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Chapter 3


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McMaster University
Ayesha Khan

CHAPTER 3 85 billion neuron cells → with up to 15000 connections each 86 billion glial cells nuclei are clusters with specific functions tracts are pathways, long distance connections between cells NEUROANATOMY: FINDING YOUR WAY AROUND THE BRAIN Describing the Location in the Brain → rostrum/rostral → beak → caudum/caudal → tail → dorsum/dorsal → back → ventrum/ventral → stomach → superior/inferior also used to refer to dorsal and ventral - coronal section is cut vertically → produces frontal view - horizontal section is cut horizontally, producing dorsal view - sagittal section is cut vertically through the length of the brain, producing a medial view - ipsilateral, contralateral, bilateral - proximal structures are nearby whereas distal are distant. - Afferent go towards the brain while efferent go away from it A WONDERLAND OF NOMENCLATURE OVERVIEW OF NERVOUS SYSTEM STRUCTURE AND FUNCTION CNS (central nervous system) is the brain and spina chord everything else is the Peripheral Nervous System The PNS has two divisions: - The somatic nervous system (SNS) → sensory organs, muscles, joint, skin, to and from the CNS - The autonomic nervous system (ANS) controls internal organs → parasympathetic nerves → rest and digest → sympathetic nerves → fight or flight SUPPORT AND PROTECTION - CNS is protected by skill and vertebrae while PNS has an easier time regenerating - Meninges are three layers of membrane between the brain and skull, protecting the brain - Brain and spinal chord are cushioned by the cerebrospinal fluid (CSF) throught the brain's 4 ventricles, the spinal column, and within the subarachnoid space in the brain's meninges - the blood-brain-barrier limits movement of chemicals from the rest of the body into the CNS (protecting it from toxic substances and infection). Glial cells (astroglia) stimulate capillaries (minute blood vessels) to form tight junctions, preventing blood-borne substances from crossing the capillaries into the CNS tissues BLOOD SUPPLY Brain receives blood supply from arteries that course up from each side of the neck: - 2 internal carotid arteries - 2 vertebral arteries These connect at the base of the brain, branch off into smaller arteries that irrigate the brainstem and cerebellum, and then give rise to 3 arteries that irrigate the forebrain: Anterior Cerebral artery (ACA): irrigates the medial and dorsal parts of the cortex Middle Cerebral artery (MCA): irrigates the lateral surface of the cortex Posterior Cerebral Artery: irrigates ventral and posterior surfaces NEURONS AND GLIA the brain originates in a single undifferentiated neural stem cell (a germinal cell) and give rise to the different types of neurons and glia in the nervous system stem cells give rise to → progenitor cell → which divides into blasts (primitive nervous system cells that do not divide) and can be glial or neuroblasts → then specialize into their forms Sensory neurons - Sensory receptors transduce sensory information into nervous system activity The simplest sensory neurons are Bipolar neurons found in the retina. They consist of a cell body with a dendrite on one side and an axon on the other. - A somatosensory neuron projects from a sensory receptor in the body into the spinal cord. Its axon and dendrite are connected → info does not have to pass through cell body → speeding information conduction Interneuron Located within the brain and spinal cord. Link up sensory-neuron and motor-neuron activity in the CNS. - Have multiple dendrites that branch extensively, only one axon - Include stellate cells, pyramidal cells and purkinje cells Motor Neurons Located in the brainstem and spinal cord Project to facial and body muscles Glial cells: Ependmal cells → line brain's ventricles and make CSF Astroglia → blood-brain-barrier, provide support and nutrition to neurons Microglia → fight infection and remove debris Oligodendroglia → insulate neurons in the CNS Schwann cells → insulate sensory and motor neurons in the PNS (insulation called myelin) GRAY, WHITE AND RETICULAR MATTER - Gray matter: neuronal cell bodies and capillaries - White matter: consists largely of axons → myelinated by oligodendrocytes and Schwann cells w/ white fatty substance - Reticular matter: mixture of cell bodies and axons LAYERS, NUCLEI, NERVES AND TRACTS Nuclei/clusters in PNS are called ganglia Tracts are large collections of axons projecting toward or away from a nucleus or layer in the CNS Fibers and fiber pathways that enter and leave the CNS are called nerves THE ORIGIN AND DEVELOPMENT OF THE CNS The developing brain originates in 3 parts. AT later stages, the front and back part expand and subdivide into 5 regions. 1- front: prosencephalon: responsible for olfaction → the anterior prosencephalon develops to form the cortex and related structures (telencephalon) → the posterior prosencephalon includes the thalamus (diencephalon) 2- middle: mesencephalon: for vision and hearing 3- back: rhombencephalon: controls movement and balance → subdivides into the metencephalon and myelencephalon Forebrain: locus of cognitive processes Brainstem: locus of regulatory functions (drinking, eating, sleeping) Spinal cord: the locus of more reflexive motor functions The brain begins as a tube that stays hollow even after full development, and is filled with CSF. The hollow parts are four ventricles, numbered 1-4. → 1 and 2: Lateral ventricles: form C-shaped lakees underlying the cerebral cortex → 3 and 4: extend into the brainstem and spinal cord - The cerebral aqueduct connects the third and fourth ventricles - CSF flows from the lateral ventricles out through the fourth ventricle to drain into the circulatory system at the base of the brain. THE SPINAL CORD 5 Regions of the spinal column (from top to tail): 1- C – Cervical → control forelimbs (arms) → spinal cord cut here leds to being quadriplegic 2- T – Thoracic → control the trunk → spinal cord cut here leads to being paraplegic 3- L – Lumbar → control the hind limbs (legs) 4- S – Sacral 5- Coccygeal 30 segments present → each forming a ring (dermatome) that encircles the spinal column Each spinal segment connects SNS spinal nerve fibers to the same-numbered dermatome, including the organs and musculature within it. (Spinal cord) - Outter cord → white matter - Inner cord → gray matter → composed largely of cell bodies and has the shape of a butterfly - Divided into posterior-anterior (dorsal-ventral) Anterior → motor → Efferent fibers exit the anterior spinal cord to carry information from the spinal cord out to the muscles, forming a strand of spinal nerve fibers → the anterior root Posterior → sensory → afferent fibers enter the posterior spinal cord to bring information in from the body's sensory receptors. These spinal nerve fibers converge as they enter forming a strand of fibers referred to as posterior root. SPINAL CORD FUNCTION AND THE SPINAL NERVES Bell-Magendie law: the principle that the dorsal/posterior roots in the spinal cord are sensory and the ventral/anterior roots are motor Spinal-Cord Injury Research on spinal cord injury has 3 objectives: 1- Following spinal-cord injury, damage in the cord takes hours to days to develop. Arresting the degenerative process can make an important contribution to sparing function. 2- Inducing fibers to regrow across the damaged section of the spinal cord can restore function. Approaches to establishing new growth involve removing scar tissue, inducing fibers to regrow by pharmacological treatments, and implanting glial cells in damaged regions to stimulate axon regrowth 3- Developing aids to movement such as brain-computer interfaces (BCI) and similar marriages of directed neural activity and technology can bypass injury and aid in restoring movement Spinal Reflexes and Sensory Integration reflexes: specific movements elicited by specific forms of sensory stimulation - Connections between the segments organize more complex movements that require the cooperation of many spinal segments - Different sensory fibers mediate different reflexes (stepping, postural support, bladder control, etc) - There are receptors for pain, temperature, touch and pressure, and the sensations of muscle and joint movement. - Generally, pain and temperature fibers are smallr - Touch and muscle sense fibers are larger - Stimulation of pain and temperature receptors in a limb produce flexion reflexes, which bring the limb inward toward the body and away from injury. → if stimulus if mild, only distal part of limb flexes, as as stimulus gets bigger so does the movements - Stimulation of fine touch and muscle reflection produces extension reflexes, which extend the limb outward, away from the body → maintains contact with the stimulus Cranial Nerve Connections 12 pairs of cranial nerves → conveying sensory and motor signals to and from the head - Afferent (sensory) and efferent (motor) functions - Connected to brainstem → posterior for sensory and anterior for motor functions → some cranial nerves, however, have both functions! - Spinal accessory and vagus nerve → make connections with many internal organs → including heart, gut, glands → via the autonomic nervous system AUTONOMIC NERVOUS SYSTEM CONNECTIONS The ANS regulates internal organs/glands - The sympathetic and parasympathetic divisions of the ANS work in opposition → sympathetic: arouses body for action → parasympathetic: calms down the body - In the sympathetic system, spinal nerves DO NOT directly control the target organs → it is instead connected to the sympathetic ganglia: collections of neural cells that function somewhat like a primitive brain to control the internal organs → forms a chain of ganglia running parallel to the spinal cord - One part of the parasympathetic system connects directly to the spinal cord – the sacral region. However, the greater part of the parasympathetic system derives from 3 cranial nerves: → The vagus nerve → calms most of the internal organs → the facial nerve → controls salivation → the oculumotor nerve → controls pupil dilatation and eye movements - The calming parasympathetic system connects with the parasympathetic ganglia near the target organs - The internal organs, although arranged segmentally in relation to the spinal cord, do not have their own sensory representation within it. Pain in thes
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