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Chapter 9 (PART TWO)

Functional Areas of the Cerebral Cortex

  • Primary Motor Areas:

    • Located in the posterior frontal lobes, anterior to the central sulcus.

    • Contains pyramidal cells, also known as upper motor neurons.

    • These neurons synapse with lower motor neurons that exit the spinal cord and reach skeletal muscles.

    • Crossover in Brainstem:

    • In motor systems, there is a crossover so that the right cerebral hemisphere controls muscles on the left side of the body.

    • Broca's Area:

    • Located in the frontal lobe (typically left side).

    • Responsible for controlling the muscle movements involved in speech.

    • Frontal Eye Field:

    • Located in the frontal lobe, controls voluntary eye movements.

The Basal Nuclei

  • Definition:

    • The basal nuclei, also known as basal ganglia, are masses of gray matter located deep within the cerebral hemispheres.

    • Key components:

    • Caudate nucleus

    • Putamen

    • Globus pallidus

  • Functions:

    • Produce the inhibitory neurotransmitter, dopamine.

    • Relay motor impulses from the cerebrum and interact with the motor cortex, thalamus, and cerebellum to help control motor activities.

    • Facilitate voluntary movement.

    • Altered activity in the neurons of these nuclei can produce signs of Parkinson's disease and Huntington's disease.

Ventricles and Cerebrospinal Fluid

  • Definition:

    • Ventricles are a series of connected cavities within the cerebral hemispheres and brainstem.

    • These cavities are continuous with the central canal of the spinal cord and subarachnoid space, filled with cerebrospinal fluid (CSF).

    • Flow of CSF:

    1. 2 Lateral ventricles

    2. Interventricular foramina

    3. Third ventricle

    4. Cerebral aqueduct

    5. Fourth ventricle (continuous with the central canal of the spinal cord and subarachnoid space of the meninges).

Choroid Plexuses

  • Definition:

    • Choroid plexuses are masses containing specialized capillaries from the pia mater.

    • Found in all four ventricles.

  • Functions:

    • Secrete cerebrospinal fluid (CSF) into the ventricles, primarily from the lateral ventricles.

    • CSF circulates through the ventricles and connecting passageways into the subarachnoid space, where it is reabsorbed into the blood.

    • The CSF surrounds the brain and spinal cord and serves both nutritive and protective (cushioning) functions.

The Diencephalon

  • Structure:

    • Lies between the cerebral hemispheres and above the midbrain.

    • Surrounds the third ventricle and mainly consists of gray matter.

  • Main Parts:

    • Thalamus

    • Hypothalamus

  • Other Portions Include:

    • Optic tracts and optic chiasma

    • Infundibulum (attachment of the pituitary gland to the hypothalamus)

    • Posterior pituitary

    • Mammillary bodies

    • Pineal gland

Functions of the Thalamus

  • Primary Functions:

    • Sorts and directs sensory information arriving from various parts of the nervous system to the cerebral cortex.

    • Channels all sensory impulses, except those for the sense of smell.

    • Produces general awareness of sensations such as pain, touch, and temperature.

Functions of the Hypothalamus

  • Homeostasis Regulation:

    • Regulates various visceral activities and links the endocrine system with the nervous system.

  • Specific Functions:

    • Regulates heart rate and arterial blood pressure.

    • Regulates body temperature, water and electrolyte balance, hunger, and body weight.

    • Controls movements and secretions of the digestive tract.

    • Helps to regulate sleep and wakefulness.

    • Stimulates the posterior pituitary gland to secrete stored hormones.

    • Produces hormones that prompt the anterior pituitary gland to secrete its hormones.

The Limbic System

  • Definition:

    • Located in the diencephalon, the limbic system controls emotional experience and expression.

  • Components:

    • Includes parts of the cerebral cortex and deep masses of gray matter (thalamus, hypothalamus, basal nuclei).

  • Functions:

    • Modifies behavior by producing feelings of fear, anger, pleasure, and sorrow.

    • Generates emotional responses that guide behavior to enhance the chances of survival.

The Brainstem

  • Components:

    • Composed of:

    • Midbrain

    • Pons

    • Medulla oblongata

    • Lies at the base of the cerebrum and connects the cerebrum, diencephalon, and cerebellum to the spinal cord.

The Brainstem: Midbrain and Pons

  • Midbrain:

    • Located between the diencephalon and pons.

    • Contains bundles of myelinated nerve fibers that convey impulses to and from higher centers of the brain.

    • Contains gray matter masses serving as centers for auditory and visual reflexes.

    • Houses main motor pathways between the cerebrum and lower portions of the nervous system.

  • Pons:

    • Lies between the midbrain and medulla oblongata.

    • Transmits impulses to and from the medulla oblongata and cerebrum.

    • Conducts impulses from the cerebrum to the cerebellum.

    • Contains centers that help regulate the rate and depth of breathing.

The Brainstem: Medulla Oblongata

  • Functions:

    • Transmits all ascending and descending impulses between the brain and spinal cord.

    • Extends from the pons to the foramen magnum.

    • Most corticospinal tracts cross over in the pyramids of the medulla oblongata.

  • Visceral Control Centers:

    • Cardiac center: alters heart rate.

    • Vasomotor center: controls vasoconstriction and vasodilation of blood vessels, thus helping control blood pressure.

    • Respiratory center: controls the rate and depth of breathing.

    • Contains nuclei that control reflexes such as coughing, sneezing, swallowing, and vomiting.

The Brainstem: Reticular Formation

  • Definition:

    • A network of nerve fibers connecting small masses of gray matter scattered throughout the brainstem.

  • Role in Consciousness:

    • Neurons in the reticular formation connect parts of the hypothalamus, basal nuclei, cerebellum, and cerebrum with major ascending and descending tracts.

    • Decreased activity results in sleep; increased activity results in wakefulness.

    • Injuries can lead to a comatose state.

  • Sensory Filtering:

    • Filters incoming sensory impulses to prioritize brain response.

Cerebellum

  • Location:

    • Positioned below the occipital lobes of the cerebrum, posterior to the brainstem.

  • Structure:

    • Consists of two hemispheres connected by the vermis.

    • Has a thin layer of gray matter (cerebellar cortex) outside a core of white matter (arbor vitae).

  • Communication:

    • Communicates with other parts of the CNS via three pairs of tracts known as cerebellar peduncles.

  • Functions:

    • Integrates sensory information about body position.

    • Coordinates skeletal muscle activity.

    • Maintains posture.

    • Ensures movement occurs as intended.

Peripheral Nervous System (PNS)

  • Definition:

    • Composed of nerves that connect the CNS to body parts.

  • Components:

    • Cranial nerves (arising from the brain)

    • Spinal nerves (arising from the spinal cord)

  • Division:

    • Contains sensory and motor divisions.

    • The PNS motor part has two portions:

    • Somatic Nervous System:

      • Connects the CNS to skeletal muscles and skin, overseeing conscious activities.

    • Autonomic Nervous System (ANS):

      • Connects the CNS to viscera, controlling subconscious activities.

Subdivisions of the Nervous System

  • Major Subdivisions:

    1. Central Nervous System (CNS):

      • Brain

      • Spinal Cord

    2. Peripheral Nervous System (PNS):

      • Cranial nerves from the brain and brainstem.

      • Sensory fibers connect peripheral sensory receptors to the CNS.

      • Somatic fibers connect to skin and skeletal muscles.

      • Autonomic fibers connect to viscera.

      • Spinal nerves from the spinal cord.

      • Sensory fibers connect peripheral sensory receptors to the CNS.

      • Somatic fibers connect to skin and skeletal muscles.

      • Autonomic fibers connect to viscera.

Cranial Nerves

  • Overview:

    • Twelve pairs of cranial nerves arise from the underside of the brain.

    • Most are mixed nerves containing sensory and motor fibers.

    • Some are only sensory or primarily motor.

  • Numbering and Arrangement:

    • The first pair arises from the cerebrum, the second from the thalamus, and the rest from the brainstem.

    • Cranial nerves are designated by number and name in order from superior to inferior.

    • Mnemonic: "Oh Once One Takes The Anatomy Final Very Good Vacations Are Happening"

Functions of Cranial Nerves - Part 1

Nerve

Type

Function

1. Olfactory

Sensory

Conducts impulses related to the sense of smell.

2. Optic

Sensory

Conducts impulses related to the sense of vision.

3. Oculomotor

Primarily motor

Conducts impulses to muscles that raise eyelids, move eyes, control light entering the eyes, and focus lenses; some sensory fibers for muscle condition.

4. Trochlear

Primarily motor

Conducts impulses to muscles that move the eyes; includes some sensory fibers related to muscle condition.

5. Trigeminal

Mixed

- Ophthalmic Division: Conducts impulses from eyes, tear glands, scalp, forehead, and upper eyelids.

  • Maxillary Division: Conducts impulses from upper teeth, gums, lip, palate, and skin of face.

  • Mandibular Division: Conducts impulses from skin of jaw, lower teeth, and muscles of mastication and lower lip. |
    | 6. Abducens | Primarily motor | Conducts impulses to muscles that move the eyes; includes some sensory fibers related to muscle condition. |

Functions of Cranial Nerves - Part 2

Nerve

Type

Function

7. Facial

Mixed

- Conducts impulses associated with taste from the anterior tongue.

Spinal Nerves

  • Overview:

    • 31 pairs of spinal nerves arise from the spinal cord, with all except the first pair being mixed nerves.

  • Grouping and Numbering:

    • Grouped based on their origin:

    • 8 pairs of cervical nerves

    • 12 pairs of thoracic nerves

    • 5 pairs of lumbar nerves

    • 5 pairs of sacral nerves

    • 1 pair of coccygeal nerves

  • Roots Structure:

    • Each spinal nerve arises from sensory posterior roots and motor anterior roots.

    • The posterior root contains a posterior root ganglion housing the cell bodies of sensory neurons entering the spinal cord.

    • The anterior and posterior roots unite to form a spinal nerve, which exits the vertebral canal via the intervertebral foramen.

Spinal Nerve Plexuses

  • Definition:

    • The main branches of spinal nerves (except in the thoracic region) form networks called plexuses.

  • Types of Plexuses:

    • Cervical Plexuses (C1 to C4):

    • Located on either side of the neck.

    • Supplies muscles and skin of the neck, including the phrenic nerve which controls the diaphragm.

    • Brachial Plexuses (C5 to T1):

    • Arise from lower cervical and upper thoracic nerves.

    • Supply muscles and skin of arms, forearms, and hands; include musculocutaneous, ulnar, median, radial, and axillary nerves.

    • Lumbosacral Plexuses (L1 to S4):

    • Arise from the lower spinal cord.

    • Supply muscles and skin of lower abdomen, external genitalia, buttocks, and legs; include obturator, femoral, and sciatic nerves.

    • Intercostal Nerves:

    • Anterior branches of the thoracic spinal nerves that do not form plexuses.

  • Function of Plexuses:

    • Sorts and recombines spinal nerve axons so that axons from different spinal nerves can extend to the same part of the body in the same peripheral nerve.

Autonomic Nervous System (ANS)

  • Definition:

    • A portion of the PNS that functions constantly and independently without conscious effort.

  • Functions:

    • Controls visceral motor functions of smooth muscle, cardiac muscle, and glands.

    • Maintains homeostasis, responds to emotional stress, and prepares the body for strenuous activity.

    • Controls heart rate, blood pressure, breathing rate, and body temperature.

General Characteristics of the ANS

  • Regulatory Mechanism:

    • Autonomic activities are regulated by reflexes that have sensory receptors in the viscera and skin.

    • Impulses are conducted to the brain or spinal cord; motor impulses travel through cranial and spinal nerves, then through ganglia, to reach effectors (muscles or glands).

  • Divisions of the ANS:

    • Sympathetic Division:

    • Active in stress or emergency conditions (fight or flight).

    • Parasympathetic Division:

    • Active under normal, restful conditions (rest and digest).

Autonomic Neurons

  • Structure of Pathways:

    • ANS neurons are all motor neurons.

    • Two-neuron pathways consist of:

    • Preganglionic Neuron:

      • Leaves the CNS and synapses with one or more neurons in an autonomic ganglion within the PNS.

    • Postganglionic Neuron:

      • Leaves the autonomic ganglion and innervates a visceral effector (muscle or gland).

Sympathetic Division

  • Preganglionic Fiber Characteristics:

    • Short fibers arise from neurons in the gray matter of thoracic and lumbar spinal cord (T1 to L2).

    • Axons exit the spinal cord through ventral roots of spinal nerves.

    • These axons leave the spinal nerves and enter the sympathetic (paravertebral) ganglia, which are chains alongside the vertebral column.

    • Synapses occur with postganglionic neurons, whose long axons return to spinal nerves before reaching visceral effectors.

    • Occasionally, preganglionic fibers bypass the paravertebral ganglia and synapse in collateral ganglia closer to target organs.

Parasympathetic Division

  • Preganglionic Fiber Characteristics:

    • Long preganglionic fibers arise from the brainstem and sacral region of the spinal cord.

    • Preganglionic fibers extend through cranial or sacral nerves and synapse in terminal ganglia near or within visceral effector organs.

    • Short postganglionic fibers then extend into the effector organs (muscles or glands).

Autonomic Neurotransmitters

  • General Facts:

    • Preganglionic fibers from both divisions release acetylcholine and are termed cholinergic fibers.

    • Parasympathetic postganglionic fibers also release acetylcholine and are cholinergic.

    • Most sympathetic postganglionic fibers release norepinephrine (noradrenaline) and are termed adrenergic fibers.

  • Differences in Effects:

    • The effects of the two ANS divisions often differ due to distinct postganglionic neurotransmitters.

    • Most organs receive innervation from both divisions, typically with opposing effects, while some effectors, like blood vessels, are only innervated by the sympathetic division.

Control of Autonomic Activity

  • Regulation:

    • The autonomic nervous system is primarily controlled by centers located in the brain and spinal cord.

    • Emotional influences from the limbic system and cerebral cortex can modify autonomic responses.