS

Gray and White Matter– Like the spinal cord, the brain contains both gray and white

matter. Unlike the spinal cord (in which gray matter forms the interior), in the brain, gray matter forms the surface.

– Specifically, gray matter (consisting of cell bodies and interneurons) covers the cerebrum and cerebellum in a layer called the cortex.

– Underneath the cortex is white matter, although gray matter exists in patches called nuclei throughout the white matter. The white matter contains bundles of axons that connect one part of the brain to another.

Meninges of the Brain

The brain contains four chambers, called ventricles

Ventricles

Two lateral ventricles arch through the cerebral hemispheres: one in the right hemisphere and one in the left.

Each of the lateral ventricles connects to a third ventricle

A canal then leads to the fourth ventricle. This space narrows to form the central canal, which extends through the spinal cord.

• A clear, colorless fluid called cerebrospinal fluid (CSF) fills the ventricles and central canal;

• It also bathes the outside of the brain and spinal cord. • CSF is formed from blood by the choroid plexus (a

network of blood vessels lining the floor or wall of each ventricle).

Cerebrospinal Fluid

Formation and Flow of CSF

• Arterial blood supply is branches from circle of Willis on base of brain

• Vessels on surface of brain----penetrate tissue• Uses 20% of our bodies oxygen & glucose needs

– blood flow to an area increases with activity in that area– deprivation of O2 for 4 min does permanent injury

• at that time, lysosome release enzymes

• Blood-brain barrier (BBB)– protects cells from some toxins and pathogens

• proteins & antibiotics can not pass but alcohol & anesthetics do

– tight junctions seal together epithelial cells, continuous basement membrane, astrocyte processes covering capillaries

Blood Supply to Brain

• The brainstem consists of the midbrain, pons, and medulla oblongata

Divisions of the Brain: Brainstem

Medulla Oblongata

Cerebellum

Diencephalon: Thalamus & Hypothalamus

Cerebrum

Frontal Lobe

Parietal Lobe

Occipital Lobe

Temporal Lobe

Cerebrum

Frontal Lobe•Central sulcus forms the posterior boundary•Governs voluntary movements, memory, emotion, social judgment, decision making, reasoning, and aggression; •is also the site for certain aspects of one’s personality

Parietal Lobe

Temporal Lobe•Separated from the parietal lobe by the lateral sulcus•Governs hearing, smell, learning, memory, emotional behavior, and visual recognition

Temporal Lobe•Concerned with analyzing and interpreting visual information

Cerebrum

Frontal Lobe Parietal Lobe•Central sulcus forms the anterior boundary•Concerned with receiving and interpreting bodily sensations (such as touch, temperature, pressure, and pain); •also governs proprioception (the awareness of one’s body and body parts in space and in relation to each other)

Occipital LobeTemporal Lobe

Insula• Hidden behind the lateral

sulcus• Plays a role in many

different functions, including perception, motor control, self awareness, and cognitive functioning

Inside the Cerebrum

Limbic System: Two key structures the hippocampus and amygdala.

Sometimes called the “emotional brain,” the limbic system is the seat of emotion and learning. It’s formed by a complex set of structures that encircle the corpus callosum and thalamus. In brief, it links areas of the lower brainstem (which control automatic functions) with areas in the cerebral cortex associated with higher mental functions.

Functions of the Cerebral CortexMotor Functions of the Cerebral CortexThe primary somatic motor area is the precentral gyrus.

Functions of the Cerebral CortexSensory Functions of the Cerebral CortexSensory nerve fibers transmit signals up the spinal cord tothe thalamus, which forwards them to the postcentral gyrus.

Functions of the Cerebral Cortex

LanguageEach aspect of language—which includes the ability to read, write, speak, and understand—is handled by a different region of the cerebral cortex.

Cranial NervesThe brain has 12 pairs of cranial nerves to relay messages to the rest of the body

Cranial Nerves

Afferent and efferent neurons

They conduct impulses from the receptors to CNS

They conduct impulses from CNS to the effector

The terminals of dendrites become modified to form receptors

The axon terminals come in contact with the motor end plate to form neuromotor junction

They are sensory in nature They are motor in nature

Grey and white matter

Greyish in color White in color due to present of fatty myelin sheath

Comprises of cell bodies, dendrites and synpases of neurons

Consists of nerve fibres (axons) arising from or to the nerve cells in grey matter

Grey matter is situated on the surface, while white matter is located deeper

In the spinal cord, white matter forms the outer layer and grey matter is located deep into the core

Cranial NervesCranial Nerve: Major Functions:

I Olfactory • smell II Optic • vision III Oculomotor • eyelid and eyeball movement

IV Trochlear • innervates superior obliqueturns eye downward and laterally

V Trigeminal • chewing face & mouth touch & pain

VI Abducens • turns eye laterally

VII Facial • controls most facial expressions

secretion of tears & salivataste

VIII Vestibulocochlear(auditory)

• hearing equillibrium sensation

IX Glossopharyngeal • taste senses carotid blood pressure

X Vagus • senses aortic blood pressure

slows heart rate stimulates digestive organstaste

XI Spinal Accessory • controls trapezius & sternocleidomastoidcontrols swallowing movements

XII Hypoglossal • controls tongue movements

Cranial Nerves

Which cranial nerve is the largest? CN V (Trigeminal)

Which cranial nerve is the only one that exits the "posterior" side of the brainstem? CN IV (Trochlear)

How many cranial nerves are responsible for eye movements? Three: CN III (Oculomotor), IV (Trochlear), and VI (Abducens).

What does "abducens" refer to? The abducens nerve carries motor impulses to the lateral rectus eye muscle which moves the eye laterally causing abduction of the eye.

Which cranial nerves carry gustatory (taste) information? CN VII (Facial), CN IX (Glossopharyngeal) and CN X (Vagus).

Which cranial nerve is the longest? CN X (Vagus) which reaches from the medulla to the digestive and urinary organs.

What two cranial nerves carry sensory information about blood pressure to the brain? CN IX (Glossopharyngeal) and CN X (Vagus).

Which cranial nerve is responsible for pupillary constriction? CN III (Oculomotor).

AUTONOMIC NERVOUS SYSTEM

• Maintain homeostasis • include such things as the secretion of

digestive enzymes, the constriction and dilation of blood vessels for the maintenance of blood pressure, and the secretion of hormones.

• Most of these activities happen independently, or autonomously.

• The ANS sends motor impulses to cardiac muscle, glands, and smooth muscle (as opposed to skeletal muscle, which is innervated by the peripheral nervous system).

• Because the ANS targets organs, it’s sometimes called the visceral motor system.

Visceral Reflexes

The ANS asserts control through visceral reflexes—similar to somatic reflexes discussed earlier, but, instead of affecting a skeletal muscle, these reflexes affect an organ.

Structure of the Autonomic Nervous System

Autonomic motor pathways (both sympathetic and parasympathetic)

Somatic motor pathways

Comparison of Somatic and Autonomic Nervous Systems

Somatic Autonomic

Innervates skeletal muscle Innervates glands, smooth muscle, and cardiac muscle

Consists of one nerve fiber leading from CNS to target (no ganglia)

Consists of two nerve fibers that synapse at a ganglion before reaching target

Secretes neurotransmitter acetylcholine

Secretes both acetylcholine and norepinephrine as neurotransmitters

Has an excitatory effect on target cells

May excite or inhibit target cells

Operates under voluntary control

Operates involuntarily

Divisions of the Autonomic Nervous SystemSympathetic Division Parasympathetic Division

Increases alertness Has a calming effectIncreases heart rate Decreases heart rate

Dilates bronchial tubes to increase air flow in the lungs

Constricts bronchial tubes to decrease air flow in lungs

Dilates blood vessels of skeletal muscles to increase blood flow

Has no effect on blood vessels of skeletal muscles

Inhibits intestinal motility Stimulates intestinal motility and secretion to promote digestion

Stimulates secretion of thick salivary mucus Stimulates secretion of thin salivary mucus

Stimulates sweat glands Has no effect on sweat glands

Stimulates adrenal medulla to secrete epinephrine

Has no effect on adrenal medulla

Has no effect on the urinary bladder or internal sphincter

Stimulates the bladder wall to contract and the internal sphincter to relax to cause urination

Causes “fight or flight” response Causes the “resting and digesting” state

Receptors

Cholinergic ReceptorsAcetylcholine may bind to one of two different types of receptors:

Receptors

Adrenergic ReceptorsThere are also two basic types of adrenergic receptors:alpha-(-)adrenergic receptors and beta-(-)adrenergicreceptors. The following principles are true most of the time:● Cells with a-adrenergic receptors are excited by NE.● Cells with b-adrenergic receptors are inhibited by NE.

Neurotransmitters and Receptors

Sympathetic Division

Neurotransmitters and Receptors

Parasympathetic Division

Impulse Conduction

Synapse

Continuous propagation (continuous conduction)

• Involves entire membrane surface

• Proceeds in series of small steps (slower)

• Occurs in unmyelinated axons (& in muscle cells)

Continuous vs Saltatory propagation

Saltatory propagation (saltatory conduction)

Involves patches of membrane exposed at nodes of Ranvier Proceeds in series of large steps (faster) Occurs in myelinated axons

Continuous vs Saltatory propagation

Unit 7 nervous system nrs 237

Education

Transcript of Unit 7 nervous system nrs 237