Ch 3 - Nervous System Organization

Neuroanatomy

Nuclei = groups of NEURONS in CNS

Tracts = groups of AXONS in CNS

→ use specific terms to describe directions & positions in nervous system

CNS = brain & spinal cord

Neuroanatomy: Direction Terms

1. Rostral-caudal = nose → tail (front to back)

• “anterior-posterior”

2. Dorsal-ventral = back to front axis of body (back to front)

• “superior-inferior ONLY for brain”

3. Medial-lateral = twd middle or twd the side

top of brain = dorsal

bottom of brain = ventral

Neuroanatomy Structure terms

Ipsilateral = on same side of body

Contralateral = on opposite sides of body

ex. feeling on R arm is sent to the contralateral side of the brain (L brain)

Proximal = close together

Distal = far apart

Afferent = movement twd the CNS

ex. sensory info to brain

Efferent = movement away from CNS

ex. motor info to muscles

Anatomical organization

Nomenclature

• many parts of the brain have a functional name + a structural name → either can be used

• many names come from latin or greek or greek variants (don’t follow greek rules)

ex. the most posterior part of the frontal lobe, along the central sulcus: KNOW BOTH

• Precentral gyrus (anatomical name)

• Primary motor cortex (functional name)

NS Structure & Function

• NS can be divided based on its structure or function:

→ CNS = spinal cord + brain

• both protected by bone (skull & vertebrae)

→ PNS = projects out to the body + ISNT protected by bone

1. Somatic NS = provides interaction w external world (afferent&efferent)

2. Autonomic system:

   1. Sympathetic branch = flight-or-flight response

   2. Parasympathetic branch = calming influence on body

Protection of the CNS

1. Bone (skull & vertebral column) surrounds the tissue

2. Meninges surround the CNS to hold it in place + to keep things out

   1. Dura mater = tough OUTER covering

   2. Arachnoid mater = weblike + connects layers + adds cushion

   3. Pia mater = tough INNER layer + adheres to surface of brain (very thin)

note: see ridges when dura mater layer is removed

3. Cerebrospinal fluid around meninges absorbs shocks + carries waste away and nutrients in (salty)

4. Blood-brain barrier for chemical protection

• formed by astroglia (astrocytes) that hold the cells of blood vessel tightly together

• regulates what molecules can exit the blood supply to enter the brain

note: blood kills neurons

Blood Supply to Brain

→ multiple arteries supply blood to brain → ensures damage to one blood vessel doesn't interrupt blood flow to the entire brain

• arteries branch to form smaller arteries → deliver to specific areas of brain

Stroke = interruption of blood supply to brain, bc artery is blocked or bursts (lack of oxygen & nutrients)

• type of deficit observed after tells the clinician abt the location of brain damage (info abt what blood vessel is damaged)

Major Cerebral Arteries:

1. Anterior = along top of brain

2. Middle = middle of brain

3. Posterior = bottom of brain

   

Nuerons & Glia

1. brain starts as neural stem cells (undifferentiated)

2. stems cells divide → neuroblast cells → neurons & glioblasts → glia

overall:

1. stem cells

2. progenitor cells

3. blast cells (produce neuroblasts & glioblasts)

4. specialized cells (neurons & glia differentiate)

note: projecting neurons are sensory or motor neurons

3 Basic Types of Neurons

1. Sensory neurons = transduce info from the environ

• PNS (afferent - twd CNS)

• bipolar (one dendrite + one axon)

• cell bodies sit outside CNS

note: somatosensory neuron has ONLY one projection from the soma (cell body)

2. Interneurons = connect sensory & motor neurons within the CNS

• often have extensive branching of dendrites

• MOST common

• some in PNS

3. Motor neurons = project to muscle to carry out movement

• PNS (efferent - away from CNS)

• found in brainstem & spinal cord (cell bodies, but axons extend outward)

• receive info @ dendrite

note: unipolar = 1 projection off of cell body

Types of Glia

1. Ependymal cell - secretes cerebrospinal fluid

• small, ovoid

2. Astrocyte = nutritive & support functions + cognition + forms blood-brain barrier

3. Microglial cell = defensive function (CNS immune system)

• used bc WBC cant provide defence since blood kills neurons

• small, mesodermally derived

4. Oligodendroglial cell = forms insulating myelin around axons in CNS (brain&spinal cord)

• can create myelin on 1+ axons at once

• asymmetrical

5. Schwann cell = forms insulating myelin arounds nerves in PNS

• creates myelin on ONLY 1 axon at once

• asymmetrical

→ oligo & Schwann cells have same fxn but diff locations, thus they look different

Gray vs. White vs. Reticular Matter

Gray matter = contains cell bodies & capillaries that supply them w blood

• outer part of cortex (CLOSEST to blood)

note: cell bodies require blood the most

White matter = myelinated axons (fat)

• underneath the cortex (inside of brain)

Reticular matter = netlike appearance & is a mix of gray/white matter

• in brainstem (middle brain)

Development of the CNS

→ NS develops from 3 enlargements of the embryonic spinal cord

1. Prosencephalon = divides to form the telencephalon (cortex/brain) & diencephalon (thalamus + hypothalamus)

2. Mesencephalon = remains and becomes the midbrain

3. Rhombencephalon = divides to form the mesencephalon (pons + cerebellum) & myelencephalon (medulla oblongata)

The 4 Ventricles & their Locations

→ brain develops as a hollow tube → hollow parts become the ventricle filled with CSF

1. L Lateral ventricle (in telencephalon)

2. R lateral ventricle (in telencephalon)

3. 3rd ventricle (midline of brain)

4. 4th ventricle (b/w cerebellum & brainstem)

→ tube continues down the centre of the spinal cord

Spinal Cord

= gray matter surrounded by white matter

• Anterior root = nerve roots branch from the cord to carry motor commands to the body

• Posterior root = nerve roots branch from the cord to conduct sensory info into the CNS

→ 4 segments of the spinal cord are named based on the vertebrae the nerve roots pass to exit the CNS:

1. cervical

2. thoracic

3. lumbar

4. sacral

→ damage at any point effects any segment 2 and below it

→ sensory info enters @ back

→ motor info exits @ front

nerves = group of axons in PNS

Dermatomes = region of the body surface innervated by each nerve

Spinal Nerves & Spinal Reflexes

1. sensory neuron (cell body outside of spinal cord) brings sensory info from sensory receptors into posterior root (back) of spinal cord

2. sensory neuron in spinal cord talks to interferon or motor neuron

• some info is sent to brain

3. motor neuron (cell body inside spinal cord) sends motor commands out the anterior root (front) to muscles

   

Reflxes = basic behaviours that occur w/o cognitive inout

• spinal reflexes are generated in the spinal cord on posterior sensory input → direct connection onto the anterior motor output pathways

• complex reflexes can use info across multiple spinal cord segments

Cranial Nerves

→ 12 pairs of nerves branch from the brain & brainstem to provide sensory & motor innervation to the head

• PNS (outside of brain)

• direct to brain (medulla) , NOT spinal cord

• some are afferent (sensory), some are efferent (motor), some are mixed

→ access fxn of these cranial nerves to diagnose issues with NS

KNOW 2 CRANIAL NERVES:

1. olfactory = smell

2. oculomotor = movement of eyes

Autonomic NS

= interacts w internal organs & glands

→ Sympathetic branch = body ready for action

• sympathetic ganglia are close to spinal cord → easier to activate than para

→ Parasympathetic branch = calms body

• parasympathetic ganglia are near the target organs

Brainstem

= hindbrain + parts of midbrain

• extends from where the spinal cords enters the skull to the forebrain

• where the cranial nerves enter the brain

→ Hindbrain:

• cerebellum = motor control & sensory integration (backup brain)

• pons = connects cerebellum with rest of brain + sleep reg

• medulla = regulations breathing & heart rate

• reticular formation (spans pons/medulla) = sends projects to the cortex to maintain alertness & arousal

  • majority of brainstem (hindbrain & mid)