nervous system overview• somatic nervous system • comprised of parts of both cns & pns that...

Nervous System Overview

Dr. Hugo Bergen, Ph.D.

Dept. Human Anatomy & Cell Science

Faculty of Medicine

E-mail: Hugo.Bergen@med.umanitoba.ca

Objectives:

1. Describe the basic organization of the central and peripheral nervous systems.

2. Describe the ventricular system within the brain

3. Describe meninges and the blood supply to the brain

4. Describe the structure and function of the spinal nerves and the cranial nerves

Basic Organization of NS:

The nervous system consists of:

1. Central Nervous System (CNS): i. Brain

ii. Spinal cord

2. Peripheral Nervous System (PNS):o Connects the CNS to tissues and organs in the

periphery. Communication via nerves is both afferent/sensory and efferent/motor.

i. Nerves connect CNS to peripheral targetsa. Cranial Nerves (12 pairs) (Brain)

b. Spinal Nerves (31 pairs) (Spinal cord)

carry out higher mental functions, integrate & coordinate signals

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Limited. Figure 1.3 and 1.4

Spinal Cord

Functional Organization

• Somatic Nervous System• Comprised of parts of both CNS & PNS that are under

voluntary control

• Uses 1 neuron connecting CNS to target tissue

• Sensory: touch, pain, temperature, position

• Motor: voluntary & reflexive movements

• Autonomic Nervous System• Involuntary innervation to smooth & cardiac muscle, and

glandular tissue

• 2 neurons connecting CNS to target tissue (pre-ganglionic & post-ganglionic)

• Divided into sympathetic & parasympathetic divisions

Sympathetic NS

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Limited. Figure 4.2

Note:Pre-Ganglionics are blue&Post-Ganglionics are green

Prevertebralganglion

Prevertebralganglion

Prevertebralganglion

Parasympathetic NS

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Limited. Figure 4.4

Note:Pre-Ganglionics are purple&Post-Ganglionics are amber

Brain

o Constitutes ~ 2% of body weight

o Contains approx. 100 billion neurons

o Specialized for:

• Reception of stimuli

• Transduction of stimuli – electrical/chemical impulse

• Conduction of impulse to various sites of CNS

Central Nervous System (CNS)

CNS = Brain & Spinal Cordo Gray Matter = Cell bodies and dendrites of

neurons. Nucleus/Nuclei = collections of nerve cell bodies and dendrites (sites of connections)

o White Matter = Axons of neurons. Tracts = bundles of axons/nerve fibers that connect neighboring or distant nuclei

o CNS is surrounded by membranous layers & fluid: provides structural support & protection

o Recovers poorly from damage / trauma

Brain - divisions

1. Cerebrum

• Telencephalon

• Diencephalon

2. Brain stem

• Midbrain

• Pons

• Medulla

3. Cerebellum

Mid-Sagittal Views

Cerebrum / Cerebral Hemispheres

Telencephalon consists of:

a. Cerebral Cortex (Gray matter) / Lobes

b. White matter (axons)

c. Ventricles (fluid-filled cavities)

d. Basal Ganglia/Basal Nuclei (Gray; Motor)

Diencephalon consists of:

a. Thalamus

b. Hypothalamus

From: The Nervous System, B. Pentland, in Medical Sciences (2nd

Ed.). © 2015, Figure 8.6 Elsevier Limited.

Frontal

Temporal

Occipital

Parietal

© Dept. HACS, U. of Manitoba

Anatomical Divisions

Cerebral Cortex Functions

Coordination of complex activities:

• Sensory perception

• Voluntary movement

• Language

• Non verbal communication

• Decision making processes / spatial relations

• Memory

• Emotions neurons

These rely on white matter connections

White Matter

• Constitutes the ‘wiring’ of the brain

• Brain function depends on connectivity

• 3 categories of connections:

1. Commissural fibres: connect the left and right cerebral hemispheres to each other

2. Association Fibres: connect cortical regions within a hemisphere (don’t cross midline)

3. Projection Fibres: connect cortex to sub-cortical regions: diencephalon, brainstem, spinal cord

1. Commissural Fibres (cross midline)

• Largest bundle (by far) is corpus callosum (CC)

2. Association Fibres (connect cortical

areas within hemisphere)

Numerous connections connecting gyri within and between lobes

3. Projection Fibres: connect cortex to

sub-cortical areas (to and from cortex)

• Includes corona radiata and internal capsule; projects to spinal cord, brainstem, etc.

Diencephalon: Components and

Functions

1. Thalamus – largest component, relay station for all sensory info. (except smell), transmits to all areas of the cortex. Also regulates consciousness and sleep

2. Hypothalamus – integrates behaviors & visceral function; controls Autonomic N.S.

3. Epithalamus – includes pineal gland: regulates circadian rhythms (releases melatonin)

4. Subthalamus – involved in the regulation of movements; part of basal nuclei

Diencephalon: location

• Deep within brain on either side of midline rostral to midbrain

Brainstem

Brainstem components are considered together:• 10 of the 12 cranial nerves attach to it (III – XII;

form component of Peripheral NS) and cranial nerve nuclei (origin/termination)

• Reticular formation: controls vegetative functions and consciousness/arousal; critical for life

o Respiratory and cardiovascular centres; autonomic reflexes; pain modulation

o Non-conscious adjustments in posture & locomotion

o e.g., large lesions may result in coma

Brainstem (cont.)

• Ascending tracts destined for cortex pass through the brainstem

o All somatosensory information (pain, temp. touch, vibration, & proprioception) from the periphery (ascending from the spinal cord) passes through the brainstem

• Descending tracts pass through brainstem

o Responsible for controlling motor activity of the body

• These tracts are essential for maintaining normal motor and somatosensory function

Cerebellum (lat.: ‘little brain’)

• Gray matter surrounds core of white matter

• Attached to midbrain, pons, and medulla

• Critical for coordinating motor activity and is important for precision and timing of movements

• Motor learning

• Important for maintaining posture

• Cerebellar lesions can result in incoordination of movements

Brainstem & Cerebellum

Medulla

Midbrain

Pons

Midbrain

C. Callosum

Medulla

Pons

© Dept. HACS, U. of Manitoba

C. Callosum

Cerebellum

Cerebellum

Cranial Nerves

Ascending and DescendingTracts

Nuclei(e.g. CNs & RF)

© H. Bergen, University of Manitoba

Cerebellum

Thalamus

Cerebellum: Inputs and Outputs

Inputs:

• Motor-related input from cerebral cortex

• Proprioceptive input from spinal cord

• Vestibular input from vestibular apparatus

Integrates sensory info. with motor info. to produce normal movement patterns

Acts like a comparator; it adjusts motor activity in response to proprioceptive feedback

Cerebellum (cont.)

Outputs:

• Projects to higher centres (cortex via thalamus) to adjust activity of descending motor pathways (from cortex to spinal cord)

• Projects to brainstem circuits to adjust posture and balance; automatic adjustments in posture and locomotion

Spinal Cord

• Rostrally it is continuous with the medulla• Contained within spinal canal of vert. column• 31 pairs of spinal nerves (component of PNS) are

attached to cord White matter surrounds core of gray matter

White Matter:o Ascending tracts carry somatosensory

information from the periphery to higher centres

o Descending tracts carry fibres that control the motor neurons that innervate voluntary muscle

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Limited. Figure

1.19

From: The Nervous System, B. Pentland, in Medical Sciences (2nd Ed.). © 2015, Figure 8.15 Elsevier Limited.

Medulla

Midbrain

Pons

Spinal Cord

Cerebellum

© Dept. HACS, U. of Manitoba

Brainstem &Spinal Cord

Vertebrae

Spinal Cord

• Spinal nerves are the source of the segmental innervation of the body

• Each spinal nerve is formed from a dorsal root (sensory fibres) and a ventral root (motor fibres)

• Dorsal root contains the cell bodies of the sensory neurons that convey pain, temp., touch, etc. (cell bodies are in dorsal root ganglion)

• Ventral root contains motor axons

AMIRSYS | Reference Centers

Dorsal RootVertebra

Dorsal RootGanglion

Ventral RootSpinal Nerve Gray Matter White Matter

Dorsal

Spinal Reflexes

Ventricular System & Cerebrospinal Fluid

• Within the cerebral hemispheres are the large bilateral C-shaped lateral ventricles (n=2) filled with cerebrospinal fluid (csf)

• These drain into the midline 3rd ventricle via the interventricular foramen (foramen of Monro)

• The 3rd ventricle drains into midline 4th ventricle via the cerebral aqueduct

• Total of 4 fluid-filled ventricles

From: The Nervous System, B. Pentland, in Medical Sciences (2nd

Ed.). © 2015, Figure 8.19 Elsevier Limited.

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Limited. Figure 6.7

Cerebrospinal Fluid

• CSF is a clear fluid derived from plasma of the highly vascularized choroid plexus

• Choroid plexus (CP) appears as tufts of material in all 4 ventricles

• Regulates extracellular environment of CNS

• Absorbed into venous circulation

Meninges

• Brain & spinal cord are covered with 3 membranous layers:

1. Dura Mater: tough, dense layer tightly attached to inner surface of skull

2. Arachnoid Mater: thinner layer attached to dura; extensions from arachnoid to pia (‘spider-web’ like)

3. Pia Mater: very thin translucent layer attached to cortical surface; follows contours of the gyri

Meninges - Functions

1. Protection (together with CSF, skull, vertebral column)

2. Compartmentalization of cranial cavity which provides structural support and suspends spinal cord within the dural sleeve

3. Conduit for cerebrospinal fluid (subarachnoid space) and venous blood (venous sinuses)

Cerebrospinal System - Meninges

Arterial Supply to the Brain

Brain consumes ~ 20% of oxygen and is supplied by 2 major pairs of arteries:

1. Internal carotids (anterior circulation): give rise to Anterior cerebral art. (ACA) & Middle cerebral art. (MCA) to supply most of telencephalon (ant., med., & lat.) and diencephalon

2. Vertebrals (posterior circulation): give rise to basilar and Posterior cerebral art. (PCA) supply occipital lobe and inferior part of temporal lobe, brainstem, cerebellum, and cervical spinal cord

Anterior Circulation

Vertebral Artery (Posterior Circ.)

• Vertebrals enter the cranium (foramen magnum) and form the single basilar artery which ascends along the pons before before splitting into posterior cerebral arteries (left and right)

• The anastomosing network of arteries at base of brain is the ‘Circle of Willis’

• The posterior communicating artery connects the anterior circulation with the posterior circulation

Middle Cerebral

Internal Carotid

PosteriorCommunicating

Vertebral

AnteriorCerebral

Anterior Communicating

Basilar

PosteriorCerebral

Dural Venous Sinuses

• A sinus is a channel formed within the dura that carries venous blood from the brain to the internal jugular vein.

Internal Carotid

Sinuses

From: Neuroanatomy: An Illustrated Colour Text (5th Ed.) by Crossman & Neary. © 2015, Elsevier Ltd.. Figure 7.9

Peripheral Nervous System

• Consists of nervous system outside of brain and spinal cord

1. Spinal Nerves (31 pairs attached to spinal cord): each nerve has sensory fibres (enter cord through dorsal root) and motor fibres (exit cord through ventral root)

2. Cranial Nerves (12 pairs attached to brain): innervation of head, neck, and viscera. May have sensory fibres only, motor fibres only, or both. Have specialized sensory functions.

From: The Nervous System, B. Pentland, in Medical Sciences (2nd

Ed.). © 2015, Figure 8.19 Elsevier Limited.

Dorsal Root

Ventral Root

© Amirsys Reference Centers

Spinal Nerve

Terminal Nerves

– Receive contributions from several levels

– Contain both motor & sensory components

Sensory Endings

Sensory system is capable of detecting • Mechanical stimuli (mechanoreceptors)• Pain (nociceptors) and • Temperature (thermoreceptors)Mechanoreceptors (many types) include• Neuromuscular spindles - receptors for stretch,

respond to change in length (basis for stretch reflex i.e. Knee jerk)

• Golgi Tendon organs - stimulated by tension in tendons (protection against damage from excessive stretch)

Spinal Nerves & Roots (cont.)

• Anterior Root = Ventral Root = Motor Root contains efferent fibres; action potentials are leaving the cord

• Motor neurons (cell bodies in the ventral horn of the cord) innervate skeletal muscle (via motor fibres) cause contractions of the muscle (LMNs)

• Descending fibres from the cerebral cortex and brainstem activate these motor neurons to produce voluntary (and reflexive) movement

Cranial Nerves

2 of the 12 attached to cerebrum (CN I and CN II)

10 of the 12 arise from brainstem:

• Midbrain: CNs III (Occulomotor) & IV (Trochlear)

• Pons: CNs V (Trigeminal), VI (Abducens), VII (Facial), and VIII (Vestibulocochlear)

• Medulla: CNs IX (Glossopharyngeal), X (Vagus), XI (Spinal Accessory), & XII (Hypoglossal)

Cranial Nerves

Cranial Nerves (cont.)

Some CNs are entirely motor, some entirely sensory, and some are mixed (motor and sensory fibres) (different than spinal nerves) and also contain visceromotor

Efferents: carry motor fibres and visceromotor (i.e., parasympathetics)

1. Motor fibres: facial expression, eye movement, chewing, swallowing, speech, and 2 neck muscles (trapezius, & sternocleidomastoid)

Cranial Nerves (cont.)

2. Parasympathetics: regulates heart, blood pressure, involuntary muscles, glands, gut

3. Afferents: carry sensory fibres (Somatosensory Special sensory, and Visceral sensations)

Somatosensory: skin and muscles of face

Special sensory: smell, vision, taste, hearing, and balance

Visceral sensations: includes blood pressure, blood oxygen and CO2, various visceral sensations of the thoracic and abdominal viscera

CN I: Olfactory Nerve

CN I = Smell

CN II: Optic Nerve

CN II = Vision

CN III, IV & VI: Occulomotor, Trochlear,

and Abducens Nerves

CN III, IV, and VI = Eye movements

CN V: Trigeminal Nerve

Motor Fibres

Somatosensory FibresCN V = Somatosensory for face and chewing

CN VII: Facial Nerve

Taste

&

Salivary Glands

Facial Expression

CN VII = Facial expression, taste, and glands

CN VIII: Vestibulocochlear Nerve

Semi-circular Canals (n=3)

Cochlea

CN VIII = Hearing and Balance

CN IX: Glossopharyngeal Nerve

CN IX = Parotid gland, taste, sensory (pharynx) and viscerosensory

CN X: Vagus Nerve

CN IX = Swallowing, speech, viscerosensory, and visceromotor

CN XI: (Spinal) Accessory Nerve

CN XI = Shrugging shoulders (trapezius) and turning head (sternocleidomastoid)

Trapezius

Sternocleidomastoid

CN X: Hypoglossal Nerve

CN XII = Tongue movements

Lesions Involving Cranial Nerves

• Deficits may be evident in motor and/or sensory function

• Most commonly involving a specific area of the face and/or neck

• Deficits in cranial nerve function are commonly associated with brainstem lesions

• Assessment Website: www.neuroexam.com